infers that if sickness occurs then there must have been a
conflict. Although this does not necessarily invalidate it, at
least one other group of researchers have found enough problems
with the cue conflict theory to propose an alternative.

Stoffregen and Riccio (1991) presented a critique of the cue
conflict theory from an ecological psychology viewpoint, leading
to an ecological theory of motion sickness (Riccio & Stoffregen,
1991). A fundamental aspect of their theory concerns the idea of
agreement among or within senses which, according to the cue
conflict theory, does not occur in sickness-provoking situations.
They referred to this agreement as redundancy and argued that the
visual, vestibular, and somatosensory systems might not
experience redundant input and such redundancy is not necessarily
expected (Stoffregen & Riccio). Input redundancy is a major
determinant of sickness according to the cue conflict theory:
nonnauseogenic situations are those in which there is redundancy
whereas nauseogenic situations are those in which redundancy is
lacking. If such redundancy is not a reliable standard for the
determination of cue conflict, Stoffregen and Riccio argued that
the conflict theory cannot distinguish between nauseogenic and
nonnauseogenic situations. In fact, they argued that sensory
conflict may not even exist! (Riccio & Stoffregen)

The ecological theory of motion sickness they propose
centers on postural stability or lack thereof. (Postural
instability, or ataxia, is a possible effect of simulator
exposure and is discussed in the next section.) Riccio &
Stoffregen (1991) hypothesized that sickness results when the
individual lacks or has not yet learned strategies for
maintaining postural stability. They argued that postural
instability both precedes sickness symptoms and is necessary to
produce symptoms. To support their theory, they described how
several provocative environments involve postural instability and
they also discuss influences on stability. As one piece of
experimental support for their theory, Riccio, Martin, and
Stoffregen (1992) discussed the results of several experiments in
which no motion sickness was reported in what should have been a
cue conflict situation. The lack of sickness in these
situations, however, is consistent with their theory.

The work of these researchers has been greatly summarized
here, but they describe their theory and the theories leading to
it in much detail. The interested reader is encouraged to see
Stoffregen and Riccio (1988), Stoffregen and Riccio (1991), and
Riccio and Stoffregen (1991) for more information.

Although the ecological theory is a competitor to the cue
conflict theory, the latter currently remains the most widely
accepted theory of simulator sickness, most likely because it has
enjoyed wide exposure in the literature and appears to be
supported by much of the data. Thus, the rest of this report

9



To avoid flicker, refresh rate must increase as both field-of-
view and luminance level increase (Farrell et al., 1988; Maxwell,
1992). At high refresh rates, luminance can be any level, but
displays with faster refresh rates cost more. Thus, if faster
refresh is not an option, dusk conditions (i.e., lower luminance)
may be simulated in a system with a slower refresh rate in order
to prevent flicker.

Resolution

Resolution is a measure of the level of detail provided by
the display and is related to both contrast and luminance level
(Pausch et al., 1992). As with contrast, any adjustment of one
may require adjustment of the other two in order to achieve a
proper visual display, especially at lower luminance ranges. At
higher luminances, these tradeoffs are not as great unless
resolution and contrast are very poor. Luminance level, however,
is related to flicker, which is associated with sickness and was
discussed earlier in this section. Thus, resolution's role in
simulator sickness is most likely to be indirect, stemming from
the relationship to flicker.

Scene content

Scene content is defined as the level of detail available
for a given scene (Pausch et. al, 1992). It affects update rate,
which is discussed later in this section for its role in
simulator sickness. Any association between scene content and
sickness is probably indirect, through the effect of scene
content on update rate.

Time lag (transport delay)

Time lag, also known as transport delay, can be associated
with the motion or visual system. It refers to the delay between
information input to and motion or visual output from the
simulator (Pausch et al., 1992). A driving simulator study by
Frank, Casali, and Wierwille (1988) concluded that visual lag is
more disruptive to both a user's performance and comfort than is
motion lag. Large lag may lead to conflict among cues from the
different simulator systems (e.g., motion, visual, and
instrument) (Pausch et al.).

Uliano et al. (1986), however, found no effect of visual lag
on sickness even though long lags were somewhat disruptive to
performance. They caution against generalizing their results,
however, on two grounds. First, the pilots in their study
performed only two tasks, which had been selected because of
their nauseogenic properties. Second, because the system was
fixed-base, there was no lag possible between visual and motion
cues. Nevertheless, Uliano et al. concluded that visual lag

30



resolution are very poor. Luminance level, however, is related
to flicker. Flicker is believed to be associated with simulator
sickness and is discussed later in this section. Thus, any role
of contrast in simulator sickness is most likely to be through
its indirect relationship to flicker.

Field of view

Field-of-view is defined as the horizontal and vertical
angular dimensions of the display (Pausch et al., 1992).
Simulators with a wide field-of-view generally exhibit higher
incidences of simulator sickness than do those with a narrow
field-of-view (Kennedy et al., 1989). This is likely due to
increased vection arising from increased stimulation of the
peripheral retina from a wide field-of-view display (Kennedy et
al., 1988). Vection plays an important role in simulator
sickness and is discussed in the next section. Anderson and
Braunstein (1985), however, induced vection using only a small
portion of the central visual field with stimuli which appeared
to have depth. This led them to conclude that the representation
of motion and texture cues in the display may actually be more
critical than the display's field-of-view.

A wide field-of-view also increases the likelihood that
flicker will be perceived (Maxwell, 1992). This is because the
peripheral visual system is more sensitive to flicker than is the
fovea (Boff & Lincoln, 1988). Thus, if flicker is to be avoided,
a wider field-of-view necessitates a faster refresh rate
(Maxwell).

Flicker

Flicker has been extensively studied. The interested reader
can find a review of primarily recent references on flicker
perception and simulator sickness in the annotated bibliography
by Rinalducci and MacArthur (1990).

The extensive literature concerning flicker reveals that
flicker is something to be avoided if at all possible since it is
distracting, induces eye fatigue, and appears to be associated
with simulator sickness (e.g., Harwood & Foley, 1987; Pausch et
al., 1992; Rinalducci & MacArthur). The perception of flicker
differs among individuals and depends on an individual's flicker
fusion frequency threshold, as discussed in the previous section.

Several aspects of the visual display affect the perception
of flicker. Of these aspects, those most applicable to the
visual displays of virtual reality systems are refresh rate,
luminance level, and field-of-view (e.g., Boff & Lincoln, 1988;
Farrell, Casson, Haynie, & Benson, 1988; Maxwell, 1992). In
order to suppress flicker, refresh rate must increase as the
luminance level increases (Farrell et al.). Refresh rate must

26



because of the restricted motion of the head. In most
experiments with virtual environments, however, subjects are
likely to be either standing or sitting. Based on their theory
of motion sickness, Riccio and Stoffregen (1991) would predict
that less sickness would occur for seated subjects because of
reduced demands on postural control.

Regan (1993) investigated this issue and compared subjects
who sat while using a VE system to subjects who stood. A total
of 44 subjects were exposed to a virtual environment for 10
minutes. Of the 20 seated subjects, 55% reported ratings greater
than 1 during the session whereas, of the 24 standing subjects,
46% reported ratings greater than 1 (Regan's rating scale is
discussed earlier in this report). Although comparison of the
mean ratings of the subjects yielded non-significant results, it
was noted that the higher ratings of moderate and severe nausea
were reported only in the sitting group.

Type of application

McCauley and Sharkey (1992) have classified VE applications
as "near" and "far". "Near" applications are those which involve
proximate objects, stationary self, and the absence of vection
(discussed later in this section). Because such applications do
not involve whole-body rotations or linear accelerations,
vestibular function is primarily limited to head movements.

"Far" applications are those which involve distant objects, self-
motion through the environment, and vection. It is in these
applications that the vestibular input does not correspond to the
visual display. Thus, McCauley and Sharkey have predicted that
sickness will occur primarily in "far" applications.

Unusual maneuvers

In addition to tasks with high rates of linear or rotational
acceleration, extraordinary or unusual situations should also be
avoided as some have been found to be unsettling (McCauley &
Sharkey, 1992). Two possibly nauseogenic maneuvers identified by
McCauley and Sharkey are abruptly freezing the simulation and
"flying" backwards. Frank & Casali (1986) also recommended that
situational reset be avoided: the scene should not be rapidly
reset forward or backward in time. They also advised that the
scene be blanked for simulator entrance and exit in order to
avoid possibly disorienting effects. These recommendations have
direct applications for HMDs: either the visual display should
be turned off or the subject should be asked to close her or his
eyes when such procedures are necessary.

Vection

One phenomenon closely involved with simulator sickness is
that of illusory self-motion, known as vection. Kennedy et al.

35



Table 1

Potential Factors Associated With Simulator Sickness in Virtual

Environments
Individual
age

concentration
level

ethnicity

experience with
real-world task

experience with
simulator
(adaptation)
flicker fusion
frequency
threshold
gender

illness and
personal
characteristics

mental rotation
ability

perceptual style

postural stability

Simulator
binocular viewing
calibration
color
contrast
field of view
flicker

inter-pupillary
distance

motion platform
phosphor lag

position-tracking
error

refresh rate
resolution
scene content

time lag
(transport delay)

update rate
(frame rate)

viewing region

Task

altitude above
terrain

degree of control
duration

global visual flow
head movements
luminance level
method of movement
rate of linear or
rotational

acceleration

self-movement
speed

sitting vs.
standing

type of
application

unusual maneuvers

vection



Kolasinski, E. M., Jones, S. A., Kennedy, R. S., & Gilson, R. D.
(1994, October). Postural stability and its relation to
simulator sickness. Poster session presented at the 38th
Annual Meeting of the Human Factors and Ergonomics Society.
Abstract appears in Proceedings of the 38th Annual Meeting of

the Human Factors and Ergonomics Society, vol. 2, 980.

Levine, M. W., & Shefner, J. M. (1991). Fundamentals of
sensation and perception. Belmont, CA: Wadsworth, Inc.

Levison, W. H., & Pew, R. W. (1993, February). Use of Virtual
Environment Training Technology for Individual Combat Simula-
tion (ARI Technical Report 971). Alexandria, VA: U.S. Army
Research Institute for the Behavioral and Social Sciences.
(AD A263 546)

Long, G. M., Ambler, R. K., & Guedry, F. E. (1975).
Relationship between perceptual style and reactivity to
motion. Journal of Applied Psychology, 60(5), 599-605.

Maxwell, C. A. (1992, November). Flicker science and the con-
sumer. Information Display, 7-10.

McCauley, M. E., & Sharkey, T. J. (1992). Cybersickness:
Perception of self-motion in virtual environments. Presence,
1(3), 311-318.

Miller, J. C., Sharkey, T. J., Graham, G. A., & McCauley, M. E.
(1993). Autonomic physiological data associated with
simulator discomfort. Aviation, Space, and Environmental
Medicine, 64(9), 813-819.

Money, K. E. (1970). Motion sickness. Physiological Reviews,
50(1), 1-39.

Owen, D. H. (1990). Perception and control of changes in self-
motion: A functional approach to the study of information
and skill. In R. Warren & A. H. Wertheim (Eds.), Perception
and control of self-motion. Hillsdale, NJ: Lawrence
Erlbaum.

Parker, D. E., & Harm, D. L. (1992). Mental rotation: A key to
mitigation of motion sickness in the virtual environment?
Presence, 1(3), 329-333.

Pausch, R., Crea, T., & Conway, M. (1992). A literature survey
for virtual environments: Military flight simulator visual
systems and simulator sickness. Presence, 1(3), 344-363.

Reason, J. T., & Brand, J. J. (1975). Motion sickness. London:
Academic Press.

45



Frank, L. H., Casali, J. G., & Wierwille, W. W. (1988). Effects
of visual display and motion system delays on operator
performance and uneasiness in a driving simulator. Human

Factors, 30(2), 201-217.

Goldstein, E. B. (1989). Sensation and perception (3rd ed.).
Belmont, CA: Wadsworth Publishing Company.

Grandjean, E. (1988). Fitting the task to the man. London:
Taylor & Francis.

Guedry, F. E., & Montague, E. K. (1961). Quantitative
evaluation of the vestibular Coriolis reaction. Aviation

Space, and Environmental Medicine, 32(6), 487-500.

Hamilton, K. M., Kantor, L., & Magee, L. E. (1989). Limitations
of postural equilibrium tests for examining simulator
sickness. Aviation, Space, and Environmental Medicine,
60(3), 246-251.

Harwood, K., & Foley, P. (1987). Temporal resolution: An
insight into the video display terminal (VDT) "Problem."
Human Factors, 29(4), 447-452.

Hettinger, L. J., Berbaum, K. S., Kennedy, R. S., Dunlap, W. P.,

& Nolan, M. D. (1990). Vection and simulator sickness.
Military Psychology, 2(3), 171-181.
Jones, S. A. (1993, April). Simulator sickness implications for

advanced training technologies. Proceedings of the 7th
International Symposium on Aviation Psychology.

Jones, S. A.,, Kennedy, R. S., Lilienthal, M. G., & Berbaum,
K. S. (Unpublished manuscript). Comparison of simulator
sickness incidence in two Navy flight trainers: A field
investigation. Orlando, FL: Essex Corporation.

Kellogg, R. S., & Gillingham, K. K. (1986). United States Air
Force experience with simulator sickness, research and train-
ing. Proceedings of the 30th Annual Meeting of the Human

Factors Society, 1, 427-429.

Kellogg, R. S., Kennedy, R. S., & Graybiel, A. (1965). Motion
sickness symptomatology of labyrinthine defective and normal
subjects during zero gravity maneuvers. Aerospace Medicine,
4, 315-318.

Kennedy, R. S., Allgood, G. O., Van Hoy, B. W., & Lilienthal,
M. G. (1987). Motion sickness symptoms and postural changes
following flights in motion-based flight trainers. Journal

of Low Frequency Noise and Vibration, 6(4), 147-154.

43



INDIVIDUAL

FACTOR

EFFECT (S)

COMMENTS

gender females may be more
susceptible to

motion sickness"

females exhibit
larger FOVs*®

illness and
personal
characteristics

many forms of
illness may result
in increased
susceptibility to
simulator
sickness‘!

possible effect of
characteristics
such as motivation,
goals, or belief of
susceptibility?

mental rotation
ability

greater ability may
be related to a
decrease in space
motion sickness”

mental rotation
must occur for an
individual to
recognize objects
which are not in
their usual
orientations”

perceptual style

* field-independent
individuals have
been predicted to
have higher
sickness rates’

¢ field-dependent
individuals have
demonstrated higher
sickness rates in
several studiesTM®

results are
inconclusive

postural stability

* an alternative to
the cue conflict
theory proposes
that postural
instability causes
motion and
simulator sickness
* good pre-
simulator postural
stability may be
associated with
less sickness”

16

possible
implications for
liability issues
concerned with
public-use VR
systems

(table continues)



SIMULATOR

FACTOR

EFFECT (S)

COMMENTS

viewing region

image distortion
increases as
distance from
design eyepoint

the optimal viewing
position within the
viewing region is
called the design

increases”’ eyepoint”®
A-6 (table continues)



REFERENCES

Arditi, A. (1986). Binocular vision. In kK. Boff, L. Kaufman, &
J. Thomas (Eds.), Handbook of Perception & Human Performance,
(Chapter 23). New York: John Wiley & Sons.

