How is tactile timing information integrated-,somatotopically or spatiotopically?
Shinobu KUROKI, Junji WATANABE, Naoki KAWAKAMI and Susumu TACHI
Graduate School of Information Science and Technology, the University of Tokyo

ABSTRACT
We are perceiving environment tactually in various situations.
Does the consciousness of tactile timing change depending on thestimulated body parts or how we touch? In this research, we
focused on how temporal information is combined under the
influence of two distances-somatotopic and spatiotopic. Whichdistance is important for temporal processing? We provide two
experiments to investigate this question. By using two stimuli
with short intervals, we can judge simultaneity. With a longinterval, two stimuli can be separated, and the time interval can beevaluated. As a result, we found that somatotopic coordinates
clearly influence these processes. Conversely, they are not basedon spatiotopic coordinates. Furthermore it was suggested that the
judgment mechanism for about 1 second of timing information
exists in the early stage of temporal processing, where even thecoordinate frame remapping of somatotopic to spatiotopic has not
yet been completed. Depends on how we touch stimuli theperceived temporal interval would be slightly changed.
INDEX TERMS: H.1.2 [MODELS AND PRINCIPLES]:
User/Machine Systems—Human information processing; H.5.2[INFORMATION INTERFACES AND PRESENTATION]: User
Interfaces—Theory and methods
1

INTRODUCTION

Human beings touch and feel the environment daily in various
ways. We do this not only with our fingertips, but also with ourhands; sometimes we use both hands. Tactile receptors are
distributed over the whole body, so in terms of nerve circuits,
stimuli are come into contact with very different places on the
body each time. This raises the following question: Whensearching for environmental information, does our perceptionchange with how we touch? Does consciousness change
depending on the stimulated body parts, even if touching same
stimulus? This question can easily be expanded to incorporate thequestion of how the brain interprets information from a lot of
sensors.
This unresolved question carries fundamental importance fordeveloping tactile displays. Our major research question asks how
space and time correlate in tactile information processing. In
vision, spatiotopic position information is coded on the retina sothat the retinotopic and spatiotopic coordinates are the same. On
the contrary, in the tactile modality, the spatiotopic location of astimulus can only be determined after information fromproprioception and other senses is integrated. Thus, there exist
two different definitions for coordinates: somatotopic andspatiotopic. Which coordinates should be paid attention when we
design tactile displays?

7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, JAPANShinobu_Kuroki@ipc.i.u-tokyo.ac.jp

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Each coordinate would affect perception at a different layer. We
focused on temporal information processing and investigatedwhich type of distance is a clue to this processing, the somatotopic
or the spatiotopic?
There must be several layers in temporal information
processing; the definition of the coordinate that influences
processing is probably different in each layer. To investigate thisfunctioning, first, we measured simultaneity, since simultaneity
judgments are assumed to be fundamental to all other types of
temporal judgments [1]. How much of a time interval is needed
between stimulus onsets so that they do not fuse together and,rather, are identified as successive? In earlier studies, simultaneity
testing involved fingers on the ipsilateral side and the bilateralside of the body. On the basis of the results from these tests, it wasthought that somatotopic distance affected simultaneity because of
the clear difference of simultaneity durations. Here, we haveanother question: How does spatiotopic distance affect the judging
of simultaneity? We experimentally investigated the influence of
these two distances at the same time.
2

EXPERIMENT 1: SIMULTANEITY

To our knowledge, no previous studies have compared influenceson simultaneity by separating somatotopic and spatiotopic
locations using the same conditions.
In Experiment 1, we examined how the window of subjective
simultaneity was influenced by the distance of the stimulus points
with two coordinate definitions.
2.1
Apparatus
To avoid the negative effect of finger skin vibration, we used anelectric stimulus in all of our experiments. The electric stimulating
method makes a potential gradient on the axon of the peripheral
tactile receptor, and produces nerve activity directly. As shown inprevious work, an electric stimulus has same capability as amechanical stimulus when used in simultaneous testing [2].

Figure 1. Experimental Equipment

Anodic stimulation, where one electrode and surrounding
electrodes served as the anode and ground, respectively, was used
(Fig. 1). In this experiment, stimuli were presented at equal
intensities by an electric stimulator and involved quite simple
impulses, so that one stimulus would not interfere with another.
The subjects placed their finger pads on the electrode arrays. The
electrodes were 1.25 mm in diameter and arranged at intervals of
2.5 mm each. Five shots of the current pulses, which were 20

microseconds in length and a maximum 4mA current, were addedin 1 msec as one group stimuli.
2.2
Procedure
Four electrode arrays were arranged for the subject's left middle
finger (A), left index finger (B), and right index finger (C), with 2
cm distance between them. The subject could only move the rightindex finger to the right (D), so that the distance from left middle
finger to right index finger was 50 cm, as shown in Fig. 2.

