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OCCIPITAL LEADS
joc

TION
r

Fievre 2. A characteristic segment of a record taken before (A) and during (B) strobe light stimulation.
Alpha pacing is shown best in the occipital leads. Lateral eye movements were not recorded during this
particular experiment so that the flash could be indicated on that channel. Flashes were indicated by manu-
ally pressing the calibrate button on the recorder; thus, the deflections indicate the onset of the flash but
do not indicate its duration which was only a few milliseconds. Note that the subject looks up slightly
with most flashes, as indicated by the small deflections of the vertical eye movement recording in (B).

5



stimulation period in the fog. His EEG con-
firms his impression.

The computer analysis of one occiptal record
showed a shift in the power of alpha from a
frequency of 10/sec to 8/sec during beacon stimu-
lation in the fog.

Experiment II, involving a 10/sec flicker, re-
vealed no evidence of seizure, no nystagmus, and
no photic driving. Six of the subjects reported
drowsiness during the period of stimulation and
in the recovery period. One subject complained
that the light was annoying. Three subjects who
claimed to be unaffected by the experimental situ-
ation showed EKG slowing consistent with light
sleep. Three of the six drowsy subjects showed
sleep spindles. One other subject who claimed
drowsiness showed parietal assymetry that was
accentuated during flicker.

Experiment III consisted of stimulation at a
frequency of 1.0 fps with a very high intensity,
brief duration strobe flash in the fog. Seven of
the subjects said that the light was irritating;
three of that group stated that they could not
have taken more than the ten minutes of ex-
posure. Three other subjects said that they were
not bothered by the light.

A peculiar pacing of the alpha rhythm was
observed in 8 of the subjects (Fig. 2). The inter-
ruption began after the flash started and per-
sisted after the flash had ended. Seven of the
people who showed alpha pacing complained of
irritation while one disclaimed discomfort.

The high-intensity flash apparently caused
some degree of dark de-adaptation and pupillary
constriction. These effects caused the instrument
panel to appear to dim then brighten as the
pupil redilated. The pupil pulsed at the fre-
quency of the light but out of phase with it.

III. Discussion.

The reported cases of seizures precipitated by
detuned television sets * + *% 5. 1° sunlight passing
through trees* ‘* and flicking light switches off
and on? have few correlates in aviation. If the
same incidence of light-sensitive individuals ex-
ists among pilots as is claimed for the general
population, then one would expect to find that
0.01% ® to 6% ** of the total number of pilots
are affected. This means that 50 to 30,000 of
the 500,000 pilots in the U.S.A. should be suf-
ficiently photically-sensitive that abnormal EEG’s
could be demonstrated. Medical certification

statistics show that up through 1963, 113 pilots
had been medically grounded by reason of
epilepsy. If the accepted ratio of 5%°* holds
true in these cases, then about 6 of those 113
pilots should be photically sensitive.

The literature seems to establish that only
peculiarly susceptible individuals are seriously
affected by intermittent light. 17 The low in-
cidence of such individuals in the population ap-
pears to be the chief reason why no accidents
in-flight have ever unequivocally been attributed
to flashing lights, One case reported by Tang
and Dille*® dealt with a general aviation pilot
who experienced vertigo while taking off into the
sun. He lost consciousness while attempting to
land and crashed, but received only minor in-
juries. Laboratory studies revealed certain ab-
normal EEG features but no adverse effect of
photic stimulation. The case was believed to be
one of true labyrinthine vertigo.

It appears, further, from the literature that
susceptible people are sensitive to only a narrow
range of frequencies, generally between 8 and 30
flashes per second,*** Most flashing lights in
aviation are below or above that range. Rotat-
ing lights and flashing strobe lights operate at a
frequency between 1 and 2 per second. Propel-
lers generally are turning at a higher frequency.
A three-bladed propeller running at 2000 rpm
would produce a flicker of 100 fps. Further, the
airplane would have to be a single engine type
and headed properly in order for the pilot to
be behind the propeller and in the exact relation
to the sun for his eyes to receive the flicker.

Further still, the intensity of the light ap-
parently enters into the effectiveness of the stim-
ulus, as indicated by some television-induced seiz-
ures.* 1°18 Most anticollision lights are mounted
out of direct sight of the pilot, thus the emitted
light from his own plane is lost in the distance.
In fog, when back-scatter is produced, the
greatest liability occurs? The experiments in
this study showed clearly that the subjects only
complained of annoyance by the lights while in
the fog. .

Finally, laboratory studies have shown that
photically-induced paroxysms occur most readily
when the eyes are closed and the flash frequency
is close to the resting rhythm,” a condition
not likely to occur under IFR conditions at
night. The flight task itself might interfere with
the triggering of seizures in pilots.


a

a

10.

