JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

1

Impact Vibration Source Localizationin Two-Dimensional Space Around Hand
Yusuke Ujitoko, Ryunosuke Tokuhisa, Sho Sakurai, and Koichi Hirota, Member, IEEE
Abstract—This study investigated the localization ability of an impulse vibration source outside the body in two-dimensional space. We
tested whether humans can recognize the direction or distance of an impulse vibration source when using their hand to detectspatiotemporal vibrotactile information provided by the propagated vibrational wave from the source. Speciﬁcally, we had users put
their hands on a silicone rubber sheet in several postures. We asked users to indicate the position of the vibration source when alocation on the sheet was indented. Experimental results suggested that the direction of the impact vibration source can be recognized
to some extent, although recognition accuracy depends on hand posture and the position of the vibration source. The best results wereachieved when the ﬁngers and palm were grounded and a vibration source was presented around the middle ﬁngertip, and the
directional recognition error in this case was 6◦ . In contrast, results suggest it is difﬁcult to accurately recognize the distance of thevibration. The results of this study suggest a new possibility for directional display where vibrotactile actuators are embedded at a
distance from the user’s hand.
Index Terms—Haptics, Vibration, Vibrotactile, Vibration Localization

F

1

I NTRODUCTIONT

HE human auditory system can identify both the direction and distance of sound sources [1], [2]. The auditory
system uses several cues for sound source localization, suchas interaural differences in time and intensity between both
ears, spectral information, and pattern matching [3], [4]. Thisknowledge of auditory mechanisms is used for simulating
sound ﬁelds [5], [6] to present spatial information in theuser’s peripheral space. Considering that sound is the perceptual result of mechanical vibrations traveling through a
medium such as air, we hypothesized that humans could
identify vibration sources outside the body not only withthe ear but also with the hand’s vibrotactile sense.
Vibrotactile stimuli as a feedback modality have been
widely explored in various application scenarios, particularly to present spatial information when the visual andauditory channels are overloaded [7] or impaired [8]. For
example, when we ride in cars, the visual and auditory
channels are not vacant, and recent research is increasingly
focusing on the presentation of spatial information using
the vibrotactile channel [9], [10]. If the position of a vibration source in relation to users were recognized, thenthis information could support the user’s activity such as
monitoring of autonomous robotic systems moving in twodimensional space. For example, humans can monitor anautonomous robotic system if the recognition of the positions of the obstacles by the autonomous robotic system in
two-dimensional space can be conveyed to the humans.

••

Yusuke Ujitoko is with the NTT Communication Science Laboratories,
Nippon Telegraph and Telephone Corporation, Kanagawa, Japan. E-mail:yusuke.ujitoko.ws@hco.ntt.co.jp
Ryunosuke Tokuhisa, Sho Sakurai, and Koichi Hirota are with
the Graduate School of Information Systems, University ofElectro-Communications, Tokyo, Japan. E-mail: {r.tokuhisa, sho,hirota}@vogue.is.uec.ac.jp

Manuscript received April 19, 2005; revised August 26, 2015.

vibration
sourcetwo-demsional medium(e.g. silicone rubber sheet)

Fig. 1: We investigate human two-dimensional localization
ability of vibration sources outside the body using spatiotemporal cues provided by propagated waves.
Previous studies investigated the localization ability ofvibration sources on the body’s surface and the use of thatability for direction information presentation. Researchers
attached a vibrator array directly to the surface areas ofusers’ bodies such as the hand [11], torso [12] or wrist [13].
This method was effective for indicating direction to theuser, but attaching a vibrator directly to a user’s body is
not always a viable option. The vibrotactile actuators mustbe laid out for the limited space in contact with the body. For
example, when it is desired to present directions with highresolution, it is necessary to arrange vibrotactile actuators at
a high density, but it is costly to miniaturize and integrate
the actuators in a limited mechnical spaces. In addition,
the heat generated by the presentation system including
the vibrotactile actuator might make it worse the tactile
impression of the display. These problems are unavoidable
when the vibrotactile actuators are in contact with the body.
This study explores another possibility of presenting
spatial information from vibration sources that are remotely
placed from the user’s body. In this setup, it is not necessary
to contact the vibrotactile actuators to the body, and thus the
problem of the layout of the vibrating actuator in a limited

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

space and the problem of heat are alleviated. In this case, thelocalized vibrator source position information emanatingfrom outside the hand might indicate the direction anddistance from the hand. However, it is still unclear how
well humans can localize the vibration source. Thus, weinvestigated the localization ability of an impact vibration
source outside the body. We made participants judge thevibrator source position when they put their hands ona silicone rubber sheet, and a location on the sheet wasindented (shown in Fig.1). In this case, the localized vibrator
source position information emanating from outside thehand might indicate the direction and distance from thehand.
The contribution of present study is the clariﬁcationthat the direction of the impact vibration source can berecognized to some extent, although recognition accuracy
depends on the hand posture and the position of the vibration source. The best results were achieved when theﬁngers and palm were grounded, and a vibration source
was presented around the middle ﬁngertip, and in thiscase, the directional recognition error was 6◦ . In contrast,
results suggest it is difﬁcult to recognize the distance of thevibration accurately. Our results open possibilities for interfaces that present direction information to a user without
requiring the attachment of a vibrator to the user’s body.