Figure 3. Experimental Procedure

Figure 2. Place Conditions

Four place conditions were examined. One was referred to as
the "same site condition," in which the two patterns weregenerated on one electrode array (A) and presented to the subject's
left middle finger pad. The other three conditions used two arrays:
the "ipsilateral condition" used (A) and (B), and the two "bilateralconditions" used (A) and (C) or (A) and (D). For the ipsilateralcondition, the subject's left middle finger pad rested on one of thetwo electrode arrays, and the subject's left index finger pad restedon the other array. For the bilateral condition, the subject's left
middle finger pad rested on one, and the right index finger pad
rested on the other. The difference between the two bilateralconditions was the distance of the subject's hands. These fingers
were chosen because the index and middle fingers are innervatedby the same spinal nerve [3]. One of the two arrays was randomly
selected and presented first stimulus and the other was stimulated
after brief stimulus-onset asynchronies (SOA). In both the samesite and the separate-site conditions, the subject's task was toindicate whether the stimuli were simultaneous or successive.
Seven SOAs, 0, 15, 30, 45, 60, 90, and 135 msec, were tested.
Simultaneity tasks generally require the participant to judge
whether a pair of stimuli are presented simultaneously orsequentially. However, this judgment is easily influenced byparticipants' bias. The definition of simultaneity varies among
different individuals and erratic. What we want to investigate here
is whether the output of peripheral sensors in connection withsimultaneity judgment is influenced according to the distance
between sensors. To avoid the effect of bias, two simultaneous
stimuli were shown to the same two arrays used in the test
stimulus as in the comparison stimulus. The subject was presented
with two groups of stimulus, then answered which group tended
to be simultaneous within a two alternative forced choice. Theinterval between the two groups was 2 seconds, long enough toreduce the masking effect. Which group was presented first, thecomparison stimulus group or test stimulus group, was changed
randomly. This procedure, shown in Fig. 3. might enable thesubjects to pick up any possible difference between stimuli, andthus might show relatively low thresholds.

Here, we have another problem to be considered. Even if theequipment shows exactly simultaneous stimuli, there is apossibility that a subject feels successive sensation because of hisor her information processing. To avoid this bias also, we fixed
stimulus order. In every test stimulus in separate-site condition,left side stimulus came first. Subject was told of stimulus order as
criterion to make decision. Each condition was tested 20 times.
The SOAs and place conditions were tested in random order, andan experimental session consisted of 4 blocks of 140 trials.
Subjects were one of the authors (S.K.) and three volunteers who
were unaware of the purpose of the experiments.
The subjects were tested individually.
2.3
Results and Discussion
Figure 4 shows the rates of "Simultaneous" responses obtained.
The horizontal and vertical axes represent the SOA of the test
stimulus, and the rate of a subject's response of "Simultaneous."
Since we used a comparative method, a rate of 0.5 represented thesubject's complete confusion of simultaneous and successive
stimuli. The red circles, blue squares, green inverted triangles, andbrown crosses represent the averages of each of 20 trials in samesite (AA), ipsilateral (AB), bilateral-near (AC) and bilateral-far(AD) conditions, respectively. The thin red line, broken blue line,
dotted green line and short-dotted brown line indicate the fitted
line with cumulative normal distribution. As the SOA increased,
the rate declined to zero. First the red and blue lines, then thegreen and brown lines approach asymptotically to zero. Thesetendencies were observed for all subjects.
As a measure of the range width of subjective simultaneity,
the standard deviation of the Gaussian function is shown in Fig.5.
for each of the place conditions. Symbols indicate individual data,
and the bars indicate the average (n = 4). When the SOA was 0,
the rate of an answer of "Simultaneous" was about 0.5.
Performance in the same site condition was significantly
better than in the other three conditions for all subjects, except for
subject K.I. In addition, in the three separate-site conditions, bothbilateral performances lagged behind ipsilateral performance forall subjects. This accorded with a previous study [4]. In which theresearchers said that ipsilateral stimulation delivered stimuli to thesame cerebral hemisphere, while bilateral stimulation resulted in
the delivery of one stimulus to each hemisphere, therefore
requiring interhemispheric transmission (IHHT) before judging
simultaneity. Our experiment showed that the somatotopic
distance between two stimulus points produced a large deleterious
effect on bilateral-site performance. On the other hand, spatiotopic
distance had relatively no effect, such as an improvement, on
either the near-site condition or on the far-site condition.