Anderson, G. J., & Braunstein, M. L. (1985). Induced self-
motion in central vision. Journal of Experimental
Psychology: Human Perception and Performance, 11(2), 122-
132.

Antonoff, M. (1993, June). Living in a virtual world. Popular
Science, 83-86, 124, 125.

Baltzley, D. R., Kennedy, R. S., Berbaum, K. S., Lilienthal, M.
G., & Gower, D. W. (1989). The time course of postflight
simulator sickness symptoms. Aviation, Space, and
Environmental Medicine, 60(11), 1043-1048.

Barrett, G. V., & Thornton, C. L. (1968). Relationship between
perceptual style and simulator sickness. Journal of Applied
Psychology, 52(4), 304-308. .

Barrett, G. V., Thornton, C. L., & Cabe, P. A. (1969).
Relationship between embedded figures test performance and
simulator behavior. Journal of Applied Psychology, 53(3),
253-254.

Barrett, G. V., Thornton, C. L., & Cabe, P. A. (1970). Cue
conflict related to perceptual style. Journal of Applied
Psychology, 54(3), 258-264.

Biocca, F. (1992). Will simulation sickness slow down the
diffusion of virtual environment technology? Presence, 1(3),
334-343.

Boff, K. R., & Lincoln, J. E. (1988). Engineering data compen-

dium: Human perception & performance. AAMRL,
Wright-Patterson AFB, OH, 166-91.

Botwinick, J., & Brinley, J. F. (1963). Age differences in
relations between CFF and apparent motion. Journal of
Genetic Psychology, 102, 189-194.

Casali, J. G. (1986). Vehicular simulation-induced sickness,
Volume 1: An overview. IEOR Technical Report No. 8501.
(NTSC TR 86-010). Orlando, FL: Naval Training Systems
Center.

4lL



disorder known as asthenopia, which Stone described as a type of
oculomotor instability.

Ebenholtz (1992) also addressed the issue of asthenopia. He
pointed out that it appears to result from the negative feedback
control of most oculomotor systems. These systems, he stated,
work to correct visual errors over a certain limited range, such
as small differences in the direction in which the two eyes
point. It is working to correct the errors, rather than the
errors themselves, which appears to be problematic (Duke-Elder &
Abrams, 1970). Thus, situations yielding error in eye movement
control and involving the error-control mechanism have the
potential to evoke symptoms of motion sickness (Ebenholtz).

Ebenholtz (1992) concluded that since the display devices of
virtual environments, just like those in simulators, call on
numerous oculomotor systems which may require error-correcting
eye movements, they can potentially produce sickness symptoms.

He noted that a prolonged need for error correction may result in
adaptive shifts of the gain, phase, and direction of eye
movements so that such error correction would no longer be
necessary. Ebenholtz included such adaptive shifts among the
possible effects of exposure to virtual environments. He further
noted that such shifts may not be limited solely to the
oculomotor system. He mentioned aftereffects such as ataxia and
flashbacks as examples of other forms of adaptive shifts
resulting from simulator exposure.

Performance changes

Kennedy, Fowlkes, and Lilienthal (1993) investigated
performance changes following simulator exposure. Subjects were
given three performance tests before and after simulator
exposure: Pattern Comparison, Grammatical Reasoning, and Finger
Tapping. The exposed pilots showed less improvement due to
practice on the Pattern Comparison and Grammatical Reasoning
tests than did the control group. Such differences were not
observed on the Tapping test. Kennedy, Fowlkes, et al. noted
that, of the three performance measures administered, Grammatical
Reasoning is the most sensitive to disruption by stressors
whereas Tapping is highly resistant to disruption. Although the
results indicated minimal and unclear effects, Kennedy, Fowlkes,
et al. nonetheless concluded that performance losses may occur
following simulator exposure.

Quantitative Tools

Measuring simulator sickness: The Simulator Sickness
Questionnaire

Questionnaires or symptom checklists are the usual means of
measuring simulator sickness. This is because of the

13



asynchrony - within the limits studied in their research - is
probably not a contributing factor to simulator sickness.

Update rate (frame rate)

Update rate, also referred to as frame rate, is defined as
the speed of the simulation: the rate at which subsequent frames
of the moving scene can be generated and rendered into the frame
buffer for display (Pausch et al., 1992). Unlike the hardware-
determined refresh rate, it can vary widely based on scene
complexity and available computing power for the simulation
(Pausch et al.). A slow update rate could lead to visual lag,
which may be associated with sickness. Thus, any effect of
update rate on sickness is likely to be indirect and due to its
effect on other aspects of the simulation.

Viewing region

Pausch et al. (1992) defined viewing region as the volume in
front of the display where an observer can be situated and still
see an undistorted, high-quality view of the simulated scene.

The optimal position for the observer is called the design
eyepoint and is located in the center of the viewing region.
Moving away from the design eyepoint increases image distortion.
Outside the viewing region of infinity-focused optics, the
graphics disappear or become of unacceptable quality. Pausch et
al. stated that the effect of this small optimal viewing region
is that some simulator users may be far away from the design
eyepoint even though they are still inside the viewing region.
Thus, simulator sickness incidence and ataxia for these users may
increase due to distorted visuals. This may be one explanation
for the different sickness incidence rates observed among
different crew members (Casali & Wierwille, 1986). Some crew
members, such as pilots, may be located at or closer to the
design eyepoint; whereas other crew members, such as co-pilots,
although still inside the viewing region, may be located away
from the design eyepoint.

Factors Associated with the Simulated Task

Several features of the particular task being simulated may
be associated with sickness. These factors are likely to differ
from task to task and may include such things as altitude above
the terrain, degree of control, duration, global visual flow,
head movements, luminance level, method of movement, rate of
linear or rotational acceleration, self-movement speed, sitting
vs. standing, type of application,” unusual maneuvers, and
vection. These are all discussed below.

31



FOREWORD

The Army has made a substantial commitment to Distributed
Interactive Simulation (DIS) and the electronic battlefield for
training, concept development, and test and evaluation. The
current DIS training System-Simulation Networking (SIMNET)—and
the next generation system-—the Close Combat Tactical Trainer
(CCTT)—provide effective forms of training for soldiers fighting
from vehicles, but these systems are unable to do the same for
individual dismounted soldiers. Virtual Environment (VE)
technology has the potential to provide Individual Combat
Simulations (ICS) for the electronic battlefield. However, our
initial research in the use of VE technology indicated that some
participants experienced simulator sickness—a pattern of symptoms
including nausea, headaches, and disorientation. This has
implications for both training effectiveness and safety. As the
first step in identifying ways to reduce the severity of these
symptoms, we reviewed the research literature on the factors
involved in simulator sickness.

This report describes the result of that literature review.
In addition to influencing future research plans, the research
has directly influenced the approach being used in technical
advisory service provided to Headquarters, U.S. Army Training and
Doctrine Command, on simulator sickness in combat vehicle
trainers.

The U.S. Army Research Institute for the Behavioral and
Social Sciences (ARI) conducts research to improve the
effectiveness of training simulators and simulations. The work
described is a part of ARI research task entitled VIRTUE-Virtual
Environments for Combat Training and Mission Rehearsal.

EDGAR M. JOHNSON
Director

Speotal



stability is related to high Disorientation subscale scores
(Jones, Kennedy, Lilienthal, & Berbaum, 1993).

At the very least, the existence of a relationship between
pre-simulator postural stability and post-simulator sickness
could shed light on the mechanism controlling simulator sickness.
Furthermore, it would provide support for the use of postural
tests as predictors of simulator sickness susceptibility. It
could, however, have specific implications for liability issues
concerned with public-use virtual reality systems. Decreased
postural stability for a given individual might be indicative of
illness, drugs, or alcohol. It is highly likely that individuals
in such states would be more likely to experience sickness,
especially with lower-quality commercial VR systems. Thus, some
specified level of postural stability could be used as a
requirement before an individual would be permitted to use those
systems.

Factors Associated with the Simulator

Jones (1993) implied that the one sure way to eliminate
visually-induced motion sickness is to shut off the visual
system. Distorted graphics, visual lags, and off-axis viewing
are just some of the many aspects of the visual display which
could be problematic (Kennedy et al., 1988). Pausch et al.
(1992) provided an in-depth review of the literature concerning
technical properties of the simulator - specifically, those
associated with the visual display - which may correlate with
sickness. They identified time lag, phosphor lag, refresh rate,
and update rate as potentially the most important aspects of a
simulation system, yet among the most difficult to measure.
Other factors they discussed which may influence sickness are
contrast, resolution, color, field of view, viewing region,
binocular viewing, scene content, and flicker. These are all
discussed in detail below. In addition to these features of the
visual display, other features of the simulator, such as
calibration, inter-pupillary distance in head-mounted displays,
motion platforms, and position-tracking error may be associated
with sickness and are also discussed.

Binocular viewing

Humans can view a display in one of several ways depending
on both the human and the display. First, either one eye or both
eyes can be used. The former can be termed monocular viewing and
the latter binocular viewing. A binocular display, such as an
HMD, can present identical or different images to each eye. The
former is referred to as a monoscopic display and the latter, a
stereoscopic display. Whereas a monoscopic display can provide
depth cues such as relative size and overlap, a stereoscopic
display permits depth perception based on binocular cues
(stereopsis). Although monocular cues are adequate for many

24



will make several references to cue conflict as it relates to
sickness. This is not intended to take sides on the cause issue
but, rather, is simply due to the fact that the cue conflict
theory underlies most of the current research.

Effects of Simulator Exposure

As mentioned in the introduction, the most common effects of
simulator exposure resemble the symptoms of motion sickness:
general discomfort, drowsiness, pallor, sweating, nausea, and,
occasionally, vomiting. These and other typical symptoms are
grouped into three major clusters of symptoms - nausea,
disorientation, and oculomotor discomfort - by the Simulator
Sickness Questionnaire (SSQ) (discussed in the next section).

Along with eye strain, which is identified by the SSQ, and
postural disequilibrium, which was discussed earlier as a post-
simulator safety concern, two additional effects of simulator
exposure - dark focus shifts and changes in performance ~ have
also received attention in the literature. These four effects
are discussed in this section.

Ataxia

A major effect of simulator exposure is postural
disequilibrium, or ataxia. Thomley, Kennedy, and Bittner (1986
suggested that ataxia is due to a disruption in balance and
coordination resulting from the visual and vestibular adaptation
to conflicting cues occurring during simulator exposure. Ina
study of Air Force pilots, Kellogg and Gillingham (1986) found
that 60.4% reported ataxia shortly after simulator exposure. For
14.6% of the pilots, disequilibrium persisted as long as 30
minutes to 10 hours. Additionally, Fowlkes, Kennedy, and
Lilienthal (1987) reported that it has been found that intensity
and duration of ataxia increases with increased simulator
exposure. Ataxia does not always result, however, (e.g.,
Kennedy, Allgood, et al., 1987) but this could be due to the
exposure time or sensitivity of the postural test. It may also
be that some simulators - such as those with motion platforms -
may be more conducive to disequilibrium than others (motion
platforms are discussed later in this report).

Baltzley, Kennedy, Berbaum, Lilienthal, and Gower (1989)
investigated the issue of postflight symptoms. They reported
that unsteadiness and ataxia are of greatest immediate concern
for safety since there have been reports of such posteffects
lasting longer than 6 hours and, in some cases, longer than 12
hours. In their study of free response data from 742 pilot
exposures from 11 military simulators, they found that
approximately half of the pilots (334) reported posteffects of
some kind: 250 (34%) reported that symptoms dissipated in less
than 1 hour, 44 (6%) reported that symptoms lasted longer than 4

10



They studied four variations of the basic tests: the Sharpened
Romberg (also called the Tandem Romberg), Stand-on-One-Leg-Eyes-
Closed, Walk-on-Rail-Eyes-Open, and Walk-on-Line-Eyes-Closed
tests. In the Sharpened Romberg, subjects stand heel-to-toe with
arms folded and eyes closed. Hamilton et al. modified the
Sharpened Romberg and had subjects walk on narrow rails to
increase the difficulty in an attempt to avoid ceiling effects.

In the first phase of the two-phase study, test-retest
reliabilities were examined. It was found that the reliability
coefficients for each test remained relatively stable. Although
learning effects were found, ceiling effects were not. The
Stand-on-One-Leg-Eyes-Closed was found to be the most reliable,
but only the Stand-on-One-Leg-Eyes-Closed and the Sharpened
Romberg reliabilities exceeded .50.

In the second phase of the study, the first phase subjects
performed each of the four tests immediately before and after two
successive 6-minute flight simulator exposures. In addition,
each subject completed a simulator sickness questionnaire. It
was found that, despite subject reports of ataxia symptoms
following the simulator exposure, only the Sharpened Romberg
substantiated the symptoms reported on the questionnaires.
Hamilton et al. (1989) concluded that none of the four tests were
sensitive enough to quantify subjective reports of ataxia and
that more sensitive measures are needed. In fact, because of the
ceiling effects and only moderate reliabilities of such postural
measures, Thomley et al. (1986) suggested that alternative
measures, such as head stability-assessment devices be developed.
The position-tracker in an HMD may be one such method for
assessing head stability.

Predicting simulator sickness: The Motion Sickness History
Questionnaire

Kennedy, Fowlkes, et al. (1992) noted that motion history
questionnaires are useful tools for predicting many forms of
motion sickness. Such questionnaires have high retest
reliabilities with low cost for materials and little
inconvenience for the subject (Kennedy, Fowlkes, et al.). Use of
these motion history questionnaires for the prediction of
simulator sickness, however, has not been as successful. Thus,
Kennedy, Fowlkes, et al. investigated the use of questionnaires
to predict simulator sickness specifically.

The motion history questionnaire employed by Kennedy,
Fowlkes, et al. (1992) was the Kennedy and Graybiel version of
the Pensacola Motion Sickness Questionnaire (used for the
development of the SSQ), referred to as the Motion History
Questionnaire (MHQ). Four different scoring keys were developed
based on different combinations of items from the MHQ (see
Kennedy, Fowlkes, et al. for the specific MHQ items included in

16



In two of their three experiments (one with students and one
with pilots), Fowlkes et al. (1993) found that subjects who were
sick usually had dark focus shifts inward, whereas subjects who
were not sick usually had unchanged dark focus scores or shifts
outward. These results were as predicted based on the increased
parasympathetic activity associated with motion sickness. In
their other experiment (also with pilots), they found different
results: subjects who were sick usually had no change in dark
focus, whereas subjects who were not sick usually had outward
shifts. These results were not as predicted.