11,

12,

REFERENCES

. Arrken, R, C .B., Ferres, H. M., and Gepyg, J. L,

“Distraction from Flashing Lights,” Aerosp. Med. 34:
302-806, 1963.

Berry, C. A. and Hastwoop, H. K., “Helicopter
Problems: Noise, Cockpit Contamination, and Dis-
orientation,” Aerosp. Med. $1: 179-190, 1960.

. Buarrner, R, J., “Photic Seizures—Television In-

duced,” Jour, Pediat. 58: 746-749, 1961.

Bower, B, D., “Television Flicker and Fits,” Ad-
vances in Diagnosis, 2: 134-138, 1963.

BurnyaM, P. J., “Stroboscopic Effect,” A Letter to
the Editor, Jour. Aviat. Med. 29: 914, 1958,

. Cons, S., “Photic Driving as a Cause of Clinical

Seizures in Epileptic Patients,” Arch. Neurol. Psy-
chiat, 58: 70-71, 1957.

Davivorr, R. A., and Jounson, L, C., “Photic Activa-
tion and Photoconvulsive Responses in a Non-epileptic
Subject,” Neurology 13 : 617-621, 1963.

Franks, W. R., “Some Physiological Factors Lead-
ing to Incidents in the Air,” Medical Aspects of
Flight Safety, NATO Advisory Group for Aeronauti-
cal Research and Development. Aeromedical Panel,
p. 46, Pergamon Press, 1959,

. GRETHER, W. F., W. A. D. O., TR 59-715,

Himusr, L, E., “Photogenic Seizures,” Indus, Med.
Surg. 31: 381-382, 1962.

Jounson, L. C., “Flicker as a Helicopter Pilot
Problem,” Aerosp. Med. 34: 306-810, 1963.

Jounson, L. C., Unerr, G. A. and Greser, G. C.,
“Studies of the Photically Stimulated EEG, Quanti-
fication and Stability of Photic Driving Patterns,”
SAM Report #57-54, 1957.

13, Kuapetex, J., “Photogenic Epileptic Seizures Pro-
voked by Television,” EEG Clin. Neurophysiol. 11:
809, 1959.

14, Livineston, S., and Torres, I. C., “Photic Epilepsy:
Report of an Unusual Case and Review of the Litera-
ture,” Neurology 3: 8304-807, 1964.

15. Mawpstry, C., “Epilepsy and Television,” Lancet 1:
190-191, 1961.

16, Munpy-Castiz, A. C., “An Analysis of Central Re-
sponses to Photic Stimulation in Normal Adults,”
EEG and Clin. Neurophysiol, 5: 1-22, 1953.

17, Oxtansky, J., “The Use of Flashing Light to Perturb
Human Behavior,” Research Paper P-172, Institute
for Defense Analyses, Research and Engineering Sup-
port Division, Contract SD-50,

18, Pattis, D. M., “Television-induced Seizures,” Lancet
1: 188-190, 1961.

19. Tana, P, ©, and Dux, J. R, “Inflight Loss of
Consciousness; A Case Report,” CARI Report 63-21,
Federal Aviation Agency, Aviation Medical Service,
September 1963.

20, Unert, G. A., “Flicker Sickness,” Arch, Ophthal. 50:
685-687, 1958.

21, Urerr, G. A., Gueser, G., Winoxur, G. and Law-
ter, A, “The EEG and Reaction to Photie Stimu-
lation as an Index of Anxiety-Proneness,” EEG and
Clin, Neurophysiol. 5 : 23-32, 1953,

22, Watson, C. W., and Hunter, J., “Detection of Light-
Evoked Cerebral Electrical Abnormalities among
Helicopter Pilot Trainees,” Progress Report to the
Research and Development Division, Office of the
Surgeon General, Department of the Army.



AM 66-39

EXPOSURE OF MEN TO INTERMITTENT PHOTIC
STIMULATION UNDER SIMULATED IFR CONDITIONS

C. E. Melton, E. A. Higgins, J. T. Saldivar
and S. Marlene Wicks

Approved by Released by
Dla Rite [USagel “0
J. Rosert Diiuz, M.D. P. V. Srecen, M.D.
Crier, Crvi, ArRoMepicaL InstTrTUTE Feperan Am Surceon
October 1966

FEDERAL AVIATION AGENCY
Office of Aviation Medicine



When all of these factors are taken into con-
sideration, the statistical probability of a sus-
ceptible pilot encountering just the right inten-
sity of light at just the right time is very slight.

Factors operating to increase susceptibility to
flicker or flash are anxiety,” repeated exposure °
and fatigue."

