Somatotopic distortion of tactile temporal interval estimation
Shinobu Kuroki1,2 , Junji Watanabe2 , Naoki Kawakami1 and Susumu Tachi3
1

Department of Information Physics and Computing, University of Tokyo, Tokyo, Japan(Tel: +81-3-5841-6917; E-mail: {Shinobu_Kuroki,naoki_kawakami}@ipc.i.u-tokyo.ac.jp)
2
NTT Communication Science Laboratories, NTT Corporation, Kanagawa, Japan(Tel: +81-46-240-3613; E-mail: watanabe@avg.brl.ntt.co.jp)
3
Keio University Graduate School of Media Design, Tokyo, Japan(Tel: +81-3-5841-6915; E-mail: tachi@tachilab.org )Abstract: Tactile timing mechanism, which is essential for accurate response to the external environment, has to compensate for the distortions of neural timing signals. Specifically, signals come from the distributed peripheral receptors
and the body parts move dynamically in a space. Since an accurate visual timing encoding lacks the precision when using
two widely separated photoreceptors, here we studied on how the tactile timing is encoded in relation to two distance;
somatotopic representation, defined by cortical topography, and spatiotopic representation, defined in the physical world.
We performed 1 second tactile interval estimation experiments, in which the spatial distance of the two stimuli was systematically changed in somatotopic and spatiotopic representations and compared. Our results showed that somatotopic
nerve distance, not real-world physical distance, plays a dominant role in the tactile timing estimations.
Keywords: Tactile, temporal interval estimation, somatotopic

1. INTRODUCTION
When you look at a saliently ticking clock, you sometimes think that the second pointer takes longet than normal to move to its next position. For a short period, theclock appears to have stopped which is a famous phenomenon called chronostasis [1]. This temporal modification by voluntary action suggests that our temporalperception is not directly reflect the timing of the neuralsignals. In other words, our time is not caliculated by”metaphysical central clock” in our brain. Another modification phenomena were reported that sluggish neuralsignal, caused by transient luminance noise [2], impaired
the accuracy of timing judgment. And also, spatial separation between receptors has effects [3].
Since neural mechanisms encoding timing events are
crucial for human sensory and motor system, we focused
whether this modification mechanism exist in touch. Do
we have metaphysical central tactile clock? Or do wehave several distributed neural mechanisms of temporalestimation? Considering the latter case, it can be speculated our timing perception is distorted by stimulus condition, such as position or attention. Here, tactile modality is unique from the viewpoint of its spatial representation since receptors are distributed over the whole body,and body parts move around dynamically with our body
movements. For example, signals from the receptors in
the finger and toe are subject to significantly different
latencies, and spatial distance between receptors in theleft and right hands dramatically changes in the physical space depending on whether the arms are opened
or closed. So the information processing of tactile timing can be performed through at least two spatial representations; somatotopic representation, defined by cortical topography, and spatiotopic representation, defined in
the physical world. Does the perception of tactile timing change depending on the stimulus position? And ifso, which coordinates should be paid attention when wedesign tactile displays? This question can easily be expanded to incorporate the question of how the brain interprets information from a lot of sensors. Since we havebeen considered this tactile coordinate problem [4], weespecially focused on time interval estimation in this paper.

2. EXPERIMENT: 1 SEC TIME
INTERVAL ESTIMATION
2.1 Apparatus
To avoid the negative effect of finger skin vibration,
we used an electric stimulus which makes a potential gradient on the axon of the peripheral tactile receptor, andproduces nerve activity directly. The participant placed
his/her finger pad on the electrode arrays. The electrodes
were 1.25 mm in diameter and arranged at intervals of
2.5 mm. Anodic stimulation method was used, in which
one electrode and surrounding electrodes served as theanode and ground, respectively, and made a potential gradient on the axon of the peripheral tactile mechanoreceptors. As demonstrated in previous studies, electric stimulus has the same capability as a mechanical vibration[5]. The duration of one electric stimulus was about one
millisecond, and it was composed of five shots of current impulses, which were 20 microseconds of duration.
The magnitude of applied current was adjusted from 2
mA and 4 mA individually due to individual differenceof perceived intensity.
2.2 Procedure
It was difficult for participants to estimate and replicate the interval of presented tactile stimuli, we used thecomparative method. The tactile temporal interval wascompared with auditory comparison stimuli. Four electrode arrays were arranged for the subject’s left middle