To reconcile the two results, Fowlkes et al. (1993
suggested that change in dark focus (using a continuous range
from outward shift to no change to inward shift) is positively
associated with severity of sickness (using a continuous range
from low severity to high severity). The range of these changes
in dark focus, however, depends upon the demands of the
situation. Thus, for a low-demand situation (i.e., the first two
experiments), as sickness severity ranges from low to high,
change in dark focus ranges from no change to an inward shift.
On the other hand, for a high-demand situation (i.e., the third
experiment), as sickness severity ranges from low to high, change
in dark focus ranges from an outward shift to no change.

From these three experiments, Fowlkes et al. (1993)
concluded that the dark focus of accommodation can undergo
systematic change due to simulator exposure. Furthermore, the
nature of this change may depend on the performance demands of
the situation and may be associated with the incidence of
sickness (Fowlkes et al.).

Eye strain

One common effect of exposure to virtual environments is eye
strain and related oculomotor problems. According to Stone
(1993), two groups of British researchers found that only ten
minutes spent wearing a HMD can result in side effects such as
what might be observed after eight hours spent in front of a
Cathode Ray Tube (CRT) display: headaches, nausea, and blurred
vision, for example. Stone expressed concern over the strain
imposed on binocular vision by HMDs. He pointed out that,
whereas binocular vision is fully developed in adults, it is not
fully developed in children under 12 and, thus, is more likely to
break down under stress, causing squinting. It is Stone's
opinion that the visual and motor system effects, although mostly
anecdotal, are potentially serious, especially for lower quality
VE systems such as those geared for entertainment. As Stone
indicated, problems such as binocular convergence, inappropriate
accommodative response to blurred images, unequal focusing
capability in each eye, and inadequate fixation or pursuit eye
movements are all evident in current Liquid Crystal Display
(LCD)-based HMDs. These problems are known to contribute to a

12



tasks, it is widely believed that depth perception is much
enhanced when binocular depth cues can be used (Levine & Shefner,
1991). Arditi (1986), however, noted that stereopsis is not
necessary for valid depth perception. This is fortunate,
perhaps, because, as Pausch et al. (1992) noted, it is difficult
to build a system which allows for true binocular vision since
binocular depth cues are difficult to simulate.

Recent ARI research compared performance between monoscopic
and stereoscopic displays (Ehrlich, Singer, & Cinq-Mars, S., in
preparation). Ehrlich et al. also measured simulator sickness
with the SSQ. Based on previous results, it was hypothesized
that increased sickness would be observed in users of the
stereoscopic display. Indeed, it was found that the mean score
on the Nausea subscale was significantly higher with the
stereoscopic presentation than with the monoscopic presentation.
This result, however, did not hold true for the other SSQ
sickness scores.

Calibration

McCauley and Sharkey (1992) discussed some reasons why
sickness will probably be more prevalent in virtual environments
than in military flight simulators. They pointed out that
commercial VE systems will probably not benefit from the regular
calibration that military flight simulators typically receive.
Lack of calibration could result in increased spatial and
temporal distortions which could set the stage for sickness due
to distorted graphics.

Color

Color is detected by the foveal visual system, whereas
motion is largely detected by the peripheral system (Levine &
Shefner, 1991). Thus, because of the role of motion detection in
simulator sickness, color is not likely to be a factor in
simulator sickness. However, color displays may have lower
resolution (Pausch et al., 1992) and resolution is discussed
later in this section as a possible factor in sickness. Thus,
any role of color in simulator sickness is most likely to be
indirect, stemming from the possible trade-off between the use of
color and display resolution.

Contrast

Contrast may be defined as the ratio of the highest
luminance provided by the display to the lowest (Pausch et al.,
1992). It is also related to resolution and, for low luminance
ranges, any adjustment of either luminance, contrast, or
resolution may require adjustment of the other two in order to
achieve a proper visual display (Pausch et al.). At higher
luminances, these tradeoffs are not as great unless contrast and

25



dwell on their physical state, Regan concluded that sickness may
be common among users of virtual environments.

Theories of Simulator Sickness

Cue conflict

As Kennedy et al. (1988) pointed out, a comprehensive model
of simulator sickness does not currently exist. The primary
theory involves discrepancies between the senses concerning
information about body orientation and motion. This theory is
usually referred to as the cue conflict theory.

Early studies with both fixed-base and moving-base driving
simulators implicated cue conflict as the source of the problems
(Casali, 1986). To reduce conflicts in fixed-base systems, it
seemed logical to add motion to the simulators. Initially,
simple random vibration was added, but this alone was not enough
to reduce the conflicts. Eventually, advances in simulation made
possible the incorporation of acceleration cues which more nearly
approximate those actually experienced during driving. Sickness
still occurred, perhaps because the visual and motion cues were
not in perfect synchrony. Thus, a conflict between the two
systems still could have resulted, possibly setting the stage for
sickness.

The theory of cue conflict is the most widely accepted
theory of simulator sickness. Cue conflict occurs when there is
a disparity between senses or within a sense. The two primary
conflicts thought to be at the root of simulator sickness occur
between the visual and vestibular senses. In a fixed-base
simulator, the visual system senses motion while the vestibular
system senses no motion. Thus, according to the cue conflict
theory, a conflict results. In a moving-base simulator, the
visual stimuli experienced may not correspond exactly to the
motion which the vestibular system registers. Thus, a conflict
can still result.

McCauley and Sharkey (1992) discussed potential sources of
cue conflict which could occur in a virtual environment. They
pointed out that ambiguities among visual, vestibular, and
proprioceptive cues may be created in a VE in the representation
of motion because these systems provide visual cues consistent
with self-motion, but no corresponding vestibular cues. Such
cues are necessary for supporting postural control and
locomotion, with vestibular cues and peripheral vision appearing
especially important for spatial orientation and self-motion
detection. Thus, with ambiguous information, a cue conflict may
develop.

Since perceptual and perceptual-motor systems are
modifiable, people can learn to function adequately despite

7



SIMULATOR

FACTOR

EFFECT (S)

COMMENTS

flicker

flicker is
distracting,
induces eye
fatigue, and
appears to be
associated with
sickness’*?%74

* to avoid flicker,
refresh rate must
increase as both
luminance and FOV
increase’?

* perception of
flicker depends on
an individual's
flicker fusion
frequency threshold

inter-pupillary
distance

fixed distance in
head mounted
displays may
contribute to
ocular discomfort’®

individual
variations may
require adjustable
system parameters

motion platform

* non-
correspondence of
visual and motion
cues may cause
sickness?’

* motion may
produce motion
sickness”®

* moving-base
simulators may
increase ataxia’

* simple random
vibration is not
enough to reduce
sickness”’

¢ most likely not
an engineering
solution to
sickness”

phosphor lag

if excessive,
causes smearing
which may lead to
sickness due to
distorted images”?

definition: the
continued glowing
of the CRT phosphor
from one frame to
the next?

position-tracking
error

may lead to cue
conflicts”

the position
tracker in a VR
system provides
information about
the location of the
user's head and
limbs in space

(table continues)



CONTENTS (Continued)

Page
FACTORS ASSOCIATED WITH THE SIMULATOR. .....-++-.--. 24
Binocular viewing .... 6 ee ee ee ee ee ew 24
Calibration . 2. 6 6 ee ee ee ee ee ee ee 25
Color a 25
Contrast 2. 6 1 ee ee ee ee ee ee ee ee 25
Field of view . 1. 1 eee ee ee ee ee ee 26
Flicker .. ee 26
Inter-pupillary ‘distance ee 27
Motion platform a a 27
Phosphor lag . . 2. ee ee ee ee ee ee eee 28
Position-tracking error ....--. eee ee eee 29
Refresh rate . 6. 1 6 6 ee ee ee wee eee ee 29
Resolution ©... 6 ee eee ee ee ee ee ee 30
Scene content rs ee 30
Time lag (transport delay) as 30
Update rate (frame rate) . ... -. 2. 2+ e+ eee eee 31
Viewing region . . 1... 1 ee ee ee ee ee ee 31
FACTORS ASSOCIATED WITH THE SIMULATED TASK ....... 31
Altitude above the terrain .........+-+e4e- 32
Degree of control Or 32
Duration . 2. 1 ee ee ee ee ee ee ee ee 32
Global visual flow .. .. 2. 6 ee ee ee ee eee 32
Head movements . 2. 2. 6 6 6 ee ee ee ew ee ee 33
Luminance level er 33
Method of movement . eee eee 34
Rate of linear or rotational acceleration oe eae 34
Self-movement speed or 34
Sitting vs. standing .. 1... ee eee ee ee 34
Type of application . ... 1. eee ee ee eee 35
Unusual maneuvers .. 2. 1 ee ee ee ee ee ee 35
Vection 2. 2. 6. 6 ee ee ee ee ee ee eee 35
SUMMARY: POTENTIAL FACTORS ASSOCIATED WITH SIMULATOR
SICKNESS IN VIRTUAL ENVIRONMENTS .......4-2+46-. 36
AREAS FOR FUTURE RESEARCH SUGGESTED BY THE LITERATURE . . 37
Correlating visual scene elements with simulator
sickness Pr 37
Eye strain ..... . : es 37
Physiological measures. of ‘simulator sickness .... 37
CONCLUSIONS . . 1 ee ee ee ee ee ee ee ee ee 38
REFERENCES . «1. 2 6 6 2 6 6 ee ee ee ee ee ee 41



or, in other words, the extent to which an individual perceives
analytically (Witkin, Moore, Goodenough, & Cox, 1977).

Several tests can be used to classify an individual's
perceptual style. The classical one is the Rod and Frame Test
(RFT). This test measures the accuracy with which an individual
can adjust a rod to the true vertical position under conditions
of visual-kinesthetic conflict. Another test of perceptual style
is the Embedded Figures Test (EFT), which measures the subject's
ability to extract a geometric pattern from a complex pattern.
Based on performance on tests such as these, an individual's
perceptual style is classified as either field-independent or
field-dependent. "Field-independent" individuals are able to
perceive items as separate from a surrounding field - such
individuals are able to adjust the rod to its true vertical with
high accuracy and can successfully extract geometric patterns
from the complex patterns. The perception of "field-dependent"”
individuals, however, is strongly dominated by the surrounding
field - such individuals are unable to accurately adjust the rod
to its true vertical and have difficulty discerning geometric
patterns from complex patterns.

It has been suggested that field-independent individuals are
more sensitive to body cues than are field-dependent individuals
(Barrett & Thornton, 1968). Because of this sensitivity and the
conflict between static body cues and dynamic visual cues in a
moving display, field-independent individuals have been predicted
to be more susceptible to simulator sickness than field-dependent
individuals (Barrett & Thornton, 1968).

Barrett and Thornton (1968) and Barrett, Thornton, and Cabe
(1969, 1970) investigated the possible relationship between
perceptual style and simulator sickness. Barrett and Thornton
(1968) found that all of the extremely field-independent subjects
left the simulator and, even though some field-dependent subjects
also became ill in the simulator, they concluded that the results
supported their prediction that field-independent individuals
would experience more discomfort. Barrett, Thornton, and Cabe
(1969), however, found no relationship between simulator sickness
and perceptual style as measured with the EFT. Barrett,
Thornton, and Cabe (1970) investigated the relationship between
perceptual style and cue conflict induced by a "haunted swing"-
like device. The results indicated that, although many subjects
did experience discomfort in the swing, it was the field-
dependent individuals who experienced the most discomfort -
opposite the Barrett and Thornton hypothesis.

Although there would be great theoretical and practical
value if a predictive relationship could be found between
perceptual-style and susceptibility to motion sickness (Long,
Ambler, & Guedry, 1975), the only clear result that can be
discerned from these studies is that there is no clear result.

22



that pilots with more flight experience and little simulator time
are more prone to simulator sickness than are those with little
aircraft flight time. Although the relationship is often
observed, this finding is not fully consistent in the literature
(Kennedy et al., 1988). Kennedy et al. (1988) suggested that,
for the cases in which such a relationship is observed, the
pilot's experience with the sensory aspects of actual flight
might lead to greater sensitivity to the discrepancies between
actual and simulated flight. Pausch, Crea, and Conway (1992)
offered two other possible explanations. Degree of control of
the task may be a factor since student pilots tend to handle the
flight controls more than the instructor pilots. Second, viewing
region may be a factor if the optimal viewing position is placed
at the student's location. Both of these factors are discussed
later in this report for their possible role in sickness. For
the cases in which a positive relationship between experience and
sickness is not observed, Kennedy et al. (1988) suggested that
the pilot's experience may result in protection through some
mechanism of adaptation or that susceptible individuals may have
been self-selected out of a career in aviation.

Experience with the simulator (adaptation)

Uliano, Lambert, Kennedy, & Sheppard, (1986) found that
pilots who experienced sickness on initial simulator hops were
able to rapidly adapt to the simulator on following hops and,
therefore, experienced less sickness over time. Thus, increased
experience with the simulator - adaptation - generally leads to a
decreased incidence of sickness. This could be the result of
building a tolerance to sickness-inducing stimuli and learning
adaptive behaviors to avoid sickness. Although this adaptation
may help reduce sickness, Kennedy and Frank (1983) pointed out
that it may cause problems when the individual returns to the
normal environment. Similarly, Regan (1993) suggested that
repeated immersions in a VE system will result in a decrease in
sickness as subjects become more accustomed to, and confident
about, interaction with the system. She added that it has been
suggested that adaptation may lead to reduced symptoms during
immersion, but greater levels of post-immersion symptoms.

Flicker fusion frequency threshold

Flicker is discussed in the next section for its role in
sickness and all of the issues raised are properties of the
display device. There is, however, another issue associated with
flicker which is a property of the individual: the flicker
fusion frequency threshold. This threshold is defined as the
point at which flicker becomes visually perceptible. Grandjean
(1988) indicated that the human flicker fusion frequency
threshold is a circadian bodily function which increases by day
and decreases by night. Thus, the threshold frequency at which
flicker is detectable is reduced at night. In addition, there is

19



SIMULATOR SICKNESS IN VIRTUAL ENVIRONMENTS

EXECUTIVE SUMMARY

Requirement:

The Army has made a substantial commitment to Distributed
Interactive Simulation (DIS) and the electronic battlefield for
training, concept development, and test and evaluation. The
current DIS training system-Simulation Networking (SIMNET)—and
the next generation system-the Close Combat Tactical Trainer
(CCTT)—provide effective training for soldiers fighting from
vehicles, but are unable to do the same for individual dismounted
soldiers. Virtual Environment (VE) technology has the potential
to provide Individual Combat Simulations (ICS) for the electronic
battlefield. However, initial research in the use of VE
technology indicates that some participants experience simulator
sickness—a pattern of symptoms including nausea, headaches, and
disorientation. This has implications for both training
effectiveness and safety. This report is the first step in the
identification of ways to reduce the occurrence and severity of
these symptoms.

Procedure:

Since the research literature of simulator sickness in VEs
is very limited, the literature on sickness in other types of
simulators and, to a lesser extent, the literature on the related
phenomenon of motion sickness, were reviewed. The factors
believed to affect the duration and severity of simulator
sickness were organized into three groups: simulator factors,
task factors, and individual factors.