2

R ELATED W ORK

We introduce previous studies on vibration localization on
the body’s surface and localization outside the body, and weclarify differences from our study.
2.1 Localization of Vibration Source on Body Surface
A number of studies have examined the localization vibrotactile stimuli applied to different areas of the body’s surface
such as hands [14], wrists [13], [15], arms [16], [17], theabdomen [12], [18], [19], waist [20], back [21], and head [22].
For example, Chen et al. [13] used a 3 × 3 array of linear
resonance actuators to investigate vibrator localization performance at the wrist. Similarly, Soﬁa et al. [23] used a 3 × 3
array on the palm.
The localization accuracy depended not only on thearea of the body but also the number of vibrators or intervibrator distance. The work [18] found that as the numberof vibrators increased from 6 (inter-vibrator distance: 140mm) to 12 (inter-vibrator distance: 72 mm), the localization
accuracy decreased from 97 % to 74 %. Compared to thesevariables, the frequency of vibration had little effect on theability to localize [24]. In addition, it was reported thathead [25] or gaze direction [26] affected the accuracy of
localization.
Localized vibration sources on body surfaces do indicate
direction, and the applicability of direction presentation
has been widely explored in various application scenarios.
These include the presentation of tactile directional navigation [27] and warning signals for drivers [28] and possibly
also for blind people.
However, attaching vibrators to the user’s body surface
may take time and effort and sometimes be uncomfortable.
We explored the possibilities for interfaces that present
direction information to the user without requiring theattachment of a vibrator to the user’s body.

2

2.2 Localization of Vibration Source Outside Body
There are some studies that investigated localization ability
when users held the edge of a one-dimensional medium(e.g., a stick) which was stimulated at some distance fromthe holding point. In this situation, the localized vibration position information indicates the distance from users.
Miller et al. [29], [30] showed that when humans actually
held a wooden stick with one hand, they could recognize
the distance of an object when the stick contacted said
object. The localization succeeded with both self-generated
action and passive reception of impact. Gongora et al. [31]investigated recognition of the point to which the impact
was applied when users held a rod with both hands. First,
they recorded the vibration when users gripped both endsof either wooden, aluminum, or oxymethylene sticks withboth hands and a location on the stick was hit. Then, theypresented users with recorded vibrations and made users
judge the location of impact. The estimated position of
impact was imprecise and depended on the kind of material. In addition, their results suggested that the amplitude
and duration of vibration were cues to recognition. Sreng
et al. [32] investigated whether users could recognize thedistance when they held a vibrator as well as when theyheld a rod by its ends. They used a vibration model ofthe Euler–Bernoulli beam or simpliﬁed decaying sinusoids.
Their experiment showed that users could associate theimpact location with vibration patterns.
In contrast to studies that investigated the localization
ability via propagating waves through a one-dimensional
medium, we focused on the localization ability via waves
propagating through a two-dimensional medium. In sucha situation, the localized point indicates not only distancebut also direction information to users. The work by [33]proposed a similar concept to ours, with a system to present
users with a vibration source in two-dimensional space,but the work did not clarify how well users can localize
the source because they did not conduct an experiment.
Our contribution is to provide experimental results anddiscussions of such a situation.
There are other studies extending phantom sensation [34], [35], which is a phenomenon of tactile illusionthat can act as a reference for the presentation of the senseof movement. In this phenomenon, the amplitude of thevibrating stimuli presented to the two points can be controlled such that the user can perceive an illusionary motionin the area on the body surface between the two points. The
work by [36], [37], [38] extended the phantom sensation andattempted to present the illusory phantom sensation in the
air where there are no body surfaces.
In these cases, as we describe in the introduction section,
the vibrotactile actuator contact with the body surface, andthus there is the problem of the layout of the vibrating
actuator or the problem of heat. Thus, we examined the possibility of localization of vibration sources that are remotely
placed from the user’s body.

3

R ESEARCH Q UESTIONThis study assumes speciﬁc situations where users place a
speciﬁc body site on a two-dimensional medium and the

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 20153

vibration originating from a single point travels to the human body site. The system for realizing such situations has
three components: vibration source, medium, and human(shown in Fig. 2). We focused on speciﬁc parameters related
to each component that could affect the localization ability.
The parameters related to the vibration source include thevibration waveform and the position of the vibrator source.
The parameters related to the medium include the materialand size. The parameters related to the human include thebody sites and posture, which indicate how to locate thebody site with regard to the medium.

••

Regarding the waveform of a vibration source, weused an impulse signal – as in previous studies
that investigated vibration localization outside thebody [29], [30], [31] – instead of a cyclic signal.
The position of the vibration source could affect what
spatiotemporal cue is obtained. This is because thebody sites that sense a propagated vibration change
depending on the position of the vibration source,
and thus the localization ability is inﬂuenced. We
regard the position of the vibration source as a variable to test how it affects the localization ability in anexperiment.

components parametersvibration
sourceLast, we focus on the parameters related to the medium.

・waveform・position

medium・material
・size

human

・body site・posture•

Fig. 2: Components and parameters that could affect localization ability.
From here, we discuss each component’s parameters.
First, we focus on the parameters related to the human.
••

Regarding the body site, this study used the handbecause the human hand has a relatively high sensitivity to vibration among all body sites and is theordinary input/output area when touching interface
devices. The hand grounded to the medium receives
spatiotactile cues given by the propagating vibration
wave (shown in Fig. 3). When the stimulus positionchanges, the spatiotemporal cues change, and it isexpected that users will be able to localize the source
position using the characterization of the cues.
The posture, or the method of locating the human
distributed tactile sensors of the hand on a medium,
could affect what spatiotemporal cue is obtained. We
regard posture as a variable to test how it affects the
localization ability in an experiment.

•

Based on these considerations, the research questions of
this study are as follows:•

vibration wave•

vibration source

•

human
vibrotactile
sensingt

Fig. 3: Vibration propagates through medium and gives
human hands spatiotemporal tactile cues.
Next, we focus on the parameters related to the vibration
source.