The results obtained in this study are in gen-
eral agreement with those in the literature that
the primary response to photic stimulation by
normal subjects is drowsiness. That sensation is
borne out by the subjects’ EEG’s. It is difficult
to decide, however, how much of the drowsiness
was due to the warm moist darkness and bor-
ing nature of the task and how much was due
to the flashing lights, In several of the cases,
drowsiness preceded the period of light stimula-
tion and the beacon caused arousal. In other
cases, drowsiness began in the pre-stimulation
phase and progressed to light sleep during stimu-
lation. Other workers * *” are of the opinion that
little of the drowsiness can be attributed to the
light. In our population there was no photic
driving of the EEG by either the Grimes beacon
or the propeller-chopped light and no EEG ab-
normalities emerged during the period of stimu-
lation. In one case parietal asymmetry appeared
to be accentuated by propeller flicker. The strobe
light did produce a peculiar pacing of some of

the subjects’ alpha rhythm. This interruption
of the resting alpha began after the flash was
over and lasted for about 1/3 of a second. Pupil-
lary constriction and redilation together with
some dark de-adaptation produced a visual sensa-
tion of fading and brightening of the surround-
ings. Since this light is of such a high intensity,
these responses are not unexpected. In fact, they
are most likely related to the experimental situa-
tion. It was not possible to drape the room com-
pletely; reflections from the floor and ceiling
threw a great deal more light into the cockpit
than one would normally expect in-flight.

The strobe light was also the most irritating
of the three forms of stimulus. That property
may be desirable since the purpose of these lights
is to attract the attention of other pilots. In
flight, the light would probably not affect the
pilot of the aircraft on which it was mounted
to the same extent that it did under these experi-
mental circumstances.

The commonest complaint of pilots about anti-
collision lights is annoyance. The manufacturers
of these lights furnish a placard stating that the
beacon should be turned off when flying in or
near clouds (Fig. 3). Adherance to that recom-
mendation seems to be the simplest solution to
the commonest problem.

ROTATING BEACON LIMITATIONS

OPERATION OF ROTATING BEACON
IN OR NEAR FOG, CLOUDS OR SNOW
IS STRICTLY PROHIBITED.

Figure 3. Placard furnished with red rotating beacon.


It seems clear from the literature that a variety
of symptoms ranging from annoyance to grand
mal seizures can be precipitated by flashing lights.
The purpose of this study was to determine
whether or not a group of normal young men
showed any electroencephalographic changes dur-
ing and following photic stimulation as it might
occur in flight.

I. Method.

Ten young men, all volunteers, who had no
history of syncope or seizure were exposed to in-
termittent photic stimulation at each of three
frequencies.

All experiments were carried out in a Beech-
craft Bonanza cockpit that was situated in an
environmental chamber. Six channels of mono-
polar EEG were recorded together with lateral
and vertical electro-oculograms. Frontal, pari-
etal and occipital leads were attached to the
scalp conventionally over each hemisphere. The
leads on each side were referenced to the ipsila-
teral earlobe. Leads were also attached at the
outer canthi and above and below the eyes. Air-
plane engine noise, taped in the cockpit of a
Beechcraft T-34, was played back through a
speaker located in the mockup cockpit through-
out each experiment. A red fixation light was
mounted on the instrument panel, which other-
wise was empty. The EEGs were recorded on a
Grass Model VI electroencephalograph located
outside the chamber. The terminal board was
attached to the back of the left seat, directly
behind the subject. The connecting cable was
passed through a small port in the chamber. One
occipital channel was recorded on magnetic tape
for computer analysis. The chamber walls were
draped with black velvet, insofar as practicable,
to reduce reflections. The mockup and a subject
are shown in Figure 1.

A Grimes red rotating beacon and an Air-
Guard Strobe Light were mounted on the top
of the mockup, out of the subjects’ direct vision.
The anti-collision lights were powered from out-
side the chamber. A 36 inch aluminum propeller
was mounted to the shaft of a large floor-stand
electric fan and placed in front of the mockup.
A 200 watt incandescent lamp was mounted be-
hind the propeller.

The Grimes light yielded a flash frequency of
1.5 fps, the strobe light a frequency of 1.0 fps,

and the propeller.a flicker of 10 fps, as deter-
mined with a stroboscope.

The chamber could be filled with fog by cool-
ing it to 60° F and then releasing steam into it;
the steam, upon condensation, raised the tempera-
ture to 70° F, where it was maintained.

Experiment I with the Grimes light was
divided into two parts, with fog and without
fog. Each part was divided into three 10 minute
phases, (1) in the darkened room with no beacon,
(2) in the dark with the beacon, and (3) a re-
covery period in the dark with the beacon off.

Experiment II with propeller flicker was car-
ried out at the same location as previously de-
scribed. However, in this experiment the room
was lighted and no fog was introduced. The
subject sat in the cockpit as before looking
through the propeller at the light.