Findings:

Although there is debate as to the exact cause or causes of
simulator sickness, a primary suspected cause is inconsistent
information about body orientation and motion received by the
different senses, known as the cue conflict theory. For example,
the visual system may perceive that the body is moving rapidly,
while the vestibular system perceives that the body is
stationary. Inconsistent, non-natural information within a
single sense has also been prominent among suggested causes.

Although a large contingent of researchers believe the cue
conflict theory explains simulator sickness, an alternative

vii



ACKNOWLEDGMENTS

The author would like to thank the following individuals for
their help with this Technical Report:

Sherrie A. Jones for reviewing the draft. Ms. Jones
provided much assistance during the entire development of this
report and the author is greatly indebted to her.

Robert H. Wright for reviewing the draft.

Bruce W. Knerr for reviewing several versions and assisting
with the entire development and preparation process of this
report.

Robert S. Kennedy for assisting with an understanding of the
details of many technical aspects associated with the information
contained in this report.

Anthony D. Andre for providing information concerning the
Vertical Motion Simulator at Ames.

Guillermo Navarro for assisting with the literature
concerning flicker.

Donald R. Lampton for supporting this undertaking.

vi



altered conditions such as visual and auditory rearrangements
(Welch, 1978). McCauley and Sharkey pointed out that adaptation
to a transformed world does not happen immediately. Furthermore,
adaptation time depends upon the type of transformation (Welch,
1986). Individuals who adapt quickly may avoid sickness whereas
those who adapt slowly may become sick before completely adapting
(McCauley & Sharkey, 1992). McCauley and Sharkey offered several
potential sources of transformation in a VE including optical
distortions, temporal distortions, and the altered correspondence
between visual and vestibular information concerning spatial
dynamics. They suggested that large transformations and longer
exposure times to virtual environments will result in an
increased incidence of sickness and will require longer
adaptation periods.

The theory of cue conflict provides one explanation for the
occurrence of sickness in certain situations, namely, those in
which there is a conflict among sensory cues. The theory does
not, however, provide any explanation for why sickness occurs
from an evolutionary or adaptive point of view. Treisman (1977)
proposed just that when he suggested that sickness could be the
result of a misapplication of an adaptive mechanism designed to
protect an organism. His hypothesis consists of three major
arguments. First, the eye-head and head-body systems involved in
controlling movement must be highly sensitive in order to carry
out their function. Second, neurotoxins in the body can affect
movement control. Thus, because of their high sensitivity, the
systems which control movement could also function as an early
warning system for the detection of toxins in the body. Third,
such toxins, if ingested by an organism, usually trigger an
emetic response. Putting these three arguments together,
Treisman hypothesized that the emetic response associated with
motion sickness may be due to a mechanism which responds to
ingested toxins but which can also be mistakenly triggered in
nauseogenic situations. Furthermore, the nausea and malaise
responses may be viewed as an aversive conditioning mechanism
which help the organism avoid future ingestion of such toxins.
Treisman's hypothesis, which suggests an adaptive benefit for the
occurrence of motion sickness, is one of the few explanations for
why such effects may occur.

Postural instability

Although cue conflict is the most widely accepted theory
associated with simulator sickness, it is not without its
critics. One problem several researchers have noted (e.g., Frank
& Casali, 1986; Stoffregen & Riccio, 1991) is that it has little
predictive power concerning sickness. According to the cue
conflict theory, sickness will occur in situations where there is
a mismatch between experienced stimuli and expected stimuli. It
does not, however, predict which situations will result in a
mismatch or how severe it will be. The cue conflict theory only

8



SIMULATOR SICKNESS IN VIRTUAL ENVIRONMENTS

CONTENTS
Page
INTRODUCTION . . 2 ee ee ee ee ee ee ew ee 1
Background .....+++-+s-s rs 4
Consequences of simulator sickness rs 5
Simulator sickness compared with motion sickness 5
Expected incidence and severity of simulator
sickness in virtual environments erry 6
THEORIES OF SIMULATOR SICKNESS .....-.-.+ ++ © 7
Cue conflict . 2... 1 ee ee ee ee ee 7
Postural instability ...... 2... ee eee 8
EFFECTS OF SIMULATOR EXPOSURE . . . .- +. + + + e+ eee 10
Ataxia .. Ce oe 10
Dark focus shifts soe ee se eee . oo. 11
Eye strain . 1... 2 ee ee ee ene * oo. 12
Performance changes er 13
QUANTITATIVE TOOLS .... ee ee ee ee ee ee 13
Measuring simulator sickness: The Simulator
Sickness Questionnaire . soe ee eee 13
Evaluating ataxia: Postural tests eee eee 15
Predicting simulator sickness: The Motion
Sickness History Questionnaire ee ee ee 16
FACTORS ASSOCIATED WITH THE INDIVIDUAL ....... 17
Age 2. eee 17
Concentration level Ce 18
Ethnicity se we rr ere er es 18
Experience with the ‘real-world task ee ee we 18
Experience with the simulator (adaptation) .. . 19
Flicker fusion frequency threshold ....... 19
Gender .... . eee eee 20
Illness and personal characteristics ers 20
Mental rotation ability eee eee ee eee 20
Perceptual style . . .. 2. . ee ee eee ene 21
Postural stability .... 1... 2 ee ee eee 23

ix



polysymptomatic nature of the sickness: measuring just one sign
or symptom would not be sensitive enough (Kennedy & Fowlkes,
1992). One frequently used questionnaire is the Pensacola Motion
Sickness Questionnaire (MSQ) (Kellogg, Kennedy, & Graybiel

1965). This questionnaire is a self-report form consisting of 23
symptoms which are rated by the subject on a 4-point severity
scale (none, slight, moderate, severe). Although the multi-
symptom scoring of the MSQ takes into account polysymptomaticity,
a major deficiency for its application to the study of simulator
sickness is that the single resultant score provides no
information about the multiple, separable dimensions of the
sickness (Kennedy & Fowlkes, 1992). This deficiency led to the
development of the Simulator Sickness Questionnaire (SSQ)
(Kennedy, Lane, Berbaum, & Lilienthal, 1993).

The SSQ was derived from the MSQ using factor analyses of
1,119 MSQs collected at 10 simulator sites. The resulting SSQ
reduced the symptom list to 16 symptoms, which are rated by the
subject on a 4-point scale (0=absent, l=slight, 2=moderate,
3=severe). Based on the results of the factor analysis, these
ratings form the basis for three subscale scores - Nausea,
Oculomotor Discomfort, and Disorientation - as well as a Total
Severity score. The symptoms making up the three subscale scores
are as follows: Nausea - general discomfort, increased
salivation, sweating, nausea, difficulty concentrating, stomach
awareness, and burping; Oculomotor - general discomfort, fatigue,
headache, eyestrain, difficulty focusing, difficulty
concentrating, and blurred vision; and Disorientation -
difficulty focusing, nausea, fullness of head, blurred vision,
dizzy (eyes open), dizzy (eyes closed), and vertigo (Kennedy,
Lane, et al., 1993). (Note that some symptoms appear on more
than one subscale; this is a characteristic of the factor
analysis procedure.) The Total Severity score uses all of the
symptoms.

As is discussed later in this report, individuals who are
not in their usual state of fitness (e.g., suffering from a cold
or flu, hangover, etc.) tend to have an increased susceptibility
to simulator sickness. Because of this, Kennedy, Lane, et al.
(1993) advised that, in administering the SSQ, such individuals
should not be included in the sample. Additionally, only post-
exposure data are typically scored since there is a high
correlation between pre- and post-exposure scores. These
restrictions probably do not pose a problem with flight
simulators because pilots form such a relatively homogeneous
group - they tend to be in good physical shape overall and are
usually in good health when they arrive for simulator training.
However, because of the more diverse user population potentially
associated with VE systems, military data may not be comparable
to general population data, especially if different scoring
systems are used. For instance, pre-exposure sickness scores may
need to be considered in interpreting post-exposure scores.

14



Technical Report 1027

Simulator Sickness in Virtual
Environments

Eugenia M. Kolasinski
U.S. Army Research Institute

May 1995

19950630 166

United States Army Research Institute
for the Behavioral and Social Sciences

DTI@ QUALITY INSPECTED 8

gY Q
Ney hos

Approved for public release; distribution is untimited,



Factors within the three categories are listed in
alphabetical order. This is to give equal importance status to
all of factors instead of implying that some factors are "above"
or "below" others on some type of scale. Furthermore, the
alphabetized list simplifies use in a reference capacity. For
more detail on a particular factor, each entry refers to a sub-
section in either the individual, simulator, or task sections.
Superscripts refer to the abbreviated references at the end of
the table. All of these references appear in complete form in
the reference list for the report.

Table Al attempts to integrate the major findings of the
current literature as it relates to simulator sickness in virtual
environments. It should be emphasized that this is a working
table. Many factors have been identified which do not have a
clear effect on sickness. Thus, this table will see many
modifications and more information can be added as the state of
the research progresses.

Areas for Future Research Suggested by the Literature

Correlating visual scene elements with simulator sickness

Kennedy, Berbaum, et al. (1993) reported on preliminary work
being done to record the visual scene in flight simulators via
video frame-by-frame decomposition. The ultimate goal of this
endeavor is to analyze the visual scene and relate elements of it
to the incidence of simulator sickness. In the initial study,
they have attempted to identify the attributes most likely to be
related to sickness. Future work will attempt to address the
relationship of those attributes to simulator sickness.

Eye strain

It was discussed earlier that Stone (1993) has expressed
concern over effects of the head-mounted display on the visual
system. Although anecdotal evidence for such effects abounds,
scientific evidence is lacking. Thus, Stone has recommended that
applied psychology and ophthalmic research be combined to form an
international standard. He has also recommended that a standard
virtual test environment be developed which could form a
foundation for an experimental paradigm, as well as provide a
means of testing new VE equipment. One such test battery, the
Virtual Environment Performance Assessment Battery, has been
developed and tested by the Army Research Institute (Knerr,
Lampton, Bliss, Moshell, & Blau, 1993).

Physiological measures of simulator sickness

The most common measures of simulator sickness are
questionnaires and postural tests. Physiological measures are
not often used, possibly because of equipment costs or effort

37



SIMULATOR SICKNESS IN VIRTUAL ENVIRONMENTS
Introduction

"virtual Reality" (VR) is one of the hottest technologies of
the '90s. Already popular in arcades for entertainment purposes,
VR (also known as Virtual Environment or VE) technology shows
many promising applications in areas such as training, medicine,
architecture, astronomy, data handling, and teleoperation (i.e.,
remote control). Work in VE research centers includes a
molecular docking system that has been used to create anticancer
medicines, VE-based radiology treatment planning, and VE
architectural walkthroughs of building interiors (Rheingold,
1991). Additional examples of VE technology include the
virtuality Dactyl Nightmare arcade game. Matsushita’s Kitchen
World (a simulated kitchen that can be explored via a virtual
environment), and the prototype ski training system by the Nippon
Electronic Company Corporation of Tokyo (Antonoff, 1993). Both
the U.S. Army and Navy are intently interested in the training
applications of virtual environments. As part of its commitment
to Distributed Interactive Simulation, the U.S. Army Research
Institute for the Behavioral and Social Sciences (ARI) is
specifically looking at VE technology in training dismounted
infantry on the electronic battlefield (Levison & Pew, 1993).

A virtual environment can be defined in many ways. The
definition used for this report is a three-dimensional,
interactive, realistic, real-time computer-generated simulation
providing direct input to the senses via a head-mounted display
(HMD), Binocular Omni-Oriented Monitor (BOOM), DataGlove and
similar devices. From the standpoint of a user, there are three
major components of a VE system. First, the user must have some
way of seeing in the virtual environment. This is usually
accomplished with an HMD. Second, the user must have some way of
moving through the VE. Joysticks, spaceballs, and wired clothing
devices are some of the current devices used to control movement
in the VE. Finally, there must be some way to identify the
user’s direction of view in the VE. This is accomplished by
means of a tracking device, often attached to the HMD.

In some ways, the above definition of a VE may be more
representative of the goal of VE technology rather than its
current state. Today’s virtual environment is not necessarily
fully three-dimensional, fully interactive, completely realistic,
nor carried out in exact real time. Future VE systems will be
defined by both technology developments and research on necessary
characteristics.

Although this new technology is very promising, there exists
a potential threat to the ultimate usability of virtual
environments: some users experience discomfort during, and
sometimes after, a session in a simulated environment. Similar

1



CONTENTS (Continued)

APPENDIX A.

POTENTIAL FACTORS ASSOCIATED WITH
SIMULATOR SICKNESS IN VIRTUAL
ENVIRONMENTS . . . ss - sone

xi



SIMULATOR

displays may result
in increased
sickness

| FACTOR EFFECT (S) COMMENTS
| binocular viewing stereoscopic definitions:

* monoscopic
display: same image
presented to both
eyes

* stereoscopic
display: slightly
different images
presented to each
eye

there are more
tradeoffs among
required contrast,
luminance, and
resolution;
luminance level is
related to flicker”?

calibration poor calibration commercial VR
may lead to various | systems might
distortions which suffer from lack of
could lead to regularly scheduled
sickness due to calibration’®
distorted graphics’®
color color displays may color is detected
have lower by the foveal
resolution” visual system but
motion is largely
detected by the
peripheral system
contrast with poor contrast definition: the

ratio of the
highest luminance
in the display to
the lowest”

field of view (FOV)

wider FOV increases
both:

¢ incidence and
severity of
sickness”

¢ likelihood of
perception of
flicker’

to avoid flicker,
refresh rate must
increase as FOV
increases’

(table continues)



Sharkey recommended that self-movement in a virtual environment
should be at high altitudes above the terrain and/or at low
speeds.

Head movements

> Reason and Brand (1975) stated that a significant reduction
in motion sickness occurs when an individual adopts a supine
position. They attributed this to restricted motion of the head.
Head motions are known to be associated with motion sickness
through the mechanisms of Coriolis and pseudo-Coriolis
stimulation. Coriolis stimulation occurs when the head is tilted
out of the axis of rotation during actual body rotation (Dichgans
& Brandt, 1973; Guedry & Montague, 1961). Pseudo-Coriolis
stimulation occurs when the head is tilted as perceived self-
rotation is induced from visual stimuli (Dichgans & Brandt,
1973).

During her study of the frequency of occurrence and severity
of sickness in virtual environments, Regan (1993) noted that some
subjects moved more slowly and cautiously through the VE and made
fewer head movements than others. To investigate this matter
further, another study was conducted in which two groups of
subjects were compared (Regan, 1993). One group underwent
actions in the VE which were designed to maximize head movements
and speed of interaction with the system. These subjects were
compared to subjects in an earlier study who controlled their own
head movements and speed of interaction. VE exposure for the
first group lasted 10 minutes and, for the control group, only
the first 10 minutes of their VE exposures were analyzed. In the
experimental condition, 50% of the subjects reported ratings
greater than 1 whereas, in the control group, only 36% of the
subjects reported ratings greater than 1 (Regan's rating scale is
discussed earlier in this report). Since mean ratings for the
two groups did not differ significantly, Regan concluded that
some factor other than head movement and speed must have been
responsible for the level of side-effects reported.