(A)

2cm(B)

25cm
50cm

4cm(C)(D)

Ipsilateral
Bilateral-Near
Bilateral-Far
Same Site

Proportion “Audio was longer”
1.0

0.8
Left

LeftRightMiddle

Index

Index

RightIndex

Body Midline

0.6

0.4

Fig. 1 Finger place conditions
0.2finger (A), left index finger (B), and right index finger(C), with 2 cm distance between them. The subject could
only move the right index finger to the right (D), so thatthe distance from left middle finger to right index finger
was 50 cm, as shown in Fig.1. Four place conditions withpair of fingers were examined. One was referred to as
the ”same site condition,” in which the two stimuli wereadded on (A). This ”same site condition” was tested asa control condition. The ”ipsilateral condition” used (A)and (B), and the two ”bilateral conditions” used (A) and(C) or (A) and (D). The difference between the two bilateral conditions was the distance of the subject’s hands.
Two beeps with stimulus-onset asynchronies (SOA) wereprovided, then left side arrays was presented first stimulus and the other was stimulated with exactly 1 second interval. The auditory stimuli presented with an interval chosen from 5 values between 850 and 1150 ms.
Participants were asked to compare the temporal interval of auditory stimuli with that of tactile stimuli, which
was always one second, and answered which interval waslonger as shown in Fig. 2.

MT085090095010001050

1100

1150

Audio Signal Interval (ms)Fig. 3 Psychometric functions obtained with participantMTticipant perceives two intervals to be the same was calculated. We consider that shift of the same site condition’s PSE from the audio stimuli interval as a corollaryof modality difference. Because each subject showed different PSE shift of same site condition, we normalizedeach subject’s results by each PSE of same site condition
as shown in Fig.4.

SKMTKITS

IpsilateralTH

One of 5 conditions
SOAAuditory
1 sec interval
TactileJudgement

two stimuli withﬁxed interval
1000 msec

Which one is longer?

Bilateral-Near

Bilateral-Far

Fig. 2 Procedure
Each condition was tested 20 times. The SOAs and place
conditions were tested in random order. Five subjects
were tested individually.
2.3 ResultFig.3 shows the result obtained with participant MT.
The horizontal and vertical axes represent SOA of Audio
signal, and the rate of a participant’s response of ”Audiowas longer”, respectively. The rates in the both bilateralconditions fell to zero with smaller SOAs than those in
the same site and ipsilateral conditions. As the SOA increased, the rate increased to one. In this case, the samesite and ipsilateral conditions increase with smaller SOAs
than those in the bilateral conditions.
Point of Subjective Equalities (PSEs) where each par-050100150200

PSE Shift from Same site condition (ms)Fig. 4 PSE shift of temporal interval from same site
condition
Surprisingly, the shift of PSE in the ipsilateral conditionis significantly small than bilateral conditions (t=-3.951,p<0.05) which were more than sixty milliseconds. In addition, there were no significant differences between thetwo bilateral conditions (t=0.536, p<n.s.). This meansthat temporal interval perception around one second is
distorted by the difference in the stimulus position on thebody, and the difference in the body positions in the realworld does not make sense.

3. CONTROL EXPERIMENT: 3 SEC
TIME INTERVAL ESTIMATION
How can the difference in the stimulus position be explained? Although there must be neural latency difference between ipsilateral and bilateral conditions, it seems
that this interval distortion we observed is different scale(see discussion). If so, the dissociation of the estimatedtime interval between the two fingers should be ascribed
not to the difference in the timing information of the sensory input but to higher processes difference such as filtering or memory mechanism. In next experiment, wetest for a possible influence of tactile memory on thelonger time interval estimation to look more closely at
the somatotopic effects of tactile timing mechanism.
The same experimental procedures as in the case of
the 1 sec time interval estimation without SOA variations
were used. Here we prolonged stimulus interval from 1
second to 3 seconds so that participants surely use tactile short term memory to judge. The interval of auditory
stimuli were varied 5 values between 2700 and 3300 ms.
The results are shown in Fig.5.