Experiment III with the strobe light was car-
ried out under the same conditions as Experiment
I, except that the part without fog was omitted.

The collected records were examined by a neu-
rologist-electroencephalographer who rendered an
opinion as to the relationship of the stimulus to
the electroencephalogram and as to the clinical
normality of the record.

The taped channel of occipital EEG was
subjected to power spectrum analysis in a com-
puter. Filtering, in this case, was done to reveal
shifts in alpha rhythm.

II. Results.

Analysis of the records taken in Experiment I
revealed no evidence of seizure, no nystagmus,
and no photic driving. Three of the subjects re-
ported that they became drowsy during beacon
stimulation; these reports were compatible with
their EEG’s. Two of the subjects had physiologi-
cally abnormal EEG’s, with the abnormalities
showing up in all experimental phases, but all
records were clinically normal. The abnormal-
ities consisted of a shifting occipital asymmetry
in one case and 1-8/sec high voltage slow waves
in the other. In the latter case, the cause of the
slow changes could not be identified but there
was no behavioral correlate; it was felt that it
was possibly an artifact related to some sort of
field change. Two other subjects volunteered
their opinion of the beacon. One said it was
annoying, the other said he was bored. One
subject felt that he may have slept in the pre-


EXPOSURE OF MEN TO INTERMITTENT PHOTIC STIMULATION
UNDER SIMULATED IFR CONDITIONS

The medical literature is replete with apoc-
ryphal, anecdotal and documented reports dealing
with the problem of alteration of consciousness,
disorientation, nausea, distraction, annoyance or
other symptoms related to flashing or flickering
lights in the environment. The possibility of a
compromise of air safety by such an effect has
naturally stimulated several investigators to ex-
amine the problem as it exists in the aviation
environment.

The problem was brought into focus by a ques-
tion raised by Burnham ® regarding the possible
effects on epileptic passengers of sunlight being
chopped by helicopter rotors. Van Wulfften
Palthe, in reply to Burnham’s query, pointed out
that the small number of light sensitive epileptics
probably did not warrant a general warning to
the public about the possible effects of flicker
in helicopters, but that elipeptics should be
warned by their physicians of the danger of hav-
ing @ seizure thus triggered. Berry and East-
wood? quote a personal communication from a
helicopter passenger who was incapacitated in-
flight by sunlight flickering through a helicopter
rotor. These authors also cite two cases of
photically-induced grand mal seizures; one was a
B-86 tail gunner who looked at the sun through
a propeller while in flight and another was a
pilot standing on the ramp who looked at the
sun through a propeller of a C-54 while the air-
plane’s engines were being shut down, Franks®
described one incident of pilot syncope encoun-
tered while landing into the sun.

Johnson * studied 102 Navy helicopter pilots
and found that 25 of them were bothered by
flicker while inflight, though none had ever ter-
minated a flight for that reason. The principal
complaints occurred as a result of operation in
or near clouds. One pilot felt that flicker con-
tributed to a near accident. Electroencephalo-
graphic examination revealed abnormalities in
only two of the subjects. Twenty-two of the
pilots became drowsy or went to sleep during

photic stimulation in the clinic and their EEG’s
were compatible with a lowered state of vigilance.
None showed paroxysmal activity during photic
stimulation. Others ® have reported similar find-
ings when subjects were exposed to flashing
lights, though they attributed the drowsiness to
the nature of the task rather than to a specific
effect of intermittent light stimulation.

Watson and Hunter” reported that 5 of a
group of 100 airplane pilots showed abnormal
EEG’s in response to flashing lights; 8 com-
plained of annoyance by propeller flicker, 2 said
discomfort was accompanied by confusion, and
12 reported being distracted by the rotating bea-
con. These authors recomended that all pilots
and trainees be tested for light sensitivity and
those found sensitive should be excluded from
flight operations.

Aitken, Ferres, and Gedye* exposed 10 sub-
jects to flashing lights of different frequencies
and allowed the subjects to express a “preference”
for one frequency, thus establishing the least an-
noying rate of flash. The highest preference was
for the lowest test frequencies of 1 and 1.83
flashes per second. These investigators further
found that change in skin conductance was lin-
early related to the flash frequency. Johnson,
Ulett, and Gleser ?* also found that the frequency
range of 85-24 flashes per second was most
effective in establishing a photic driving response
in the EEG. These workers also reported that
there was no correlation between the amount of
resting activity and that evoked by photic stimu-
lation,

Orlansky,” in a review of the literature,
reached the conclusion that the behavioral effects
of photic stimulation were shown primarily by
epileptics. He further expressed the opinion
that such epileptics only show the effect in the
laboratory, and then only when resting and co-
operating in the experiment. Orlansky was of
the opinion that flashing lights pose a negligible
threat to normal people.



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