Luminance level

Luminance is defined as the intensity or brightness of the
light coming from the display and is related to both contrast and
resolution (Pausch et al., 1992). Thus, any adjustment of one
may require adjustment of the other two in order to achieve a
proper visual display, especially at lower luminance levels
(Pausch et al.). At higher luminances, these tradeoffs are not
as great unless contrast and resolution are very poor.

Luminance level is related to flicker which, as explained in
the previous section, is believed to be associated with simulator
sickness. To avoid flicker, refresh rate must increase as
luminance level increases (Farrell et al., 1988) and, with high

33



Parker and Harm (1992) stated that mental rotation is
important for efficient goal-directed locomotion - a common task
in a VE - since a person must orient in order to locomote
efficiently. They cited several examples which support their
claim that the ability to perform mental rotation is important
for competent function and the reduction of motion sickness.

The first study involved cosmonauts during a Soviet space
mission. These cosmonauts were trained in a mental rotation
procedure prior to flight and, during the mission, they
significantly improved their performance on the procedure.
Parker and Harm (1992) argued that, during the mission, the
cosmonauts learned to locomote in their microgravity environment,
a task which required mental rotation ability. They suggested
that by improving their performance on this complex task,
performance on the easier mental rotation procedure was also
improved.

As a second piece of supporting evidence for their theory,
Parker and Harm (1992) pointed out that different astronauts
appear:to have different methods of dealing with the sensory
disturbances experienced in microgravity. Astronauts who appear
to deal with the absence of gravity by paying more attention to
internally-generated orientation vectors - especially the one
associated with their Z-body axis (up-down) - are termed Type IZ.
Parker and Harm suggested that these astronauts, who generally
report little or no motion sickness during their space flights,
are able to ignore visual cues for upright. They have conducted
their own research on the matter of mental rotation and space
motion sickness using the Device for Orientation and Motion
Environments Preflight Adaptation Trainer (DOME-PAT), a
microgravity simulator.

These results have several implications for virtual
environments (Parker & Harm, 1992). First, mental rotation tests
could be employed to identify individuals who may be less likely
to experience sickness in virtual environments. As an
alternative, VE users could receive training to improve their
mental rotation abilities. For example, a VE system could be
adapted to produce stimulus rearrangements, thus allowing users
to practice mental rotations. Lastly, mental rotation skills
learned in one virtual environment will likely transfer to other
virtual environments.

Perceptual style

The field-dependence/independence dimension of cognitive
style, commonly referred to as perceptual style, has been well
represented in the literature. Classification of perceptual
style is an indicator of the extent to which a surrounding field
affects an individual's perception of an item within the field

21



involved with their use. Additionally, the reliabilities and
sensitivities of physiological measures are often low or unknown.
Money (1970), in a comprehensive review of the signs and symptoms
of motion sickness, found no clear results concerning
physiological data.

As part of the construction of a physiological monitoring
system for simulator sickness, Miller, Sharkey, Graham, and
McCauley (1993) demonstrated the sensitivity of physiological
measures to the severity of sickness as measured by self-reports
from U.S. Army helicopter pilots. Five physiological measures
were employed: tachygastria, normal myoelectrical gastric
activity, skin conductance level, vagal tone, and heart period.
Tachygastria and normal myoelectrical gastric activity were both
reductions of energy estimates from discrete Fourier-transformed
electrogastrogram (EGG) data - tachygastria were those in the 4
to 9 cycles per minute (cpm) range and normal myoelectrical
gastric activity were 3 cpm. Skin conductance level was measured
from electrodes placed on the flexor portion of the forearm.
Vagal tone was derived from a reduction of electrocardiogram
(ECG) data and provided an index of parasympathetic activity,
measuring the middle component (0.12 to 0.40 Hz) of respiratory
sinus arrhythmia. Another reduction of the ECG data provided the
reciprocal of heart rate, heart period. Of the five
physiological measures compared, heart period, tachygastria, and
skin conductance level were found to be more sensitive to
simulator sickness than were vagal tone and normal myoelectrical
gastric activity.

Miller et al. (1993) plan to continue their research on
physiological measures of simulator sickness and hope to develop
them to increased sensitivity. Physiological measures, if found
to be both reliable and valid, would offer objective measures of
simulator sickness. Ideally, such measures would correlate with
the subjective techniques typically used today. Kennedy et al.
(1988) pointed out that accurate measurement of simulator
sickness is not a trivial matter. A solid combination of
objective and subjective measures may offer the best solution to
the measurement issue.

Conclusions

The simulator sickness literature forms an excellent
foundation for the study of sickness in virtual environments and
many potentially important factors have been identified from the
literature reviewed in this report. The tentative relationships
presented should be used as a guide for future research to
clarify the role of these factors in sickness. Unfortunately,
due to the high cost of VE research, studies usually attempt to
address as many research questions as possible and studies
addressing only the issue of sickness may not be practical at
this time. However, since many VE researchers recognize the

38



wide individual variability in the threshold along many
dimensions such as gender, age, and intelligence (e.g., Botwinick
& Brinley, 1963; Maxwell, 1992; Wilson, 1963).

Gender

Biocca (1992) reported that men and women do not differ in
their sensory response to motion stimuli, yet women tend to be
more susceptible to motion sickness. He pointed out that this
may be due to underreporting of susceptibility by men in self-
reports, but added that research has shown hormonal effects. For
instance, susceptibility may change during pregnancy and
menstruation. Kennedy and Frank (1983), however, noted that
women exhibit larger fields of view than men and, as is discussed
later in this report, wide fields of view tend to result in
increased incidence of simulator sickness.

Illness and personal characteristics

Illness has also been identified as a potential factor
related to simulator sickness susceptibility. Kennedy, Berbaum,
et al. (1987) advised against simulator exposure for subjects who
are not in their usual state of fitmess and Kennedy, Lane, et al.
(1993) advised that only individuals in their usual state of
fitness should be included in the sample when administering the
SSQ. This includes subjects who are suffering from fatigue,
sleep loss, hangover, upset stomach, emotional stress, head
colds, ear infection, ear blocks, upper respiratory illness, or
the flu; as well as those taking certain medications or having
just received a flu shot. Additionally, Biocca (1992) suggested
that personal characteristics such as neuroticism, anxiety,
arousal, and introversion may be related to sickness
susceptibility. The exact nature of those effects, however,
requires further research.

Mental rotation ability

Parker and Harm (1992) discussed the ability to mentally
rotate objects and the possible role of this ability on VE
sickness. Mental rotation is what a person must do in order to
be able to recognize objects when they are not in their usual
orientations. Parker and Harm's work is discussed in terms of a
microgravity environment but they argue that since a VE, like
microgravity, produces stimulus rearrangements, the results are
applicable to virtual environments as well. They defined
stimulus rearrangements as alterations or disturbances of the
normal spatial relationships among stimuli that contribute to
orientation. Such a rearrangement may occur when, for example, a
subject "walks" forward in a virtual environment while remaining
still in the real environment. Thus, stimulus rearrangements set
the stage for cue conflicts.

20



Table Notes

Reason & Brand (1975)

?Regan (1993)

3gtern, Hu, LeBlanc, & Koch (1993)

‘Kennedy, Berbaum, Lilienthal, Dunlap, Mulligan, & Funaro (1987)
SUliano, Lambert, Kennedy, & Sheppard (1986)
*’Kennedy & Frank (1983)

‘Grandjean (1988)

®Botwinick & Brinley (1963)

Maxwell (1992) .

MWilson (1963)

NBiocca (1992)

“Parker & Harm (1992)

Barrett & Thornton (1968)

“Barrett, Thornton, & Cabe (1970

MLong, Ambler, & Guedry (1975)

Riccio & Stoffregen (1991)

MKolasinski, Jones, Kennedy, & Gilson (1994)
“Ehrlich, Singer, & Cinq-Mars (in preparation)
icCauley & Sharkey (1992)

20Pausch, Crea & Conway (1992)

"Levine & Shefner (1991)

Kennedy, Lilienthal, Berbaum, Baltzley, & McCauley (1989)
Harwood & Foley (1987)

*4Rinalducci & MacArthur (1990)

Farrell, Casson, Haynie, & Benson (1988)
?*Regan & Price (1993)

Casali (1986)

?8kennedy, Allgood, Van Hoy, & Lilienthal (1987)
Kennedy, Fowlkes, & Lilienthal (1993)
Sharkey & McCauley (1992)

“Thorell & Smith (1990)

Kennedy, Berbaum, & Smith (1993

Fowlkes, Kennedy, & Lilienthal (1987)

Owen (1990)

SDichgans & Brandt (1973

%Guedry & Montague (1961)

Hettinger, Berbaum, Kennedy, Dunlap, & Nolan (1990)
Anderson & Braunstein (1985)

“Kennedy, Hettinger, & Lilienthal (1988)

A-10



theory was reviewed as well. Forty factors shown or believed to
influence the occurrence or severity of simulator sickness were
identified. Future research is proposed.

Utilization of Findings:

This literature search provides a framework that can be used
to conduct future research to reduce the occurrence of simulator
sickness in’ virtual environments. In addition, it has directly
influenced the approach being used in technical advisory service
provided to Headquarters, U.S. Army Training and Doctrine
Command, to reduce simulator sickness in combat vehicle trainers.

viii



hours, and 28 (4%) reported that symptoms lasted longer than 6
hours. There were also 4 (1%) reported cases of spontaneously
occurring flashbacks. Since typical post-simulator duties and
debriefing are not usually time-consuming, one hour is probably
the longest period a pilot would ordinarily be expected to remain
at the simulator site. Thus, longer-lasting aftereffects,
especially those such as flashbacks and dizziness, pose a safety
risk to both the pilots and to others.

Dark focus shifts

As discussed.in the next section, a frequently used measure
of simulator sickness is the Simulator :Sickness Questionnaire
(SSQ) or some variation of it. Surveys such as the SSQ are of a
self-report nature and, thus, represent subjective measures of
sickness. From a measurement standpoint, it may be desirable to
also have some type of objective measure of sickness, such as a
physiological measure. One such physiological measure, changes
in dark focus, is discussed in this section. Additional measures
are discussed later in this report.

Dark focus is the physiological resting position of
accommodation. Accommodation is the process in which the ciliary
muscles at the front of the eye tighten. This increases the
curvature of the lens, making it fatter so that near objects can
be brought into focus (Goldstein, 1989). Dark focus is the
resting state of this process: the point of focus in the absence
of effective visual stimulation (e.g., the dark) (Fowlkes,
Kennedy, Hettinger, & Harm, 1993).

Accommodation is controlled by the autonomic nervous system
(ANS). Motion sickness symptomatology is characteristic of the
parasympathetic nervous system, a division of the ANS. Fowlkes
et al. (1993) pointed out that increased parasympathetic activity
results in an inward shift in dark focus and lens accommodation
for near vision. They identified two important consequences of
these changes. First, dark focus shifts may serve as an
objective measure of the occurrence and severity of simulator
sickness. Second, changes in dark focus may have adverse
implications for visual performance during and immediately
following exposure to virtual environments. For example, an
inward shift in focus might render a VE user unable to
successfully meet the accomodative demands of distant viewing
necessary for detecting targets.

Fowlkes et al. (1993) examined the relationship between dark
focus and simulator sickness. In their study of both college
students and pilots, dark focus was measured before and after
exposure to simulator sickness-inducing conditions (a projected
motion scene for the students and a simulator flight for the
pilots). Simulator sickness was measured with a questionnaire.

11



Appendix
Potential Factors Associated with Simulator Sickness in Virtual
Environments
[ INDIVIDUAL
FACTOR EFFECT (S) COMMENTS
motion sickness related to

age

susceptibility
greatest between
the ages of 2 and
12; after 12,
susceptibility
decreases until
almost nonexistent
after age 50!

experience with the
real-world task

concentration level

higher levels of
concentration may
be associated with
lower levels of
sickness?

further research
needed

|

ethnicity

Asian individuals
may be more
susceptible to
visually-induced
motion sickness

possibly due to
environmental
factors or genetic
differences in
central
catecholamine
release’

experience with
real-world task

experienced pilots
have greater
incidence of
sickness than do
novices‘

results not fully
consistent in the
literature

experience with
simulator
(adaptation)

increased simulator
experience usually
results in

decreased sickness?

May cause problems |
upon return to the
normal environment®

licker fusion
frequency threshold

|

* decreases at
night’

* substantial
individual
variability’?

definition: the
point at which
flicker becomes
perceptible

(table continues)



TASK

| FACTOR

EFFECT (S)

COMMENTS

luminance level

at low luminances,
there are more
tradeoffs with
required contrast,
luminance, and
resolution;
luminance level is
related to flicker”?

« definition: the
intensity or
brightness of the
light coming from
the display”

* to avoid flicker,
refresh rate must
increase as
luminance level
increases*®

* dusk conditions
may be simulated in
systems with slower
refresh rates”

method of movement

may contribute to
cue conflict’

upcoming ARI
experiment will
couple movement on
treadmill to
movement in VE

rate of linear or
rotational
acceleration

high rates may
increase sickness’®

reduce or limit
aggressive
movements until
full adaptation to
VE has occurred’®

self-movement speed

to decrease effect
of global visual
flow on sickness,
self-movement
should be at low
speeds’?

the relationship
between speed and
self-motion
perception is an
inverted "U"

sitting vs.
standing

sitting should be
less conducive to
sickness’®

results thus far
inconclusive’

(table continues)



during very long simulator exposures, due to adaptive changes in
postural control.

Despite the possibility that motion bases may cause motion
sickness and may result in increased ataxia, a motion base is
still considered by many to be a cure for conflict between the
visual and vestibular systems. Sharkey and McCauley (1992)
addressed this issue and concluded that, despite the intuitive
appeal of this belief, a motion base is not an engineering
solution to the sickness issue. They conducted their research
using the NASA-Ames Research Center's Vertical Motion Simulator
(VMS), the world's largest motion-base simulator. In their
study, pilots experienced motion sickness in the motion-base
condition equal to that experienced in the fixed-base condition.
In an analysis of their results, they noted three important
considerations related to this finding. First, a power analysis
convinced them that their measures were sensitive enough to
detect meaningful differences if such existed. Second, since it
is unlikely that the motion-cuing capabilities of future flight
trainers will be significantly greater than those of the VMS, if
the VMS can not reduce cue conflict, future trainers probably
will not be able to either. Lastly, sickness in the motion-base
condition and sickness in the fixed-base condition may have had
different causes. In the fixed-base condition, sickness may have
been due to the lack of motion, whereas in the motion-base
condition, it could have been due to washout. In such a system,
there are limits which define the simulator's range of motion.
Motion washout is an acceleration applied to the simulator cab to
keep it from approaching those system limits or to return it to
the center of its range of motion (Sharkey & McCauley). They
noted that this acceleration may produce false motion cues which
may have interacted with the simulated motion cues, thus
producing sickness.