which memorizes estimated tactile interval is swayed bystimulated position.
According to the first hypothesis, the somatotopic effects we measured would arise from the other tactile
temporal tasks such as simultaneity judgment. Actually,
this somatotopic distortion is well reported by previousstudies[6], showing that the ranges of simultaneity werewider in the bilateral conditions than that in the ipsilateral condition. Most of previous tactile simultaneity studies explained this somatotopic distortion of simultaneity
judgment by neural transduction latencies. In the ipsilateral stimulation, the two stimuli are delivered to thesame cerebral hemisphere, while in the bilateral stimulation they are presented to each hemisphere and interhemispheric transmission time (IHTT) is needed for the task.
So are these tactile temporal estimation distortions perfectly explained by neural latency such as IHTT? We experimentally measured somatotopic distortion in simultaneity judgment with using same experimental setup as
our time interval estimation. The result is shown in Fig.6.
SK

Ipsilateral

SKMTKITSKI

Same siteMTTSTHTH

Ipsilateral

Bilateral-Near

Bilateral-NearBilateral-Far

Bilateral-Far
0050100150200250

20406080100

Half width of the Gaussian function (ms)PSE Shift from Same site condition (ms)Fig. 6 Mean ranges of subjective simultaneity

Fig. 5 PSE shift of temporal interval from audio signal

What is interesting here is that although one-second timeperception seems at a glance to be a complicated process
for neural filtering, the simultaneous perception showed
the same tendency. No spatiotopic effect, but only somatotopic distance had effect again. From the view ofinformation encoding, the accuracy of signal/noise segregation becomes worse for bilateral stimuli. In visual
psychophysical studies, it was reported that sluggish neural signal, caused by spatial separation between receptors [3] or by transient luminance noise [2], impaired theaccuracy of timing judgment. The reduced accuracy inthe tactile simultaneous task can also be explained fromthe view of information encoding as in the visual timing
tasks. However, the IHTT estimated in our simultaneity
experiment was around 30 milliseconds. The mean valueof the IHTT reported in previous study was around 20milliseconds, which was equivalent or even shorter thanour results, and it was too short to explain the differencein the time interval estimation, around 80 milliseconds.
When the time interval of two stimuli is short, the information can be processed by a low level mechanism

The same site condition also showed obviously shorter
result than other conditions, however, no significant difference was observed between ipsilateral and bilateralconditions in this 3 second estimation case. Again, nosomatotopic distortion in this control experiment. The
PSE of bilateral near condition looked taking bigger thanother condition, however, this difference is not significant
and considering the percentage of variance, it was much
more smaller than the result of 1 sec experiment.