Figure 5. The Range Width of Subjective Simultaneity

Comparing two bilateral conditions, some participants evenshowed shorter standard deviation in far condition than nearcondition. This was caused by participant's attention bias probably.
What accounts for the irrelevance of spatiotopic coordinates insimultaneity? As is well known, changing stimulus points'
spatiotopic coordinates, such as through crossing hands, certainly
affects the judgment of tactile temporal order [5, 6]. The tactile
stimuli are remapped into a spatiotopic frame of reference as a
final form for our perception. Perhaps the two available
somatotopic and spatiotopic frames are integrated, and bothcontribute to performance equally in higher order processes. We
hypothesize that judging simultaneity is a primitive process intactile information flow, which maybe occur before the remapping
process. There are two foundations for this hypothesis. One is that,considering the advantage of ipsilateral conditions, simultaneity
must be judged before the integration of both hemispheres'information. The other is the accuracy of the simultaneity
judgment: we can recognize successiveness with such a small
SOA difference as shown in Fig. 5.
To ensure consistency with the results of TOJ, we need toensure careful consideration; however, some other researchers
have argued that although the two tasks superficially appear to be
similar, they may actually be qualitatively different [1]. When
judging temporal order tasks, we have to change the coordinates
to answer "which side" was stimulated first. What is more, withsimultaneity judgments, no deficit in temporal processing is seen
when the hands are crossed [4]. For these reasons, we again
suppose that simultaneity is primitive enough that only
somatotopic distance has an influence on it.
3

Figure 4. Experiment 1 (Simultaneity)

EXPERIMENT 2: TIME INTERVAL

The results of Experiment 1 suggest that simultaneity is judged ateach hemisphere, and can be called a primitive judgment. In
Experiment 2, we examined two coordinates' effect on a more
complicated temporal judgment. By setting a second-scale interval,
two stimuli can be totally separated, and a time interval can beevaluated. In all cases, subjects judged the interval of two stimuli.
Generally, when checking the interval sensations, the subject's
task was to reproduce the interval by using the push button orMorse keys. The use of such a contact device results in fatigue byphysical touch or in change of condition and, what is more, lacks
accuracy. Here, we use a second (not so long) interval and try to
clarify the small difference between different placement
conditions; thus, we again chose a comparative method inExperiment 2. We assumed that tactile interval perception is not
so stable, and therefore used auditory comparison stimuli.
Two beeps were sequentially provided in the same way astactile stimuli. Actually, we set the interval difference for theauditory stimuli and subjects to compare them with tactile stimuli,
which had exactly a 1 second interval. In each trial, the auditory
stimuli always preceded tactile stimuli and presented with aninterval chosen from 5 values between 850 and 1150 ms. The
subject's task was to compare the two stimuli groups and tochoose which interval was longer than the other. The otherexperimental conditions were the same as Experiment 1. The
same four experienced psychophysical observers as participated in
the previous experiment, as well as two more subjects,
volunteered for this study.
3.1
Results and Discussion
Figure. 6. shows the rates of the "Sound was longer" responses
obtained. The horizontal and vertical axes represent the interval
difference between the auditory interval and the tactile interval,
and the rate of a subject's response of "Sound was longer."
When the auditory stimuli interval was 850 ms, the rate of ananswer of "Sound was longer" was around 0 in almost allconditions. On the other hand, when an interval of 1150 ms waspresented, the rate increased to 1. These tendencies were observed
for all subjects. To a greater or lesser extent in each subject, first
red and blue lines then green and brown lines increased
asymptotically to 1.

Figure 6. Experiment 2 (Time Interval)

The Point of Subjective Equality was calculated for all place
conditions. We consider that shift of the same site condition's PSE
from the audio stimuli interval as a corollary of modality
difference. Because each subject showed different PSE shift ofsame site condition, we normalized each subject's results by each
PSE of same site condition. The shifts of PSEs in three place
conditions from same site condition are shown in Figure. 7. The
symbols indicate individual data, and the bars indicate the average(n = 6).