Regarding the material of the medium, we usedmaterials such as urethane foam, plastic, aluminum,and silicone rubber sheets. When we used urethanefoam, users could not feel slight vibrotactile cues
from faraway points owing to vibration attenuation.
When we used plastic or aluminum plates, the propagated vibration could reach the user’s body, but
we felt that the entire medium vibrated at the sametime, and it was difﬁcult to localize the source. Thiswas owing to the high speed of vibrations travelingthrough hard materials such as plastic or aluminum
plates. On the other hand, when we used a soft
silicone rubber sheet, we felt that we could localize
the vibration source owing to the slow travel speedof the vibration. We used a silicone rubber sheet as
the medium in the experiment.
Regarding the medium size, we compared several
silicone rubber sheets that were different in thickness. The thicker the silicone rubber sheet, the better we could locate the vibration source. When thesilicone rubber sheet was thin, the vibration easily
attenuated at the place ﬁrst indicated by the user,
and there were fewer vibrational cues at other bodysites. By contrast, when the silicone rubber sheet was
thick, the vibration did not easily attenuate.

4

How well can humans recognize a vibration source’s
direction and distance from the body?
How does the hand posture on the medium affect the
localization ability?
How does the vibration source position affect the
localization ability?

E XPERIMENT

Twelve participants (12 male, all right-handed, with amean age of 23.1 (SD: 1.6) years) participated. All participants were naive to the purpose of the study. Written
informed consent was obtained from each individual before the experiments were performed. This study was approved by the ethics committee of the University of ElectroCommunications (approved number: 20053).

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

4

Participants were seated comfortably on a chair (shownin Fig. 4). Visual information, such as the change in thereﬂection of the sheet’s illumination when the sheet wasindented, was recognizable to the participants, so the participants were blindfolded during the task with a sleep mask.
They wore noise-canceling headphones playing white noise
to mufﬂe external sounds. Their right arm was placed in
an armrest. Their hand was set on the designated area on
the silicone rubber sheet under several posture conditions.
The size of the silicone rubber sheet was 500 mm×500 mm,
and the thickness was 10 mm. The hardness of the silicone
rubber sheet was 30 (tested by a durometer of type Shore
A). We drew a red grid on the sheet to make it easier forthe experimenter to read the subject’s answer position. The
unit of the red grid was 10 mm. The silicone rubber sheet
was placed on soft urethane foam (INOAC CORP, ECZ,
25 % ILD was 80) to diminish the inﬂuence of vibration
propagation via the ground. The size of the soft urethane
foam was 500 mm×500 mm, and the thickness was 100 mm.
The solenoids (CBS10290100, TAKAHA KIKOU Co.) wereplaced inside the urethane foam. The diameter of the contactor at the top of the solenoid was 3 mm. The soft urethane
foam was put on a desk.
sleep masknoise canceling headphone

impulse generating device…

siliconerubbersheet
urethanesheetsolenoidbase

arm rest

The solenoids were actuated by a driver circuit (SB-656501, TAKAHA KIKOU Co.). When conﬁguring amplitude of
impulse, we found that it was difﬁcult to localize the source
when the amplitude was too small, but it was too intense
to use as a user interface when the amplitude was too large.
We adjusted the amplitude to be as large as possible without
being too intense. We measured the displacement of thesilicone rubber sheet indented by the solenoid contactor
using a laser displacement sensor (ZX2-LD50, OMRONCo.). The measured displacement is shown in Fig. 6, whichshows that the maximum displacement was approximately
1 mm and the duration was approximately 0.2 s at the
point of contact of the solenoid contactor. In addition, Fig. 7
provides information about attenuation of acceleration. We
set the acceleration sensor (MPU-6050) at the center of thesilicone rubber sheet and set another sensor (MPU-6050) on
one of the discrete 24 points. We recorded acceleration at
2kHz individually when the 24 points were individuallyindented. The maximum amplitude of the accelerations issummarized in Fig. 7. The 0 mm means the measurement
at the point of indentation. The measurements at “130”,“180”, and “230” mm are the data measured at the centerof the sheet. This suggests the stability of stimuli from any
direction at the same distance. We also show the power
spectral density of the propagated wave shown in Fig. 8
as supporting information.
displacement [mm]4.1 Apparatus and Stimulus

time [s]Fig. 6: Displacement of silicone rubber sheet at the pointof contact of the solenoid contactor, 50 mm away from thepoint of contact, and 100 mm away from the point of contact
when indented by solenoid’s contactor.

Fig. 4: Apparatus.
There were 24 points that were indented by thesolenoid’s contactors in eight directions on three circumferences (shown in Fig. 5). The radii of the three circumferenceswere 130 mm, 170 mm, 230 mm. Participants did not knowthat only those discrete 24 points would be stimulated. They
only knew that some location on the whole area of thesilicon rubber sheet would be stimulated.
50mm
50mm

130 mm
500 mmcontactor

silicone rubber sheet

Fig. 5: Layout of 24 points of solenoid’s contactors.

(error bar: ±1SEM)

Fig. 7: Maximum acceleration of silicone rubber sheet atthe point of contact of the solenoid contactor, 130 mm, 180mm, and 230 mm away from the point when indented bysolenoid’s contactor.
In this experiment, there were three different conditionsof hand posture. We call these “posture conditions.” Theconditions were related to touch with “two-ﬁngertip”, “ﬁveﬁngertip”, and “hand” (shown in Fig. 9).

• two-ﬁngertip condition
Under the two-ﬁngertip condition, users groundedJOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

5

4.2

Fig. 8: Power spectral density of the propagated wave from
130 mm distant place.
the tips of their index and middle ﬁngers on the
sheet. This simulated the human ear situation interms of the time difference and intensity difference
between two sensing points. Of course, this only
worked on the premise that the ﬁngertip could be
regarded as a point (although the ﬁngertip was
actually a surface with a small area).