Sharkey and McCauley (1992) concluded that if the system
features of even the VMS are not enough to eliminate cue
conflict, then perhaps efforts should be spent on issues other
than motion bases. McCauley and Sharkey (1992) suggested that
less expensive alternatives to motion-bases, such as vibration
seats, might provide sufficient "noise" to the vestibular and
proprioceptive senses to reduce the conflict with the visually
implied motion. However, simple random vibration alone was not
enough to alleviate sickness in driving simulators (Casali
1986).

Phosphor lag

Phosphor lag is defined as the continued glowing of the
phosphor on the CRT screen from one frame to the next (Pausch et
al., 1992). Excessive phosphor lag causes smearing of a moving
image and, possibly, visible after-images of previous frames.

28



These distorted images may be disturbing and may contribute to
simulator sickness (Pausch et al.).

Position-tracking error

Biocca (1992) discussed the possible effect of position-
tracking error on sickness. The position-tracker in a VE system
provides the computer with information about the location of the
user's head and, possibly, limbs in space. This information is
used by the system to construct a graphical representation of the
user inside the VE. If this information is in error, tracked
objects may appear to be places they are not. If these tracked
objects are part of the user's body, the user may be disturbed by
the discrepancy between where the graphical representations of
the objects appear in the visual display and where the user
thinks they should appear. The result may be a breakdown of the
illusion of the simulation, possibly resulting in sickness-
related symptoms such as dizziness and lack of concentration
(Biocca) .

Position-tracking errors, therefore, create a form of cue
conflict. Biocca identified three kinds of conflicts. The first
conflict occurs between a visually represented limb and ‘the felt
position of the limb. Slight discrepancies are unlikely to
disturb users, but conflicts between visually represented and
felt positions may vary depending on the location of the user in
the virtual space. This space, as calculated by the current
technology of position trackers, is often slightly distorted.
Thus, large, potentially disturbing, discrepancies are possible.

The second conflict occurs due to lags in updating body,
limb, or head position. This conflict occurs when users move
their head or limbs and their view of the VE drags noticeably
behind. In such instances, users may minimize movements such as
rapid head turning and tilting in order reduce the period between
motion input and motion output.

Lastly, position-tracking errors may also cause jitter or
oscillations of represented body parts and users may find this
unsettling. A result by Hettinger, Berbaum, Kennedy, Dunlap, and
Nolan (1990) indicated that visual or physical oscillation in the
range of 0.2-0.25 Hz may be the most nauseogenic.

Refresh rate

Refresh rate is defined as the frequency with which the
CRT's electron beam relights the phosphor pixels (Thorell &
Smith, 1990). Slow refresh rates promote flicker and may lead to
phosphor lag, both of which may be associated with sickness
(Pausch et al., 1992), as discussed earlier in this report.
Furthermore, refresh rate combines with both field-of-view and
luminance level in their effect on flicker (Pausch et al., 1992).

29



Kennedy, R. S., Berbaum, K. S., Lilienthal, M. G., Dunlap, W. P.,
Mulligan, B. F., & Funaro, J. F. (1987, March). Guidelines

for alleviation of simulator sickness symptomatology.
(NAVTRASYSCEN TR-87007) (AD-A182 554 NAVTRASYSCEN TR-87007).

Orlando, FL: Naval Training Systems Center.

Kennedy, R. S., Berbaum, K. S., & Smith, M. G. (1993). Methods
for correlating visual scene elements with simulator sickness
incidence. Proceedings of the 37th Annual Meeting of the

Human Factors and Ergonomics Society, 2, 1252-1256.

Kennedy, R. S., & Fowlkes, J. E. (1992). Simulator sickness is
polygenic and polysymptomatic: Implications for research.

International Journal of Aviation Psychology, 2(1), 23-38.

Kennedy, R. S., Fowlkes, J. E., Berbaum, K. S., & Lilienthal,
M. G. (1992). Use of a motion sickness history
questionnaire for prediction of simulator sickness.
Aviation, Space, and Environmental Medicine, 63(7), 588-593.

Kennedy, R. S., Fowlkes, J. E., & Lilienthal, M. G. (1993).
Postural and performance changes following exposures to
flight simulators. Aviation, Space, and Environmental
Medicine, 64(10), 912-920.

Kennedy, R. S., & Frank, L. H. (1983). A review of motion sick-
ness with special reference to simulator sickness. Paper
presented at the National Academy of Sciences/National
Research Council Committee on Human Factors, Workshop on
Simulator Sickness, 26-28 September 1983, Naval Postgraduate
School, Monterey, CA.

Kennedy, R. S., Hettinger, L. J., & Lilienthal, M. G. (1988).
Simulator sickness. In G. H. Crampton (Ed.), Motion and
Space Sickness, Chapter 15, 317-341. Boca Raton, FL: CRC
Press.

Kennedy, R. S., Lane, N. E., Berbaum, K. S., & Lilienthal, M. G.
(1993). A simulator sickness questionnaire (SSQ): A new
method for quantifying simulator sickness. International
Journal of Aviation Psychology, 3(3), 203-220.

Kennedy, R. S., Lilienthal, M. G., Berbaum, K. S., Baltzley,
D. R., & McCauley, M. E. (1989). Simulator sickness in U.S.
Navy flight simulators. Aviation, Space, and Environmental
Medicine, 60(1), 10-16.

Knerr, B. W., Lampton, D. R., Bliss, J. P., Moshell, J. M., &
Blau, B. S. (1993, July). Human performance in virtual
environments: Initial experiments. Proceedings of the 29th

International Applied Military Psychology Symposium. Wolfson
College, Cambridge, UK.

44



each of the four scoring keys). The first scoring key - the
original scoring key for the MHQ - had been validated on a sample
of student pilots exposed to Coriolis forces. Coriolis
stimulation is experienced when the body is being rotated and the
head is tilted out of the axis of rotation (Dichgans & Brandt,
1973; Guedry & Montague, 1961). Similarly, pseudo-Coriolis
stimulation is experienced when the head is tilted during
illusory self-rotation induced by moving visual stimuli (Dichgans
& Brandt, 1973). In a preliminary study with a small sample, the
MHQ - as scored with the original key - did not correlate
significantly with the reported sickness symptomatology
(Kennedy, Fowlkes, et al., 1992). Thus, two additional scoring
keys were later developed. These scoring keys had been validated
on a sample of college students exposed to highly provocative and
mildly provocative simulated ship motion (VLF vibration). The
fourth scoring key, the simulator sickness scoring key (SS), was
empirically derived and cross-validated as part of the study.

The goal of this research was to compare the original MHQ
scoring key, the VLF scoring keys, and the empirically derived Ss
scoring key in terms of their relative predictive abilities for
simulator sickness susceptibility. Two dependent variables were
used in the study: a score on the MHQ taken after simulator
exposure (POST) and the difference between scores on the MHQ
taken before and after simulator exposure (DIFF). The SS scoring
key was developed by examining the correlation of each of the
individual items of the MHQ with both the POST and DIFF scores.
If the correlation for POST, DIFF, or both was significant at the
0.05 level, that MHQ item was included in the SS scoring key. To
compare the four scoring keys, correlations between the four
scoring keys and the two dependent variables were examined. All
of the scoring keys were found to be predictive of reported
symptoms of simulator sickness, but the highest correlations (.33
for both dependent variables) were obtained with the SS key.

Factors Associated with the Individual

There are very large individual differences in
susceptibility to simulator sickness. Such individual difference
factors include age, concentration level, ethnicity, experience
with the real-world task, experience with the simulator
(adaptation), the flicker fusion frequency threshold, gender,
illness and personal characteristics, mental rotation ability,
perceptual style, and postural stability. These factors are all
discussed below. .

Age
One source of individual differences is age. Reason and

Brand (1975) reported that motion sickness susceptibility is
greatest between the ages of 2 to approximately 12 years. It

17



Thomley, K. E., Kennedy, R. S., & Bittner, A. C. (1986).
Development of postural equilibrium tests for examining
environmental effects. Perceptual and Motor Skills, 63,

555-564.

Thorell, L. G., & Smith, W. J. (1990). Using computer color
effectively. Englewood Cliffs, NY: Englewood Cliffs.

Treisman, M. (1977). Motion sickness: An evolutionary hypoth-
esis. Science, 197, 493-495.

Tyler, D. B., & Bard, P. (1949). Motion sickness.
Physiological Review, 29(10), 311-369.

Uliano, K. C., Lambert, E. Y., Kennedy, R. S., & Sheppard, D. J.
(1986). The effects of asynchronous visual delays on

simulator flight performance and the development of simulator

sickness symptomatology (NAVTRASYSCEN 85-D-0026-1). Orlando,
FL: Naval Training Systems Center.

Welch, R. B. (1978). Perceptual modification: Adapting to
altered sensory environments. New York: Academic Press.

Welch, R. B. (1986). Adaptation of space perception. In kK.
Boff, L. Kaufman, & J. Thomas (Eds.), Handbook of perception
and_human performance, (Vol. 1, Chap. 24). New York: John
Wiley.

Wilson, T. R. (1963). Flicker fusion frequency, age, and
intelligence. Gerontologia, 7, 200-208.

Witkin, H. A., Moore, C. A., Goodenough, D. R., & Cox, P. W.
(1977). Field-dependent and field-independent cognitive
styles and their educational implications. Review of
Educational Research, 47(1), 1-64.

47



-———

TASK
FACTOR EFFECT (S) COMMENTS
type of application | "far" applications definitions’:
may be more * "far"
sickness-inducing applications
than "near" involve distant
applications” objects, self-
motion through the
environment, and
vection
* "near"
applications

involve proximate
objects, stationary
self, and no
vection

unusual maneuvers

may be unsettling’’

turn off HMD or ask
viewer to close
eyes if unusual
maneuvers are
necessary

vection

* believed to be a
precursor to
sickness”

* increases as FOV
increases”

* can also be
produced by stimuli
having 3D cues,
presented to a
small, central FOv®

* definition:
visually induced
perception of self-
motion

¢ visual
representations of
motion can affect
the vestibular
system*?

(table continues



Technical Report 1027

Simulator Sickness in Virtual Environments

Eugenia M. Kolasinski
U.S. Army Research Institute

Simulator Systems Research Unit
Stephen L. Goldberg, Chief

Training Systems Research Division
Jack H. Hiller, Director

U.S. Army Research Institute for the Behavioral and Social Sciences
5001 Eisenhower Avenue, Alexandria, Virginia 22333-5600
Office, Deputy Chief of Staff for Personnel
Department of the Army
May 1995

Army Project Number Education and Training
20262785A791 Technology

Approved for public release; distribution is unlimited.



(1988) stated that visual representations of motion have been
shown to affect the vestibular system. Thus, they conclude that
the motion patterns represented in the visual displays of
simulators may exert strong influences on the vestibular system.

Kennedy, Berbaum, et al. (1993) stated that the impression
of vection produced in a simulator determines both the realism of
the simulator experience and how much the simulator promotes
sickness. They suggested that the most basic level of realism is
determined by the strength of vection induced by a stimulus. For
a stimulus which produces a strong sense of vection,
correspondence between the simulated and real-world stimuli
determines whether or not the stimulus leads to sickness.

Displays which produce strong vestibular effects are likely
to produce the most simulator sickness (Kennedy, et al., 1988).
Thus, Hettinger et al. (1990) hypothesized that vection must be
experienced before sickness can occur in fixed-base simulators.
While viewing each of three 15-minute motion displays, subjects
rated the strength of experienced feelings of vection using a
potentiometer. In addition, before the first display and after
each of the three displays, the subjects completed a
questionnaire which addressed symptoms of simulator sickness. Of
the 15 subjects, 10 were classified as sick, based on their
questionnaire score. As for vection, subjects tended to report
either a great deal of vection or none at all. In relating
vection to sickness, it was found that of the 5 subjects who
reported no vection, only 1 became sick; of the remaining 10
subjects who had experienced vection, 8 became sick. Based on
their results, Hettinger et al. concluded that visual displays
that produce vection are more likely to produce simulator
sickness. It is also likely that individuals who are prone to
experience vection may be prone to experience sickness.

It was mentioned earlier in this report that a wider field-
of-view produces more vection and, thus, is believed to increase
the incidence and severity of sickness (Kennedy et al., 1989).
Anderson and Braunstein (1985), however, induced vection using
only a small portion of the central visual field and 30% of their
subjects experienced motion sickness.

Summary: Potential Factors Associated with Simulator Sickness in
Virtual Environments

This report has presented three global categories of
factors—subject, simulator, and task—that may be associated with
simulator sickness in virtual environments. Many factors were
identified in each of these categories. The factors, as well as
their known effects on simulator sickness and predicted effects
on sickness in virtual environments, are summarized in Appendix
Table Al. :

36



refresh rates, luminance can be any level. However, faster
refresh rates are associated with higher display costs. Thus,
dusk conditions (i.e., lower luminance) may be simulated in
systems with slower refresh rates in order to prevent flicker
(Pausch et al., 1992).

Method of movement

Regan (1993) suggested that the method used to move through
the virtual environment may be associated with sickness
incidence. An unnatural form of movement - such as her 3D mouse
- might create a cue conflict situation between inputs to the
visual, vestibular, and proprioceptive systems. She suggested
that the movement of a subject in a VE could be coupled to
movement on.a treadmill. This might provide relatively normal
vestibular motion cues and lessen sickness. An upcoming ARI
experiment will investigate the use of a treadmill as a device
for traversing virtual terrain.

Rate of linear or rotational acceleration

McCauley and Sharkey (1992) pointed out that the flight task
must be considered when assessing the adequacy of the motion
provided by a motion base. Simulation of aggressive maneuvers
suffers from the physical limits of the simulator to represent
the acceleration cues of the maneuver. The visual display
system, however, is not as limited. Although data relating the
maneuvering intensity and sickness are not completely consistent
and no clear conclusions can be drawn, McCauley and Sharkey
suggested that increased maneuvering aggressiveness may result in
increased incidence of sickness. Thus, they recommended that
tasks requiring high rates of linear or rotational acceleration
should be avoided or kept brief until full adaptation to the
virtual environment has been achieved.

Self-movement speed

Global visual flow is a function of velocity through the
virtual environment. Extremely slow speeds provide no indication
of movement, whereas extremely high speeds result in blur.
McCauley and Sharkey (1992) recommended that self-movement in a
virtual environment should be at low speeds to reduce the effect
of global visual flow on sickness. Although either extreme
reduces vection, the feeling of presence in the VE may also be
reduced. Thus, a breakdown in the user's acceptance of being
"in" the virtual environment may occur.

