Exp Brain Res (2007) 180:577–582
DOI 10.1007/s00221-007-0979-z

R ES EA R C H N O T E

Tactile motion aftereVects produced by appropriate presentation
for mechanoreceptors
Junji Watanabe · Seiichiro Hayashi ·Hiroyuki Kajimoto · Susumu Tachi · Shin’ya Nishida

Received: 9 March 2007 / Accepted: 5 May 2007 / Published online: 5 June 2007
© Springer-Verlag 2007

Abstract Tactile motion perception is one of the mostimportant functions for realizing a delicate appreciation of
the tactile world. To explore the neural dynamics of motion
processing in the brain, the motion adaptation phenomenon
can be a useful probe. Tactile motion aftereVects (MAE),
however, have not been reported in a reproducible fashion,and the indistinctive outcomes of the previous studies canbe ascribed to the non-optimal choice of adapting and testing stimuli. Considering the features of the stimuli used in
the studies, the stimuli activated the diVerent mechanoreceptors in the adapting and testing phase. Consequently, wetested tactile MAE using appropriate combinations ofadapting and testing stimuli. We used three pins to generatesensation of apparent motion on the Wnger cushion. They
were sequentially vibrated with the frequency of 30 Hz
both in adapting and testing phases. It is expected that thisprocedure ensured stimulation for the same mechanoreceptor(Rapid-Adapting mechanoreceptor) in both the adaptation

J. Watanabe (&)
PRESTO Japan Science and Technology Agency,
3-1, Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japane-mail: watanabe@avg.brl.ntt.co.jp
J. Watanabe · S. Nishida
Human and Information Science Laboratory,
NTT Communication Science Laboratories, NTT Corporation,
3-1, Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan
S. Hayashi · S. Tachi
Graduate School of Information Science and Technology,
The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku,
Tokyo, 113-8656, Japan
H. Kajimoto
Department of Human Communication,
The University of Electro-Communications,
1-5-1, Chofugaoka, Chofu, Tokyo, 182-8585, Japanand test phases. Using this procedure, we found robust tactile MAEs in the various tactile motions such as the shortdistance motion within the Wngertip, the long-distancemotion from the Wnger base to the Wngertip, and the circularmotion on the Wngertip. Our development of a protocol that
reliably produces tactile MAEs will provide a useful psychophysical probe into the neural mechanisms of tactile
motion processing.
Keywords Tactile motion aftereVect ·Tactile apparent movement · Vibro-tactile stimulation ·Somatosensory coding

Introduction
One way humans explore the external world is through thesense of touch. The Wngers are actively moved across thesurface of the object, and various kinds of features, such as
shape, roughness, and compliance, can be perceived. Whenthe object is slipping out of the hand, its posture can immediately be adjusted thanks to the direction-sensitive mechanism for the tactile motion (e.g., MaceWeld et al. 1996). The
information of the tactile motion between the Wngertip andthe object should play an important role in realizing thedelicate appreciation of the tactile world.
The adaptation phenomenon has been useful to investigate the mechanism of information processing in the brain
without imaging or neurophysiology. Especially, humanvisual perception of motion has been studied by the visualmotion adaptation phenomenon. After prolonged observation of a waterfall, an illusory upward motion can be seen
in a static environment. This phenomenon is referred to as
Motion AftereVect (MAE) (e.g., Wohlgemuth 1911;
Mather et al. 1998). MAE in vision is a robust and rigid