• ﬁve-ﬁngertip condition
Under the ﬁve-ﬁngertip condition, users grounded
the tips of all their ﬁngers (of the hand in question)on the sheet. This simulated a situation with ﬁve
sensing points, and recognized the time and intensity difference between the ﬁve points.
• hand condition
Under the hand condition, users put their hand ﬂaton the sheet. This simulated the situation as if there
are multiple distributed sensing points.
five-fingertip

two-fingertip
23°

hand

23°

23°

23°

23°

23°
55°

55°

78°

78°

This experiment used a within-participants design. At the
beginning of the experiment, the participants were presented with written instructions that described the situationand tasks of the experiment. After reading this, participants
moved on to the experiment. The experiment was composedof three blocks corresponding to three posture conditions.
Each block comprised a familiarization phase and a test
phase.
In the familiarization phase, all 24 points were stimulated sequentially at 5-s intervals. The order of stimulation
was randomly assigned. No information about the positions
which were stimulated was given to the participant. After
this was completed, the test phase started.
In the test phase, one of the 24 points was stimulated ineach trial. Participants answered where the stimulus point
was by touching the point with their non-dominant hand’s
index ﬁnger. We adopted this method following previous
work by [39], which investigated localization on the handsurface. To prevent the participant from noticing that thesolenoids were not present under their ﬁngers or hands, wetold participants that the vibration might occur anywhereon the sheet, and participants were allowed to point to any
area including the area where their hands or ﬁngers were.
The answered points were recorded. The experimenter stimulated each point once, and thus there were 24 trials thatparticipants answered. The order of stimulation positions
was randomly assigned. After all trials, there was a 3-minbreak before the next block started. Participants were notallowed to move their grounded hand for the duration ofeach block. The order of posture conditions assigned tothe three blocks was randomized across participants. There
were 24 trials for each block, and there were 3 blocks. Thus,
the total number of trials was 72 per participant.
4.3: grounded area
: not-grounded area

Fig. 9: Posture conditions.
We measured the width and height of the hand for eachparticipant when the participant placed the hand under the“hand” posture condition. The measured width and heightof the hands are summarized in Table 1. We deﬁned thecenter of the hand as the middle point of its width andheight. Participants set the center of their hand to the centerof the silicone rubber sheet. Then, the angle of the ﬁngers
was adjusted as shown in Fig. 9. For all of the hand postures,
participants placed their hands naturally without exerting
any force on the silicone rubber sheet.
TABLE 1: Measured width and height of participant’s hand.

width [cm]height [cm]min max

mean ±SD181915.6 ±1.417.9 ± 0.6

13
17

Task

Results Comparing Three Posture Conditions

Fig. 10 (a) shows the mean answer position for each stimulus
point. The start point of the arrow represents the stimulus
position, and the arrowhead represents the answer position.
For each condition, the absolute value of positional
error, which is the absolute value of the difference in thepositions of answer and stimulus, was averaged acrossparticipants. Fig. 10 (b) shows the absolute value of thepositional recognition error for each stimulus point. In theﬁgure, the value of the points which were not stimulated
was linearly interpolated from the near stimulus points.
Fig. 11 (a) shows the absolute positional error averaged
across all stimulus points. After checking normality (bya Shapiro-Wilk test) and sphericity (Bartlett’s test) of distribution, a one-way repeated measure ANOVA was conducted with the posture condition as a within-subject factor.
The main effect of the posture condition was signiﬁcant(F (2, 22) = 77.22, p < 0.0001). All post-hoc Tukey HSD
tests on the pairs of posture conditions were signiﬁcant(p < 0.01).
For each condition, the absolute value of the directional
error, which is the absolute difference in the direction of theanswer point from the center and the direction of the stimulus point from the center, was averaged across participants.
Fig. 10 (c) shows the absolute directional error for each

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

690°
45° 135°230

45° 135°
230
180

135°

180°

130

130

130
180

270°

270°

180°

90°
45° 135°
230
180

130

75

50

**

**

**

**

two five hand

**

**

**

two five hand*twofive hand

two: two fingertip, five: five fingertip** :p<0.01, * :p<0.05

180°

hand
90°

45° 135°230
180

45° 135°
230
180
130

130

45°
230
180
130
0°

315°225°

225°

225°

270°

270°

absolute
directional 80
error [°
]

70

60

50

(d) absolute distance error
two fingertip

40

30

315°
270°

20

10

five fingertip

hand

90°

90°

90°

45° 135°230
180

45° 135°
230
180
130

130

45°
230
180
130
0°

80

270°

270°

270°
70

60

50

315°

315°225°

225°

225°

40

30

Fig. 11: (a) Absolute positional error, (b) absolute directional
error, and (c) absolute distance error for each posture condition.

25

90°

90°

absolute
distance
error [mm]

**

100

post-hoc Tukey HSD tests on the pairs of posture conditions
were signiﬁcant (p < 0.01).
For each condition, the absolute value of the distanceerror, which is the absolute difference in the distance of
the answer point from the center and the distance of thestimulus point from the center, was averaged across participants. Fig. 10 (d) shows the absolute distance error foreach stimulus point. Fig. 11 (c) shows the absolute distanceerror averaged across all stimulus points. After checking thenormality and sphericity of distribution, a one-way repeatedmeasure ANOVA was conducted with the posture condition
as a within-subject factor. The main effect of the posture
condition was signiﬁcant (F (2, 22) = 6.99, p = 0.0045).
Post-hoc Tukey’s HSD test revealed that there was a signiﬁcant difference between the two-ﬁngertip and ﬁve-ﬁngertip
conditions (p = 0.03), between two-ﬁngertip and handconditions (p < 0.001), and between the ﬁve ﬁngertip andhand conditions (p = 0.007).
4.4 Results Focusing on “Hand” Posture Condition
From the results shown in Fig. 11, participants could localize
the impact vibration source more accurately under the handposture condition than under other conditions, and thus we
focused on the hand posture condition for further analysis.