Sitting vs. standing

As was noted earlier in this section, Reason and Brand
(1975) stated that a significant reduction in motion sickness
occurs when an individual adopts a supine position, possibly

34



Within each category, factors are presented in alphabetical
order. This is done for several reasons. First, there is no
obvious order in which they should be presented. It would be
difficult to assign a ranking of "importance" - however one
should wish to define this - because a factor's effect is often
unknown. In addition, the effects of various factors are
different and, therefore, difficult to place along some type of
continuum. Thus, factors are arranged alphabetically to give
them all equal status in importance. Secondly, factors are
alphabetized to aid in using this report as a reference. Every
entry refers to a subsection in the text and every factor
discussed in the text has a corresponding entry. Details are
supplied in the text and Table Al summarizes important points.

Background

Simulator sickness was initially documented by Havron and
Butler in 1957 in a helicopter trainer (Casali, 1986). It is
similar to motion sickness, but can occur without actual motion
of the subject. The most common symptoms resemble those of
motion sickness: general discomfort, apathy, drowsiness,
headache, disorientation, fatigue, pallor, sweating, salivation,
stomach awareness, nausea, and vomiting. Postural instability,
flashbacks (a sudden recurrence of symptoms), retching, and
vomiting have also been known to occur. Although the potential
discomfort to the subject alone makes simulator sickness a
problem, additional drawbacks include adverse consequences to
training and user acceptance.

Kennedy and Fowlkes (1992) noted that simulator sickness is
properly called a syndrome because of the complex signs and
symptoms associated with it. They further noted that some people
exhibit all the signs and symptoms, others exhibit only a few,
and some exhibit no symptoms at all. Additionally, among people
who are symptomatic, no single symptom predominates. Because of
the variety of symptoms associated with simulator sickness,
Kennedy and Fowlkes described it as being polysymptomatic.
Although this characteristic makes sickness difficult to measure,
the polysymptomatic nature has an advantage in that symptom
differences and changes in symptomaticity may be diagnostic
(Kennedy & Fowlkes). For example, if more eye strain is suddenly
associated with usage of a particular simulator, it might suggest
that something is wrong with the visual display.

Kennedy and Fowlkes (1992) also described simulator sickness
as being polygenic since no single factor can be identified as
the cause. Instead, as this report will reveal, many factors are
involved.



Regan, E. C. (1993). Side-effects of immersion virtual reality.
Paper presented at the International Applied Military
Psychology Symposium, July 26-29.

Regan, E. C., & Price, K. R. (1993). Some side-effects of
immersion virtual reality: An investigation into the
relationship between inter-pupillary distance and ocular
related problems. Army Personnel Research Establishment

Report 93R023.

Rheingold, H. (1991). Virtual reality. New York: Simon &
Schuster.

Riccio, G. E., Martin, E. J., & Stoffregen, T. A. (1992). The
role of balance dynamics in the active perception of orien-
tation. Journal of Experimental Psychology: Human
Perception and Performance, 18(3), 624-644.

Riccio, G. E., & Stoffregen, T. A. (1991). An ecological theory
of motion sickness and postural instability. Ecological

Psychology, 3(3), 195-240.

Rinalducci, E. J., & MacArthur, M. (Unpublished manuscript).
Annotated bibliography on the effects of flicker on simulator
sickness. Institute for Simulation and Training, University
of Central Florida, Division of Sponsored Research, Orlando,
FL.

Sharkey, T. J., & McCauley, M. E. (1992). Does a motion base

prevent simulator sickness? Proceedings of the ATAA/AHS
Flight Simulation Technologies Conference (pp. 21-28) (Report
Number AIAA-92-4133-CP). Washington, DC: AIAA.

Stern, R. M., Hu, S., LeBlanc, R., & Koch, K. L. (1993).
Chinese hyper-susceptibility to vection-induced motion

sickness. Aviation, Space, and Environmental Medicine, (9),
827-830.

Stoffregen, T. A., & Riccio, G. E. (1988). An ecological theory
of orientation and the vestibular system. Psychological
Review, 95(1), 3-14.

Stoffregen, T. A., & Riccio, G. E. (1991). An ecological
critique of the sensory conflict theory of motion sickness.

Ecological Psychology, 3(3), 15-194.

Stone, B. (1993, October/November). Concerns raised about eye
strain in VR systems. Real Time Graphics, 2(4), 1-3, 6, 13.

46



SIMULATOR

FACTOR

refresh rate

EFFECT (S)

slower refresh
rates promote
flicker and may
lead to phosphor
lag; refresh rate,
FOV, and luminance
level interact in
their effect on

COMMENTS

¢ definition: the
frequency with
which the CRT's
electron beam
relights the
phosphor pixels*TM

* to avoid flicker,
refresh rate must

flicker” increase as both
luminance and FOV
increase***
resolution with poor resolution is a

resolution there
are more tradeoffs
among required
contrast,
luminance,
resolution;
luminance level is
related to flicker’®

and

measure of the
level of detail
provided by the
display”

scene content

affects update
rate”?

definition: the
level of detail
available for a
given scene”?

(transport

| eine lag

delay)

increased lag may
lead to increased
sickness”

can occur between
information input
and motion or
visual output”?

update rate (frame

rate)

slow update rate
could lead to
visual lag

* definition: the
rate at which
subsequent frames
of the scene can be
generated and
rendered into the
frame buffer for
display”

* affected by scene
complexity and
available computing
power”?

(table continues)



was predicted, pilot reports of sickness increased after exposure
to the former simulator whereas exposure to the latter simulator
had virtually no effect on reports of sickness. One conclusion
from this research is that simulator sickness occurring in
motion-base simulators may actually be true motion sickness,
since VLF vibration may occur in simulators (Kennedy, Fowlkes,
Berbaum, & Lilienthal, 1992). Nonetheless, vision plays an
important role in both motion and simulator sickness (Kennedy,
Hettinger, & Lilienthal, 1988) - especially because of its
influence on orientation and perceived self-motion. In fact,
Kennedy et al. (1992) maintained that simulator sickness is
primarily visually-induced.

Expected incidence and severity of simulator sickness in virtual
environments

In an analysis of data from 10 U.S. Navy and Marine Corps
flight simulators, Kennedy, Lilienthal, Berbaum, Baltzley, and
McCauley (1989) found that approximately 20% to 40% of military
pilots indicated at least one symptom following simulator
exposure. McCauley and Sharkey (1992) pointed out that pilots
tend to be less susceptible to motion sickness than the general
population due to a self-selection process based on their
resistance to motion sickness. Since VE technologies will be
aimed at a more general population, such selection against
sickness may not occur. Thus, McCauley and Sharkey suggested
that sickness may be more common in virtual environments than in
simulators.

McCauley and Sharkey (1992) also noted that commercial users
of VE systems may differ from the typical user of a military
flight simulator in terms of their physical and psychological
state. Some commercial users may be under the influence of
medications, drugs, or alcohol. It is possible that such
substances may increase susceptibility to sickness.

Regan (1993) documented the frequency of occurrence and
severity of sickness in a commercial, off-the-shelf virtual
environment. For 20 minutes, civilian (n=80), military personnel
(n=20), and firefighters (n=50) were immersed in a VE consisting
of different rooms containing various objects. Subjects were
allowed to explore the rooms and interact with the objects. In
addition to completing pre- and post-immersion questionnaires,
subjects also rated themselves at 5 minute intervals on a 1-6
malaise scale (1 = no symptoms; 2 = any symptoms, but no nausea;
3 = mild nausea; 4 = moderate nausea; 5 = severe nausea; 6 =
being sick). At some stage during the 20 minute period, 61% of
the 146 subjects reported a highest rating greater than 1. Thus,
only 39% of the subjects reported no symptoms. In addition,
symptoms were found to be greatest at the 20 minute mark, when
45% of the subjects reported some symptoms. Although she noted
that the experimental procedure may have encouraged subjects to

6



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13. ABSTRACT (Maximum 200 words)

Virtual Reality (also known as Virtual Environment or VE) technology shows many
promising applications in areas of training, medicine, architecture, astronomy, data
handling, teleoperation, and entertainment. A potential threat to using this
technology is the mild to severe discomfort that some users experience during or
after a VE session. Similar effects have been observed with flight and driving
simulators. The simulator’ sickness literature forms a solid background for the study
of sickness in virtual environments and many of the findings may be directly appli-
cable. This report reviews literature concerning simulator sickness, motion sick-
ness, and virtual environments. Forty factors that may be associated with simulator
sickness in virtual environments are identified. These factors form three global
categories: subject, simulator, and task. The known and predicted effects of these
factors on sickness in VEs are discussed. A table summarizes the information pre-
sented in this report. The information can be used as a guide for future research
concerning simulator sickness in virtual environments.

74. SUBJECT TERMS Postural stability 15. NUMBER OF PAGES
Virtual environments Simulator sickness in virtual 68
Virtual reality environments 16. PRICE CODE
Simulator sickness Motion sickness --
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also increase as field-of-view increases, since a large field-of-
view increases the likelihood that flicker will be perceived
(Maxwell). This is due to the fact that the peripheral visual
system is more sensitive to flicker than is the fovea (Boff &
Lincoln, 1988).

* Thus, in selecting a visual display, several trade-offs are
necessary. In order to suppress flicker, refresh rate must
increase as both luminance level and field-of-view increase.
However, displays with faster refresh rates cost more. Thus,
slower refresh rates may be employed in an effort to keep costs
down. Slower refresh rates, however, promote flicker and require
more persistent phosphors. But long-persistence phosphors
promote phosphor lag, which may lead to disturbing smeared images
(Pausch et al., 1992). Trade-offs can also be made with
luminance specifications and this is discussed in the next
section.

Inter-pupillary distance

Bypical LCDs in a head-mounted display are a fixed distance
apart. In light of this, Regan and Price (1993) hypothesized
that if a subject has an inter-pupillary distance which is
markedly greater or smaller than the system configuration,
potential eyestrain, headaches, and associated visual system
problems may result. For a group of 53 subjects as a whole, this
hypothesis was not supported. However, when only subjects with
an inter-pupillary distance less than the system configuration
(which was the majority of the subjects) were considered, there
was some suggestion that the subjects who had the greater
deviations from the system configuration were those who
experienced ocular problems. Considering only those individuals
does have some basis. Diverging one's eyes is likely to cause
greater ocular stress than would converging. Individuals with an
inter-pupillary distance less than the system configuration would
have to diverge their eyes to conform to the system. Thus,
considering this reduced group may be appropriate to identify
visual system problems resulting from the fixed inter-pupillary
distance. Despite the somewhat muddy results, Regan and Price
concluded that the fixed inter-pupillary distance in HMDs may
play a role in ocular discomfort.

Motion platform

Motion was added to early driving simulators in an attempt
to reduce cue conflict (Casali, 1986). Sickness, however, still
occurred. Kennedy, Allgood, et al. (1987) noted that this
sickness may actually be true motion sickness. With a motion
platform, however, conflicts between visual and motion cues are
possible and these conflicts could lead to sickness (Casali,
1986). In addition, Kennedy, Fowlkes, et al. (1993) noted that
motion bases may also aggravate the problem of ataxia, especially

27



Once SSQ scores are determined in a given situation, there
are several ways the results can be used. Kennedy, Lane, et al.
(1993) pointed out that the total severity factor may reflect the
overall extent of symptom severity and, as such, provides the
best index of whether a given simulator has a sickness problem.
Additionally, the subscale scores can provide diagnostic
information as to the specific nature of the resulting sickness
(Kennedy, Lane, et al.). Thus, the data can be looked at on
their own since all four scores have a natural zero (i.e., no
symptoms) and increase in value as severity increases.
Additionally, Kennedy, Lane, et al. supplied the original data so
that, for new scores, percentile points can be determined (based
on the original data) and means and standard deviations can be
compared (to those of the original data). Lastly, scores in one
situation can be compared to scores from other or similar
situations (e.g. one simulator can be compared to another, or one
simulator can be compared with itself at different points in
time, such as prior to or following calibration adjustments) .

Evaluating ataxia: Postural tests

Some studies (e.g., Baltzley et al., 1989, discussed
earlier) have evaluated ataxia by means of free response survey
data (i.e., self-report). Ataxia can also be measured with
postural tests. Four of the basic tests are Stand-on-Preferred-
Leg, Stand-on-Nonpreferred-Leg, Stand-Heel-to-Toe, and Walk-Heel-
to-Toe. In each of these tests, the subject is instructed to
either stand or walk in a specified manner for a specific amount
of time or number of steps. The postural measure is the amount
of time the subject is able to stand (up to the specified
maximum) or the number of steps the subject is able to take (up
to the specified maximum). Additional postural tests can be
created from these basic four by adding such variations as eyes
open vs. closed, arms outstretched vs. folded across chest, and
different standing positions.

Thomley et al. (1986) evaluated the reliability of the four
basic tests for repeated measurement of ataxia. They studied the
tests under baseline conditions: before and after playing
approximately 30 minutes of video games. The use of the games
was for other experimental purposes and was not expected to have
postural effects. For all tests, subjects had arms crossed and
eyes closed. Based on an analysis of means and variances, as
well as a correlational analysis, Thomley et al. recommended a
Stand-on-Leg test with the Stand-on-Nonpreferred-Leg being
marginally superior to the Stand-on-Preferred-Leg. It should be
noted that ceiling effects were seen on all four tests, even from
the first trial, and that such postural tests typically exhibit
learning effects (i.e., performance improves with practice).

Hamilton, Kantor, and Magee (1989) also evaluated several
ataxia tests to determine their sensitivities and reliabilities.

15



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FOR THE BEHAVIORAL AND SOCIAL SCIENCES

A Field Operating Agency Under the Jurisdiction
of the Deputy Chief of Staff for Personnel

EDGAR M. JOHNSON
Director

Technical review by

Sherrie Ann Jones
Robert H. Wright

NOTICES

DISTRIBUTION: Primary distribution of this report has been made by ARI. Please address
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importance of studying sickness, some type of related data are
usually gathered during the course of an experiment. Although
these measures are usually not the primary focus of the

experiment, they provide at least some basis for further study.

From the inception of its research program on virtual
environments, ARI has recognized the importance of measuring
aftereffects. Specifically, ARI includes both a pre- and post-
immersion SSQ, or some variation of the SSQ, as part of its
research. In addition to SSQ data, ARI also measures changes in
postural stability to determine ataxic decrements due to
exposure. These data are gathered using both traditional
measures, such as the ataxia tests discussed earlier in this
report, and head tracking technology, which may provide a more
sensitive measure.

Sickness in virtual environments clearly warrants further
investigation and the means to do so can be reasonably
incorporated into any VE experiment, regardless of its primary
research question. Questionnaires and postural tests - both
commonly performed on military pilots as part of simulator
training - can be administered simply and in a minimal amount of
time. Such data collection is especially easy during the post-
exposure data collection segment since, for safety purposes,
subjects are commonly kept for a period of time after simulator
exposure.