Figure 7. The PSE Shift of Subjective
Temporal Interval from Same site Condition

Surprisingly, this result shows that time sensation was alteredby the finger position. Note that we used multi modal comparisonin this experiment, and exact PSE time does not have any meaning.
Thus, the focus of this study is to compare results between
conditions. Based on the same-site condition's PSE which showed
the most negative value, the ipsilateral condition showed a morepositive value, and bilateral conditions showed a much morepositive value. There are no significant differences between thetwo bilateral conditions. What is interesting here is that althoughtime sensation seems at a glance to be a higher order process, it
showed the same tendency as the simultaneous results. Is thispossible?
With the long history of research into the relation between
spatial and temporal dimensions, it has been established that theperception of brief temporal intervals is influenced by the context
in which they are presented. Using three light flashes indicating
two distances and two time intervals, it was demonstrated that
judgments of these two intervals varied according to the two
distances. Furthermore, distances varied by intervals. The former
phenomenon has been called "kappa effect," and the latter is the"tau effect" [7, 8]. These have been shown by many researchers inmany modalities, including tactile perception, and are indisputable
results. Although these appear to be contradictory arguments at a
glance, here we advocate that a simple temporal judgment is notdirectly affected by spatial information. As a matter of fact, there
is one old study about the tactile kappa effect, which carefully andcleverly tested this effect by using postural illusion [9]. As aconclusion, the researchers said that the definition of distance,
which takes account of temporal judgment, is not spatiotopic, andnot even somatotopic, but rather perceptual distance. Thisconclusion is really interesting and impressive and, from anotherstandpoint, this result shows that the kappa effect emerges in thehigh-order function of tactile information flow. When one tries to
produce the kappa effect, one needs a context of stimulus. Two
stimuli cannot make any context, more than three stimuli need to
be used. We guess that both the tau effect and the kappa effectwork in relation to context. It looks like a kind of fitting function,
which fits the middle signals to ideal temporal position into
context of stimuli group. Now, therefore, we insist that simple
temporal interval judging is made at the primitive order of tactile
information processing, not at the same order as the kappa effect.
There is a clear PSE disjunction between ipsilateral tasks andbilateral tasks in Fig. 7. Surprisingly, this result suggests that thiskind of temporal interval might be judged in each hemisphere, atan earlier stage. We consider that there are some different levels in
temporal interval tasks, and the temporal interval task used in our
experiment exists between the two kinds of tasks. The shorter(hundreds of milliseconds) task is coded at a low-level neuron
nerve circuit at a primitive stage. The longer (a few seconds) taskneeds help from memory. To investigate what happens between
these two task levels, we designed our experimental conditions insuch a way that our subjects cannot answer automatically but do
not have to think carefully. It is easy to assume that time sensationis created under the influence of many factors in many different
layers of perception processing. Considering PSE in each place
condition, we can conclude that a prototype of time sensation isbuilt in the early stage of the brain (even at the hemisphere level),
where the coordinate frame remapping of somatotopic tospatiotopic is not complete. Earlier than the kappa effect, the tau
effect, and even TOJ, the temporal interval is judged once in theprimitive stage as the same as simultaneity. Interestingly, 1 second
is enough to judge with certainty which of two stimuli camepreviously, although, a judgment of the time interval of this 1
second is made at an earlier process than a temporal orderjudgment task. Again, temporal interval judgment -a prototype oftime sensation- is affected by somatotopic distance, not by
spatiotopic distance. Since this temporal interval is judged at earlystage, it is slightly changed with how we touch the stimulus.
4

CONCLUSION

Perception information processing in a short time (around 1
second) is important for humans' sensation and movement
mechanisms. Especially in tactile sensation, it is essential when
judging the simultaneity of events and getting a movement of
encounter objects. Our major research question asks how
information about the time of a tactile sense is integrated. In thispaper, we tested the influence of finger position on simultaneity
judgments and temporal interval judgments.
When the time interval of two stimuli is short, it is thoughtthat the information is processed by a low mechanism such as a
spatio-temporal filter. Actually, there is a clear thresholddifference between ipsilateral and bilateral tasks; this fact suggeststhat the simultaneity filter exists at an early level mechanism. We
can conclude that, in case of judgement of simultaneity, the main
factor was somatotopic. On the other hand, if the stimulus
presentation time is long and becomes a stage of judgment of
temporal order, it has been established that the spatiotopic factor
is influencial. Then, consider the longer time process whichcannot process with a filter in nerve; the question then becomes
which factor is the main one: the somatotopic or the spatiotopic?
Results showed different processing times between within
hemispheres task and across hemisphere tasks, and this suggests
the priority of the somatotopic. The judgment mechanism of abouta 1 second interval temporal sensation might exist in eachhemisphere, though in a location where coordinate frame
remapping of somatotopic to spatiotopic is not completed. Thismechanism differs structurally from time processing, which
straddles the hemispheres.

These simultaneity and time interval tasks showed the sametendency, although they look totally different in quality; they wereaffected by somatotopic distance, not by spatiotopic distance. To
ensure consistency with previous works, we carefully sorted eachtask into stages of tactile information processing. Finally, ourresults suggest that when a subject received two stimuli, he or she
judged the simultaneity, and the interval immediately. Although
there must be many disturbance inputs in the higher stages, these
prototypes of time sensation are first judged at an earlier stage.
Following the addition of other tasks, further careful studies
investigating the efficiency limit of somatotopic distance are
required. After through investigation of temporal sensation, we
will be able to sort out the stages of temporal judgments, and thiswill lead to the reasonable design of tactile displays.
ACKNOWLEDGEMENTS
This research was financed by Grant-in-Aid for JSPS Fellows
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