20

Fig. 10: (a) Average answer position, (b) absolute positional
error, (c) absolute directional error, and (d) absolute distanceerror between stimulus and answer for each stimulating
point.

stimulus point. Fig. 11 (b) shows the absolute directional
error averaged across all stimulus points. After checking thenormality and sphericity of distribution, a one-way repeatedmeasure ANOVA was conducted with the posture condition
as a within-subject factor. The main effect of the posture
condition was signiﬁcant (F (2, 22) = 75.93, p < 0.0001). All

4.4.1 Absolute Positional Error
stimulus radius [mm]225 200 175 150 125 100

135°

315°
270°

270°

(c) absolute directional error
two fingertip
five fingertip

180°

160

45°
230
180
130

315°225°

225°
270°

135°

(error bar: ±1SEM)

0°

225°

absolute
positional
error [mm]

315°

hand

90°

45° 135°
230
180
130

(c) absolute
distance error(error bar: ±1SEM)

140

0°

270°

(b) absolute positional error
two fingertip
five fingertip
90°(b) absolute
directional error(error bar: ±1SEM)

120

315°225°

225°

225°

135°

45°
230
180

(a) absolute
positional error

absolute distance error [mm]

hand

90°

absolute directional error [°]five fingertip

90°

absolute positional error [mm](a) average answer position
two fingertip

230

148±24 131±25 95.6±12 109±33 159±13 122±8

180

91±17

77±13 49±7

130

70±9

40±8

0

45

174±37 151±21

58±11 73±10

73±8

64±8

92±15

29±6

38±6

46±5

44±5

32±5

69±10

90

135

180

225

270

315

stimulus direction [°]

180
160
140
120
100
80
60
40
20

absolute
positional
error [mm]Fig. 12: Absolute positional error for each stimulus radiusand direction under hand posture condition.

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 20157absolute positional error (error bar: ±1SEM)
**

**

*

130 180 230

0stimulus radius [mm]4590

135 180 225 270 315

absolute directional error [°]****absolute directional error [°]absolute positional error [mm]absolute positional error [mm]**absolute directional error (error bar: ±1SEM)

130 180 230

stimulus direction [°]** :p<0.01, * :p<0.05

045

stimulus radius [mm]90135 180 225 270 315

stimulus direction [°]** :p<0.01, * :p<0.05

Fig. 13: Left: absolute positional error for each stimulus
radius. Right: absolute positional error for each stimulus
direction.

Fig. 15: Left: absolute directional error for each stimulus
radius. Right: absolute directional error for each stimulus
direction.

Fig. 12 shows the absolute positional error for eachstimulus radius and direction. The smallest positional error
was 29 mm, recorded when the stimulus radius was 130 mm
and the stimulus direction was 90◦ .
We conducted a two-way repeated measure ANOVA ofthe stimulus radius and direction on the absolute positional
error (shown in Fig. 10 right). We checked the normality
and sphericity of each distribution in advance. There wasa signiﬁcant effect for the radius (F (2, 22) = 187.7, p <0.0001). There was also a signiﬁcant effect for the direction(F (7, 77) = 2.7, p = 0.0158). There was no signiﬁcantinteraction effect (F (14, 154) = 0.9, p = 0.61).
Fig. 13 (a) shows the absolute positional error for eachstimulus radius. Post-hoc Tukey HSD tests on all of theradius pairs were signiﬁcant (p < 0.01). Fig. 13 (b) shows
the absolute positional error for each stimulus direction.
According to a post-hoc Tukey HSD test on the pairs ofdifferent stimulus directions, there was no signiﬁcant difference between all pairs of stimulus direction (p > 0.05).

We conducted a two-way repeated measure ANOVA ofthe stimulus radius and direction on the absolute directional
error after checking the normality and sphericity of each
distribution. There was a signiﬁcant effect for the radius(F (2, 22) = 68.1, p < 0.0001). There was also a signiﬁcanteffect for the direction (F (7, 77) = 2.37, p = 0.03). There
was no signiﬁcant interaction effect (F (14, 154) = 1.0, p =0.49).
Fig. 15 (a) shows the absolute directional error for eachstimulus radius. Post-hoc Tukey HSD tests revealed signiﬁcant differences between the two radii pairs of 130◦ 230◦ (p < 0.01) and 180◦ -230◦ (p < 0.01). There was no
signiﬁcant difference between the pair of 130◦ -180◦ (p = 0.9)
Fig. 15 (b) shows the absolute directional error for eachstimulus direction. According to post-hoc Tukey HSD tests
on the pairs of different stimulus directions, there was asigniﬁcance between the pair of 0◦ -90◦ (p = 0.018).

230

43±11 34±13 14±4

180

20±6

18±5

27±5
0

130

25±13 36±6

23±4

50±15

36±8

6±2

12±3

16±4

18±3

11±4

24±6

12±3

7±2

7±3

15±2

17±2

9±2

20±5

45

90

135

180

225

270

315

stimulus direction [°]

50
45
40
35
30
25
20
15
10
5

stimulus radius [mm]stimulus radius [mm]4.4.2 Absolute Directional Error

4.4.3

Absolute Distance Error

230

86±16

80±10

75±9

53±11

86±10

80±10

62±11 70±13

180

48±10

51±11

41±7

44±10 45±9

50±8

46±8

56±11

130

31±6

25±4

23±5

33±6

29±6

21±4

23±4

40±6

0

45

90

135

180

225

270

315

stimulus direction [°]

90
80
70
60
50
40
30
20

absolute
distance
error [mm]absolute
directional
error [°]

Fig. 16: Absolute distance error for each stimulus radius anddirection under hand posture condition.