If the necessary research cannot be performed in a virtual
environment, the next best thing would be to continue to study
relevant issues in the flight/driving simulator environment and
extrapolate the results to virtual environments. Although much
is known about simulator sickness, many of the results are
contradictory. As is the case with simulator sickness, there
appears to be no one single cause of sickness in virtual
environments. As a large part of this report has demonstrated,
both individual factors and technical problems are likely to
interact. Thus, as the search for cause and cure goes on, there
remain many unanswered questions and many possible research
areas.

To aid VE research, this report has presented a review of
the literature covering many aspects of simulator sickness in
virtual environments. Simulator sickness is a matter of concern
to the new field of VE technology, partly because simulator
sickness has been such a major concern in flight simulators. The
significance of simulator sickness was underscored by the
identification of four major consequences of it: decreased
simulator use, compromised training, ground safety, and flight
safety. Cue conflict was presented as the primary explanation
for the occurrence of simulator sickness and is currently the
primary explanation for the occurrence of sickness in virtual
environments as well. Postural instability was presented as an

39



Frank and Casali (1986) reviewed additional studies examining the
relationship between perceptual style and simulator sickness.
They also concluded that little convincing evidence exists to
support the theory that field-independent individuals are more
susceptible than field dependent-individuals. As they pointed
out, perhaps perceptual style is unrelated to simulator sickness
susceptibility!

Several points can be made about the perceptual style
literature. Frank and Casali (1986) noted that, in order for
perceptual style to be a meaningful predictor, the entire range
of the perceptual style continuum must be considered. Many
studies, however, focus only on the extremes of field
independence and dependence. In addition, Ebenholtz (1977)
suggested that the visual system of field-dependent subjects may
be peripheral-dominant whereas the visual system of field-
independent subjects may be foveal-dominant. Since the periphery
is more sensitive to motion and since the perception of motion in
the periphery may induce vection, this would imply that field-
dependent subjects would be more likely to experience a conflict
between visual and proprioceptive stimuli. By this reasoning,
field-dependent individuals should be more susceptible to
simulator sickness. Clearly, the relationship between perceptual
style and simulator sickness, if one exists, is not an obvious
one.

Postural stability

As was discussed earlier in this report, postural
instability - ataxia - is a well documented effect of simulator
exposure. Postural stability is often measured before and after
simulator exposure to determine decrements in stability due to
exposure. Based on available literature, it does not appear that
postural stability has ever been used as a predictor of simulator
sickness. Recent research, however, suggests that there may be a
relationship between pre-simulator postural stability and post-
simulator sickness (Kolasinski, Jones, Kennedy, & Gilson, 1994).

Kolasinski et al. (1994) hypothesized that individuals who
are less posturally stable will be more likely to experience
simulator sickness or will experience more severe sickness;
conversely, individuals who are more posturally stable will be
less likely to experience simulator sickness or will experience
less severe sickness. To investigate this hypothesis, pre-
simulator postural stability and post-simulator sickness data
from Navy helicopter pilots were analyzed. It was found that
pre-simulator postural stability was most strongly associated
with the Nausea and Disorientation subscale scores on the SSQ.
Postural stability did not appear to be associated with the
Oculomotor subscale score. This result complements previous
results which have shown that poor post-simulator postural

23



Altitude above the terrain

Kennedy, Berbaum, & Smith (1993) noted that altitude has
been found to be one of the strongest contributors to sickness.
Altitude is related to global visual flow (discussed later in
this section). At low altitudes, the visual flow cues indicating
movement are greater than those at high altitudes. To minimize
sickness during flying tasks, McCauley and Sharkey (1992)
recommended that self-movement in a VE should be at high
altitudes above the terrain.

Degree of control

Sickness incidence is often less for pilots and drivers than
for co-pilots and "passengers" (Casali, 1986). These
observations may be explained in part by results such as those of
Casali and Wierwille (1986), who noted that simulator sickness
susceptibility among aircrews can be a function of the member's
degree of control in the simulator cockpit. Pilots, as Pausch et
al. (1992) pointed out, generally control more of the motion and
visuals than do other flight crew members. Similarly, Pausch et
al. reported that previous results have found that subjects who
generated input themselves were less susceptible to motion
sickness. This is likely because controlling allows one to
anticipate future motion so that any possible cue conflict can be
reduced or eliminated.

Duration

It was stated earlier that intensity and duration of ataxia
increases with increased simulator exposure (Fowlkes et al.,
1987). McCauley and Sharkey (1992) also suggested that longer
exposure times to virtual environments will result in an
increased incidence of sickness and will require longer
adaptation periods. Furthermore, as was discussed earlier, there
appears to be an effect of motion bases on ataxia during very
long simulator exposures (Kennedy, Fowlkes, et al., 1993). As
was noted earlier, this is likely due to disruptions in normal
postural control.

Global visual flow

Global visual flow is defined as the rate at which objects
flow through the visual scene (McCauley & Sharkey, 1992).
Maximum global visual flow rate is the observer's velocity
divided by the observer's eyeheight above the terrain surface
(Owen, 1990). It has a value of zero for images at the. horizon.
Thus, global visual flow is directly related to velocity and
inversely related to altitude and visual range. As was mentioned
earlier, altitude has been found to be one of the strongest
contributors to sickness (Kennedy, Berbaum, et al., 1993). To
reduce sickness by minimizing global visual flow, McCauley and

32



decreases rapidly from about 12 to 21 years and then more slowly
thereafter. After around 50, sickness is almost nonexistent.

Related to age is experience with the real-world task, which
plays a critical role in the cue conflict theory of simulator
sickness: conflicts are thought to occur between the actual
pattern of stimuli and the expected pattern of stimuli. The
expected patterns likely result from repeated experiences, which
Reason and Brand (1975) suggest may follow the same long-term
learning pattern seen with other types of learning. Age and
experience are correlated and, as is discussed later in this
section, experience with the real-world task is positively
correlated with sickness.

Concentration level

Regan (1993) observed that higher levels of concentration
may be associated with lower levels of sickness. Without any
formal measurement of concentration level, she observed that some
subjects need to concentrate more than others while in the VE,
especially when picking up and manipulating objects with the 3D
mouse.

Ethnicity

Stern, Hu, LeBlanc, and Koch (1993) compared susceptibility
to visually-induced motion sickness among different ethnic
groups. The subjects, all female, formed three groups: Chinese,
European-American, and African-American (the Chinese were born in
China, as opposed to the other two groups which were born in the
United States). A circular vection drum was used to induce
vection (illusory self-motion, discussed later in this report)
while electrogastrography (EGG) signals were measured and
subjective symptoms of motion sickness were noted as they were
volunteered. It was found that the Chinese group reported
significantly more nausea and other symptoms of motion sickness
than either of the other two groups, which did not differ in
their reports. A similar result was seen with the EGG signals
during the drum rotation period. These results support the
researchers' hypothesis that Chinese women are hyper-susceptible
to motion sickness when compared with European-American and Afro-
American women - a hypothesis which they developed from
observations of subjects in their laboratory. Two theories were
put forth to explain the differences obtained in the experiment:
environmental factors (all of the Chinese subjects had been in
the USA for less than 3 years) and genetic differences in central
catecholamine release.

Experience with the real-world task

Based on findings in the field dating back to.1957, Kennedy,
Berbaum, Lilienthal, Dunlap, Mulligan, and Funaro (1987) stated

18



reactions have been observed in driving simulators and military
flight simulators. This phenomenon is called simulator sickness
and it is similar to motion sickness. There is a direct link
between simulator sickness and sickness in virtual environments:
both are forms of visually induced motion sickness. Thus, the
abundant simulator sickness literature, as well as the motion
sickness literature, forms an excellent background and starting
place in the study of sickness in virtual environments. Although
most of the simulator sickness research involves military pilots
and flight simulators, many of the findings may be directly
applicable to VE systems. These findings can help identify
potential factors involved in sickness, as well as suggest ways
to combat it.

The goal of this report is to identify factors involved in
simulator sickness in virtual environments so that such sickness
can be avoided or, at least its severity and duration reduced.

To accomplish this goal, much simulator sickness, as well as some
motion sickness literature, is reviewed. Some literature
specifically applicable to virtual environments is also reviewed,
but such research is still scarce because of the newness of the
field.

The potential factors fall into three major categories:
individual factors, simulator factors, and simulated task
factors. These factors are summarized in Appendix Table Al,
grouped into the three categories. An outline of Table Al can be
found in Table 1 on the following page.



TASK

FACTOR

EFFECT (S)

altitude above
terrain

to decrease
sickness in flying
tasks, decrease
global visual flow
by using higher
altitude!’

contributors to
sickness*

degree of control

sickness rates
usually lower for
persons with higher
degree of control”

controlling allows
for anticipation of
future motion so
that cue conflict
can be reduced or
eliminated

duration

as duration
increases:

¢ intensity and
duration of ataxia
increases*

* sickness
incidence may
increase’®

* adaptation time
may increase!’

effect of motion
base on ataxia
increases as task

COMMENTS

one of the

strongest

duration increases”

global visual flow

to minimize
sickness, self-
movement should be
at high altitudes
and/or at low
speeds’?

definition: maximum
global visual flow
= velocity / eye
height above
surfaceTM

head movements

rapid and intense
head movements may
be associated with
increased sickness;
results
inconclusive thus
far?

known to be
associated with
sickness because of
Coriolis and
pseudo-Coriolis
stimulation**:%

{table continues



alternative explanation. There are many results/symptoms of
simulator sickness, but the major results/symptoms pointed out in
this report are those identified by the SSQ (i.e., nausea,
oculomotor discomfort, and disorientation), as well as ataxia,
flashbacks, eye strain, performance changes, and dark focus
shifts. Along with flashbacks, ataxia is especially problematic
since it can last several hours after simulator exposure and,
theoretically, after exposure to a virtual environment.
Additionally, flashbacks can occur suddenly, long after the
simulator experience has ended - another important result
possibly applicable to virtual environments. Three major
quantitative tools used in the study of simulator sickness were
discussed: the Simulator Sickness Questionnaire (SSQ), ataxia
tests, and the Motion Sickness History Questionnaire. Areas for
future research suggested by the literature as well as
suggestions for future research were discussed.

The bulk of this report was devoted to identifying and
discussing three categories of factors which are potentially
involved with simulator sickness in virtual environments: those
associated with the individual, the simulator, and the simulated
task. From the literature presented, the factors within these
three categories, along with their possible effects, were
organized into a table. It should be emphasized that many of the
relationships identified are tentative. Nevertheless, this
report can serve as a guide for future research to clarify the
role of these factors and, possibly, to identify additional
factors involved in simulator sickness in virtual environments.

40



Casali, J. G., & Wierwille, W. W. (1986). Vehicular simulation-
induced sickness, Volume III: Survey of etiological factors
and research facility requirements. IEOR Technical Report
No. 8503. (NTSC TR 86-012). Orlando, FL: Naval Training
Systems Center.

Crowley, J. S. (1987). Simulator sickness: A problem for Army

aviation. Aviation, Space, and Environmental Medicine,
58(4), 355-357.

Dichgans, J., & Brandt, T. (1973). Optokinetic motion sickness
and pseudo-coriolis effects induced by moving visual stimuli.
Acta Otolaryngologica, 76, 339-348.

Duke-Elder, S., & Abrams, D. (1970). Eye-strain. In S. Duke-
Elder (Ed.), System of ophthalmology (Vol. V, Chap. XV). St.
Louis: C. V. Mosby.

Ebenholtz, S. M. (1977). Determinants of the rod and frame
effect: The role of retinal size. Perception &
Psychophysics, 22(6), 531-538.

Ebenholtz, S. M. (1992). Motion sickness and oculomotor systems
in virtual environments. Presence, 1(3), 302-305.

Ehrlich, J., Singer, M. J., & Cing-Mars, S. (In preparation).
Stereoscopic and Monoscopic Displays in Virtual Environments
(ARI Technical Report). Alexandria, VA: U.S. Army Research
Institute for the Behavioral and Social Sciences.

Farrell, J. E., Casson, E. J., Haynie, C. R., & Benson, B. L.
(1988, July). Designing flicker-free video display
terminals. Displays, 9(7), 115-122.

Fowlkes, J. E., Kennedy, R. S., Hettinger, L. J., & Harm, D. L.
(1993). Changes in the dark focus of accommodation
associated with simulator sickness. Aviation, Space, and
Environmental Medicine, 64(7), 612-618.

Fowlkes, J. E., Kennedy, R. S., & Lilienthal, M. G. (1987).
Postural disequilibrium following training flights.

Proceedings of the 31st Annual Meeting of the Human Factors
Society, 488-491.

Frank, L. H., & Casali, J. G. (1986). Simulator sickness: A

review of its costs, countermeasures, and prediction
(Technical Paper 861782). Warrendale, PA: Society of
Automotive Engineers.

42



Consequences of simulator sickness

To emphasize the significance of simulator sickness, Crowley
(1987) identified four important aeromedical and operational
consequences: decreased simulator use, compromised training,
ground safety, and flight safety. Decreased simulator use may
result from pilots who have experienced symptoms and are
unwilling to repeat the experience. Training may be compromised
in one of two ways. First, symptoms in the simulator may
distract the pilot during the session, thus interfering with the
training process. Second, pilots may adopt behaviors to avoid
symptoms in the simulator which, if transferred to the actual
aircraft, may be detrimental. Ground safety in terms of exiting
the simulator or driving away from the site may be jeopardized by
aftereffects from the simulator such as postural disequilibrium
(ataxia) and flashbacks. Such aftereffects, along with any
adaptive behaviors (which may have negative transfer effects),
may also compromise flight safety after simulator exposure.

Simulator sickness compared with motion sickness

Although simulator sickness and motion sickness have similar
symptomatology, they are not the same thing. Tyler and-Bard
(1949) stated that "Motion sickness is a specific disorder which
is evoked in susceptible persons and animals when they are
subjected to movements which have certain characteristics" (p.
311). Thus, because of the role of the motion itself in motion
sickness, any sickness experienced in simulators incorporating
motion (i.e., moving-base simulators) may be true motion
sickness. However, many simulators (i.e., fixed-base simulators
do not involve operator motion yet still provoke sickness, since
visual stimulation alone can also induce sickness. Thus,
although the symptoms of motion and simulator sickness may be
similar, their causes may be quite different. Whereas vestibular
stimulation alone is usually sufficient to cause motion sickness
(Money, 1970), there is no one exact cause of simulator sickness.
Simulator sickness is more likely a result of the compounding of
the visual and motion cuing and not due to merely the motion
alone.

Because of its similarity to motion sickness, any review of
the simulator sickness literature necessarily includes some
references to motion sickness research. Early studies by Wendt
and his associates during the 1940s (cited in Kennedy, Allgood,
Van Hoy, & Lilienthal, 1987) related Very Low Frequency (VLF)
vibration exposure to motion sickness. In a similar vein,
Kennedy, Allgood, et al. (1987) rélated post-simulator motion
sickness symptoms to VLF vibration in moving-base flight
simulators. Motion sickness symptoms were assessed with a
questionnaire. Of the two simulators investigated, one exhibited
motion profiles which, based on other research, fell into a
nauseogenic category, whereas the other simulator did not. As

5