4. DISCUSSION
The somatotopic distortion of tactile temporal interval
estimation we report have at least three possible origins.
First possibility is that the somatotopy we observed could
be simply a consequence of the neural latency difference.
Second, we may have measured a truly somatotopic effect, in that the time interval estimation is linked to thesomatotopic position where we receive the pair of stimuli.
A third possibility insists that our short memory itself,such as a spatiotemporal filter. However, our experimentused enough long interval so memory mechanism should
be involved in judgement. To verify the third hypothesis”whether stimulated position does have effect on memory
or not” we made control experiment with more long interval. In a control experiment where 3 seconds interval
was used instead of 1 second, the somatotopic distortionis disappeared, indicating that this somatotopic effect ontime interval estimation is due to the estimation mechanism itself rather than the memory distortion.
Here, we support the second hypothesis. Although
the same perception such as ”1 sec” is obtained, encoding levels between ipsilateral task and bilateral task canbe different. Temporal relationship task with bilateral
stimuli may be judged in higher order process than taskwith ipsilateral stimuli and/or may be cognitive level. Itis, however, unclear at which level the difference could
be generated; it could be in the level of neural signal,encoding, or higher cognitive strategy. So why do wehave parallel temporal estimation mechanisms? Is there
any validity or demand in our daily life? There is a famous trend called simultaneity constancy, which insiststhat the perceived timing of sensory events is not determined exclusively by the transduction latencies of theneural signals [9]. This flexible mechanism enables us
to compensate some temporal differences between sensory processes and accurately decide whether they originated from a single event. For example, when two distantlights flick together, we can judge true simultaneity despite differences in the neural latency. This was widelydemonstrated in vision, however, it was reported that thisconstancy was not perfect in touch. When the different
parts of the body were touched simultaneously, they weresuccessively perceived [7]. When the foot stimulus wastouched earlier than the hand, they came to be perceived
simultaneous. Apparently it means lack of compensation mechanism. However, the observed time differencesof simultaneity in their experiments cannot fully be explained by the differences of neural latencies. The compensation mechanism somehow works but is not perfect[10]. We also hypothesize there is a compensation mechanism of tactile temporal perception, which resulted inour experimental observation.
Not only somatotopic but also spatiotopic effect is wellreported in vision, so there might be a strategy difference
between tactile and vision. This discrepancy which isalso observed in constancy system may be raised fromdifference of spatio-temporal resolution of the receptors
and dynamics of the sensory organs. In touch, the temporal resolutions of mechanoreceptors are high enoughcomparing the duration of the limb movement, while thetemporal resolution of photoreceptors and the duration ofrapid eye movements are in the same time scale. Therefore the complicated information process such as coordinate transformation may be needed in touch. In addition, if we dynamically change our strategy by positionof body parts in space for better estimation of objects,
recognize of our own body may collapse.

5. CONCLUSION
Perception information processing in a short time(around 1 second) is important for humans’ sensationand movement mechanisms. Our major research question
asks how information about the time of a tactile sense isintegrated. In this paper, we tested the influence of finger position on temporal interval judgments. From ourexperiment, 1 sec temporal interval estimation is affectedby somatotopic distance, not by spatiotopic distance. The
judgment mechanism might exist in earlier stage, though
in a location where coordinate frame remapping of somatotopic to spatiotopic is not completed. Following the addition of other tasks, further careful studies investigating
the efficiency limit of somatotopic distance are required.
After though investigation of temporal sensation, we will
be able to sort out the stages of temporal judgments, andthis will lead to the reasonable design of tactile displays.

REFERENCES[1] Yarrow K, Haggard P, Heal R, Brown P, Rothwell
JC, “Illusory perceptions of space and time preserve
cross-saccadic perceptual consinuity”, Nature, Vol.
414, pp.302-305.
[2] Terao M, Watanabe J, Yagi A, Nishida S “Reduction of stimulus visibility compresses apparent temporal intervals”, Nature Neuroscience, Vol.11, pp.
541-542, 2008.
[3] Motoyoshi I, “The role of spatial interactions in perceptual synchrony”, Journal of Vision Vol. 4, pp. 352361, 2004.
[4] Kuroki S, Watanabe J, Kawakami N, Tachi S, “Howis timing information integrated-, somatotopically orspatiotopically”,WorldHaptics2009, SaltLakeCity.
[5] Sherrick CE, “Temporal ordering of events in hapticspace”, IEEE transactions on man-machine systems,
Vol.11, No.1, pp.25-28, 1970.
[6] Geffen GM, Rosa V, Luciano M, “Effect of preferredhand and sex”, Journal of Clinical And Experimental
Neuropsychology, Vol. 22, pp.219-231, 2000.
[7] Vanessa H, Laurence R H, “Simultaneity constancy:detecting events with touch and vision”, Experimental Brain Research Vol. 166, pp. 465-473, 2005.
[8] Victor JD, Conte MM, “Temporal phase discrimination depends critically on separation”, Vision Research Vol. 42, pp. 2063-2071, 2002.
[9] Kopinska A, Harris LR, Lee I, “Comparing centraland peripheral events: compensating for neural processing delays”, Journal of Vision Vol. 3, p. 751a,2003.
[10] Watanabe J, Nakatani M, Ando H, Tachi S, “Haptic localization for vibro-tactile onset and offset aredissociated”, Experimental Brain Research Vol. 193,
pp. 483-489, 2009