Fig. 14: Absolute directional error for each stimulus radiusand direction under hand posture condition.

Fig. 16 shows the absolute distance error for each stimulus radius and direction. We found that the smallest distanceerror was 21 mm, recorded when the stimulus radius was
130 and stimulus direction was 225◦ .
We conducted a two-way repeated measure ANOVA ofthe stimulus radius and direction on the absolute distanceerror after checking the normality and sphericity of eachFig. 14 shows the absolute directional error for eachstimulus radius and direction. We found that the smallest
directional error was 6◦ , recorded when the stimulus radius
was 130 or 180 mm and the stimulus direction was 90◦ .

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

absolute distance error

directional bias (error bar: ±1SEM)(error bar: ±1SEM)****
directional bias [°]directional bias [°]absolute distance error [mm]absolute distance error [mm]

****

8

130 180 230

4590

135 180 225 270 315

stimulus direction [°]** :p<0.01, * :p<0.05

Fig. 17: Left: absolute distance error for each stimulus
radius. Right: absolute distance error for each stimulus
direction.

135 180 225 270 315

distance bias (error bar: ±1SEM)**

4.4.5 Distance Bias
In this subsection, we focus on the distance bias and test
whether the answered distance is biased from the stimulus
distance. Before we conducted a two-way repeated measure ANOVA of the stimulus radius and direction on the
distance bias, we tested the normality and sphericity ofeach distribution. We found that the normality was violated(p > 0.05) when the stimulus distance was 130 or 180, or
when the stimulus direction was either 90◦ , 135◦ , 180◦ ,90

stimulus direction [°]** :p<0.01, * :p<0.05

225◦ , or 270◦ . Then, we conducted the Kruskal-Wallis test
and revealed that the stimulus distance affected the distancebias (H(288) = 89.8, p < 0.0001), but the stimulus direction
did not affect the distance bias (H(288) = 7.34, p = 0.39).

**

4.4.4 Directional Bias
In this subsection, we focus on the directional bias andtest whether the answered direction was biased from thestimulus direction. We conducted a two-way repeated measure ANOVA of the stimulus radius and direction on the
directional bias after checking the normality and sphericity
of each distribution. There was no signiﬁcant effect of theradius (F (2, 22) = 2.59, p = 0.09), direction (F (7, 77) =1.86, p = 0.09), or interaction effect (F (14, 154) = 1.24, p =0.25).
Fig. 18 shows the directional bias for each stimulus
radius or for each stimulus direction. To test whether the
directional bias deviated from 0, we conducted a t-test (withBonferroni correction) for each stimulus radius and stimulus
direction. There was signiﬁcance only under the conditionof stimulus direction 270◦ (corrected p < 0.05).

45

Fig. 18: Left: directional bias for each stimulus radius. Right:
directional bias for each stimulus direction.

**

distribution. There was a signiﬁcant effect of the radius(F (2, 22) = 32.8, p < 0.0001). There was no signiﬁcant effect of the direction (F (7, 77) = 1.5, p = 0.18). There was no
signiﬁcant interaction effect (F (14, 154) = 1.66, p = 0.069).
Fig. 17 (a) shows the absolute distance error for eachstimulus radius. Post-hoc Tukey HSD tests revealed signiﬁcant differences between all radii pairs (p < 0.01). Fig. 17(b) shows the absolute distance error for each stimulus
direction. Because there was no main effect of stimulus
direction from the ANOVA results, we did not conduct a
post-hoc multiple comparison test.

0

stimulus radius [mm]distance bias [mm]stimulus radius [mm]0distance bias [mm]130 180 230

*

**

**

**

130 180 230

stimulus radius [mm]

0

45

90

135 180 225 270 315

stimulus direction [°]** :p<0.01, * :p<0.05

Fig. 19: Left: distance bias for each stimulus radius. Right:
distance bias for each stimulus direction.
Fig. 19 shows the distance bias for each stimulus radiusor for each stimulus direction. A Steel-Dwass post hoc test
showed a signiﬁcant difference between all pairs of stimulus
distances (130–180 (p < 0.0001), 180–230 (p < 0.0001), and
130–230 (p < 0.0001)).
To test whether this bias deviated from zero signiﬁcantly,
we used 10000 bootstrap samples [40] to calculate the conﬁdence interval (CI) of biases for each stimulus radius. If
the Bonferroni-corrected 95% CI did not overlap to zero, we
could conclude that the bias was statistically signiﬁcant. As
a result, under all of the three stimulus radii, the CI did
not overlap to zero and we conclude that the biases weresigniﬁcant. This means that it is difﬁcult to recognize an
accurate distance in two-dimensional space with the setup
we used.

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

Actually, the participant’s recognized distances were
120 ± 32.0 mm, 138.1 ± 40.1 mm and 157 ± 43.7 mm andthese were far from each stimulus radius (shown in Fig. 20).
To test whether the answered distances were different between each stimulus radius or not, we conducted a SteelDwass test on the answered distances between three pairsof stimulus radii. We obtained signiﬁcant results between
all pairs between 130-180 (p = 0.0012), 130-230 (p = 0.001),
and 180-230 (p = 0.013).
(error bar: ±1SEM)****

answered distance [mm]**130 180 230** :p<0.01,* :p<0.05

stimulus radius [mm]Fig. 20: Answered distance for each stimulus radius.

4.5

Results Focusing on Hand Size ParameterAs shown in Table 1, the participants’ hand sizes weredifferent. There is a possibility that the hand size parameter
affected the localization ability under hand conditions. As
hand size parameters, we used hand width, height, and area
which is calculated by multiplication of width and height.
First, we tested whether the absolute directional error
was affected by hand size parameters. The hand size parameters such as hand width could affect directional recognition
with three stimuli radii and eight stimuli directions in different ways. For example, there is a possibility that hand heightaffects directional recognition when the vertical direction
is stimulated but it does not affect directional recognition
when the horizontal direction is stimulated. Thus, we independently tested how the three hand size parameters
were correlated with directional recognition results withthree stimuli radii and eight stimuli directions. In total,
we conducted correlation analyses 33 times (=3 [hand size
parameters such as height, width, area] x 11 [3 stimuli radii
and 8 stimuli directions]). We set the signiﬁcance level at
0.05 with Bonferroni correction. However, as a result, we
could not ﬁnd any correlation under these 33 conditions.
Second, we also tested whether the absolute distanceerror was affected by hand size parameters. We conducted
correlation analyses 33 times (=3 [hand size parameters] x
11 [3 stimuli radii and 8 stimuli directions]) but we could
not ﬁnd any correlation.
4.6

Discussions

9

4.6.1

Differences between Posture Conditions

By comparing the results with the three posture conditions,the main effect of the posture condition was conﬁrmed
on all three viewpoints of error (absolute positional error,
absolute directional error, and absolute distance error). Posthoc multiple comparisons clariﬁed that the hand condition
was the best. In addition, the ﬁve-ﬁngertip condition was
better than the two-ﬁngertip condition.
Under the two-ﬁngertip condition, the localization error
was better near the area of the two ﬁngertips and worse in
the area furthest from the ﬁngertips (see Fig. 10). According
to Fig. 10 (a), participants recognized the far indentation
points as if they were close to their two ﬁngertips. Thissuggests that the area of the human body in contact with themedium surface was an important factor for the localization.
It was in accordance with the better results under the ﬁveﬁngertip or hand conditions in which the hand was incontact with a greater surface area than under the twoﬁngertip condition.
When comparing the results between the ﬁve-ﬁngertip
condition and the hand condition, the difference in boththe absolute directional and distance error was signiﬁcant.
The difference in posture was the presence of a groundedarea of the ﬁngers (for the ﬁve-ﬁngertip condition) insteadof both ﬁngertips and palm (for the hand condition, which
had those extra grounded areas). Thus, in the case of thehand condition, the grounded areas were larger, and participants could use this for vibrotactile sensing to recognize
the spatiotemporal cues illustrated in Fig. 3, which led tosmaller errors.
On the other hand, under the hand posture condition,
the hand size parameters between participants were notcorrelated with directional or distance errors. We consider
the hand size effect was small since the difference in handsize was relatively small compared with the differences
between posture conditions. Since this study is preliminary
at this point, more investigation would be required to clarify
this.
4.6.2 Analysis Focusing on Hand Condition and Directional
Recognition
From here, we will focus on the hand condition, and we
discuss directional recognition in this section.
Regarding the absolute directional error, there was amain effect of the stimulus radius. The absolute directional
error became larger when the stimulus radius was 230 mm
rather than 130 or 180 mm. We assume that the participants
recognized the direction mainly by judging where they felt
the intensive vibration in their hand and we speculate thatthe reason for the deterioration was that the intensity cue
would be reduced when a more distant point was stimulated (see Fig. 7). There was also a main effect of the stimulus
direction. The performance was better in the 90◦ direction
and worse in the 0◦ direction. In the 90◦ direction, where
the performance was better, the ﬁngertips of the middle
ﬁnger, index ﬁnger, and ring ﬁnger were placed. It is known
that the ﬁngertips are better in terms of sensitivity [41] oracuity [42] than the palm area, and this knowledge can
explain the better performance of the 90◦ direction. On theother hand, the 0◦ direction, where the performance wasJOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 201510

poor, is where the hypothenar eminence was located. We
surveyed previous studies that investigated the vibrotactile
sensitivity or acuity of the hypothenar eminence, but wecould not ﬁnd the information. However, in the palm area,
the hypothenar eminence has a smaller mechanoreceptor
density than the thenar eminence, which is thought to have
led to this poor result in the 0◦ direction.
Regarding the directional bias, there was no main effectof either stimulus radius or direction. However, as a resultof a t-test to examine whether the directional bias deviatedfrom 0, we found that there was a signiﬁcant bias when
the stimulus direction was 270◦ . The reason for the bias
was unclear, but there is a possibility that this is owing to
the answer method we adopted. We asked participants toindicate the estimated position with the index ﬁnger of their
non-dominant hand, which was a method similarly adopted
by work [39]. In this experiment, all participants were righthanded, and they answered with the index ﬁngers of theirleft hand. Around 270◦ is the space under the participant’s
right arm, and there is a possibility that the pointing ﬁnger
position veered to the left-hand side when pointing to thatspace.

4.6.4 Limitations
Attention has to be paid to the possibility that the presenceof the bias originated from the experimental design.
Since we needed to exclude the effect of visual information of the sheet deformation cue on the localization
result, we asked participants to wear sleep masks. Note that
previous studies suggested that blindfolded participants
could have a bias when pointing their ﬁnger at speciﬁc
locations [43] and thus there is a possibility that the obtained
values of absolute errors and biases were affected by the
pointing bias.
Participants could see the silicone rubber sheet withthe red grid before they put on their sleep masks during
the familiarization phase of the ﬁrst block. The red gridcovered all areas of the silicone sheet including areas whereparticipants put their hands or ﬁngers. Thus, we consider
that the illustration of the hatch marks itself did not give additional cues. However, considering the difﬁculty of manual
read by the experimenter of the point under participants’ﬁngers or hands where the red grid is hidden, we can notcompletely exclude the possibility that participants might
have assumed the vibration location would be outside thearea of their hands.

4.6.3 Analysis Focusing on Hand Condition and Distance
Recognition

4.6.5 Use of Vibration Localization Outside Body in Applications
Among haptic displays, the vibrotactile display is the mostwidespread class of haptic devices, found in a variety of
consumer electronics devices [44], and thus it is expectedthat the human ability to localize vibration outside the bodywill be used in applications.
We found that the hand condition was better than thetwo- or ﬁve-ﬁngertip conditions in terms of the absolute
error of position, direction, and distance. From another pointof view, considering that this vibration localization can beused as the user interface for some objectives, it is importantthat users be less tired while using it. The hand condition
has the largest grounded area and is less likely to induce fatigue. By contrast, the ﬁve- or two-ﬁngertip conditions havesmaller grounded areas, and users can easily become tired.
Therefore, the hand condition is considered to be suitable
for the UI from the viewpoint of localization accuracy andresistance to fatigue.
Under the hand posture condition, we found that it is
difﬁcult for users to accurately recognize the distance ofthe stimuli. In contrast, participants were able to recognize
the stimulus direction with an error of approximately 150◦
if we stimulated nearer than 180 mm. This is informative
to the case where it is difﬁcult to attach a vibrator to theuser’s body and it is required to embed the vibrator at a
distance. For example, considering we assume the chairtype vibrotactile device, if we attach the vibrator on thesurface contacting back or buttock, the tactile feeling of
the vibrator makes the chair less comfortable. Thus, it isconceivable that it could be embedded in the back of theseat or deep in the seat surface and transmit the direction
information by vibration presentation from that position. Of
course, this study only investigates directional recognition
when using the dominant hand, which is more sensitive tovibration, and we have to investigate whether directionalrecognition is possible using other body sites.

According to Fig. 19 and Fig. 20, distance recognition was
worse as compared to direction recognition. Though theanswered distances were different between stimuli radii(see Fig. 20), it was difﬁcult to accurately recognize thedistance in two-dimensional space with the setup we used(see Fig. 19).
In previous research investigating distance recognition
in a one-dimensional medium [29], [30], [31], [32], participants were able to recognize the distance. There areseveral differences between our work and theirs. First, in
their experimental setup, participants actively move themedium and voluntarily cause the impact. For example, theparticipant in the work by [31] moves the rod and causes
the collision with the virtual ﬂoor. The active sensing could
provide additional cues other than the vibrotactile feedback.
Thus, if we make participants actively lift up and put down
their hands and provide impact feedback in time with theputting down, there is a possibility that distance recognition
would be better.
Second, the type of wave pattern was different. In previous studies such as [29], the touch information was encodedin the standing wave. On the other hand, according to ourmeasurement of acceleration in Fig. 6, the wave attenuated17.4 dB with 130 mm. Since the attenuation was large, it
is considered that there was almost no standing wave inthe entire sheet. It suggests that the touch information was
encoded in the traveling wave in our setup. In addition,
ﬁg. 8 shows that the peak of the propagated waves wasaround 70 Hz. According to this, we consider that our
setup activated both RA (rapidly adapting) channel and PC(Pacinian) channels with our setup. There is a possibility
that distance recognition would be changed if we activate
mainly either one of the channels using cyclic waves of 30
Hz or 230 Hz.

JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, AUGUST 2015

5

C ONCLUSIONS AND F UTURE S TUDYThis study evaluated the two-dimensional localization of
an impulse vibration source outside the body. Participants
reported the position of the impulse vibration by sensing
the propagated vibrational wave from the source.
The results suggested the following characteristics:•••

localization errors are smaller when the entire handis grounded than when only the ﬁngertips are
grounded.
localization positional and directional errors are
smaller when the 90◦ (around index, middle, andring ﬁngertips) area of the hand is stimulated. They
are larger when the 0◦ (around the hypothenar eminence) area of the hand is stimulated.
Though the answered distances were signiﬁcantly
different between stimuli radii, it was difﬁcult toaccurately recognize the distance in two-dimensional
space with the setup we used.

In this study, we used impulse stimuli to stimulate themedium. We are interested to know whether the same
results would be reproduced if we used other waveforms
such as cyclic ones (e.g., a sinusoid). In addition, this study
used the hand as the body part to be grounded. As a userinterface, there may be situations where it is more naturalfor other parts (e.g., the back or foot soles) to be grounded.
Thus, we are also interested in the two-dimensional localization results when other parts are grounded.

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Yusuke Ujitoko is a researcher of NTT Communication Science Laboratories since 2020.
He used to be a member of R&D Group in
Hitachi, Ltd from 2016 to 2020. He received
PhD. in engineering at the University of ElectroCommunications, Japan in 2020. He received
a B.E. degree in mechanical engineering and
M.A.E. degree in inter-disciplinary information
studies from the University of Tokyo, Japan in2014 and 2016. His research interests include
applied haptic perception and haptic interfaces.

Ryunosuke Tokuhisa is a graduate student of
the University of Electro-Communications.

12

Sho Sakurai received a B.E. degree in social
and information studies from the Gunma University in 2007, a M.A.E. degree in inter-disciplinary
information studies in 2010 and a PhD degree
in Engineering in 2014 from the University ofTokyo. Currently, she is a project assistant professor in Graduate School of Information Systems, the University of Electro-Communications.
Her research interests are multi-modal/crossmodal interfaces and human-computer interaction. She is also active as a manga artist for introduction of the latest research of virtual reality and artiﬁcial intelligence.

Koichi Hirota received a BS degree from the
University of Tokyo, Japan, in 1988. He received
a PhD degree from the University of Tokyo, in
1994. He was then an assistant professor with
the Toyohashi University of Technology in 1995.
In 2000, he was an associate professor with the
University of Tokyo. Currently, he is a professor
in the Department of Human Media Systems,
Graduate School of Information Systems, University of Electro-Communications. His researchinterests include haptic rendering and humaninterfaces. He received several academic awards.