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phenomenon. However, although previous physiologicalstudies indicated the presence of the tactile motion detectors with directional selectivity (e.g., Hyvarinen and Poranen 1978; Constanzo and Gardner 1980), the tactile MAE
has not been reported in a reproducible fashion (Thalman
1922; Hollins and Favorov 1994; Lerner and Craig 2002).
Thalman (1922) reported a number of conditions thatmight be expected to generate tactile MAEs. Since then, little
research has been performed on this issue, and the conditions
that produce reliable tactile MAE are even presently underdiscussion. Hollins and Favorov (1994) reported robust tactile MAEs with the use of a rotating metal cylinder, 9 cm indiameter and 10.7 cm in length. After 120 s of stimulation,
the participants were instructed to lift their hands up, and thehand was then placed back on a stationary cylinder for 60 s.
All Wve participants reported tactile MAEs in the most partsof the trials. However, in the study by Lerner and Craig(2002), 50 participants performed the replicated experiment,
and no MAE was reported in about half of the trials. In addition, negative MAE was only about 10% of perceived MAEs,
though one would expect the aftereVects to be in the negative
direction, that is, opposite to the direction of adaptation.
Lerner and Craig also performed experiments using OPTACON, a reading aid for the blind. Pin array consisted of 12
rows and six columns covered an area of 14.1 £ 11.4 mm2.
Each row of OPTACON was sequentially activated to create
tactile motion. After adaptation for 120 s, the participants
held their Wngers in the air for 60 s. Also in this method, clear
negative MAEs were not observed for seventeen participants.
Apparently, these previous results indicated that the tactile
MAEs were not robust or reproducible phenomenon. However, considering the mechanoreceptors stimulated in theiradaptation and test phases, these indistinctive outcomes of
the tactile MAEs can be ascribed to the non-optimal choiceof adapting and testing stimuli.
Tactile mechanoreceptors can be classiWed into two categories, slow-adapting mechanoreceptor (SA) and rapidadapting mechanoreceptor (RA). When a fast rotating cylinder
was presented to the palm (Hollins and Favorov 1994; Lernerand Craig 2002), mainly RA would be stimulated, though SA
could slightly be activated. A static cylinder, however, which
was used as the test stimuli, would activate only SA. As aresult, the adapted mechanoreceptor (RA) was not stimulated
in the test phase. When the OPTACON was used as the stimulating device (Lerner and Craig 2002), the perceived motion
was generated by the successive stimulation with rows ofpins. It was reported that the OPTACON array would not
activate SA, but RA in a barbiturate-anesthetized monkey
study (Gardner and Palmer 1989). In Lerner’s experiments,
however, the participants held their Wngers in midair in thetest phase; thus no mechanoreceptor was activated after theadaptation phase. Considering these statuses of the mechanoreceptors in the adaptation and test phases, the diverging

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reports in the cylinder experiments and the absence of MAE
in the OPTACON experiment can be related to the non-optimal choice of adapting and testing stimuli. Consequently,
here, we tested tactile MAEs using appropriate combinationsof adapting and testing stimuli. We used successive stimulation of three pins to generate sensation of apparent motion on
the Wnger cushion, and they were sequentially vibrated withthe frequency of 30 Hz both in adapting and testing phases. It
is expected that this procedure ensured stimulation for thesame mechanoreceptor (RA mechanoreceptor) in both theadaptation and test phases. Using this procedure, we foundrobust tactile MAEs in the various tactile motions such as theshort-distance motion within the Wngertip, the long-distancemotion from the Wnger base to the Wngertip, and the circularmotion on the Wngertip.

Materials and methods
Apparatus
Three pins were arranged with 5 mm spatial intervals, andthey were vertically vibrated through a hole in a metalboard. The participant placed his/her right index Wnger on
the board, as illustrated in Fig. 1A, and participant’s Wnger
was kept in contact with the pins. The pins were driven by avibration generator (EMIC Inc., Kyoto, Japan, 511-A) withthe frequency of vibration 30 Hz, maximum amplitude
0.06 mm, and duration 200 ms. The wave form was modulated with sinusoidal function. The pin material was piano
wire of 1.0 mm diameter. When the pins were sequentiallydriven, the participant perceived apparent motion on the
Wnger cushion. For example, pin “c” sticks out Wrst, then“b” and Wnally “a”, as in Fig. 1B; participants thus perceive
the tactile motion to the Wngertip. The magnitude of theperceived motion was controlled in the experiments byvarying the Inter Stimulus Onset Interval (ISOI).
Participants
Six participants (one male author, three naive males, andtwo naive females), aged from 22 to 25 years, performed
the Wrst experiment, and the four participants from the six(one male author, and three naive males) participated in theother all experiments. Participants could not hear the soundof vibrations, and performed experiments with eyes open to
maintain their arousal level.
Procedure
The experimental procedures are described in Fig. 1C. In theadaptation phase, the tactile motion was presented at 100 ms
ISOI (that is, the duration of the motion was 400 ms). This

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experiments consisted of 18 trials (each ISOI included two
trials). Ten blocks were performed for each condition. Initial
30 s adaptation was made at the beginning of each block.
Participants took enough rest more than 5 min between
blocks. A total of 540 trials were performed for each participant (18 trials £ 10 blocks £ 3 conditions).

A

c
B

a

b a

ResultsaVertical motion
200 ms

5 mm

b

b
c

c

Time

ISOI

C

10 times (10 sec)

Adaptation
2 sec interval
One of 9 conditions

Test

Judgement

Upward or
Downward ?

Fig. 1 A Stimulus arrangement. B Time chart. C Experimental
procedure

ISOI was decided by the pilot experiments to generate clear
sensation of apparent motion, and this value is consistent
with the values obtained by the previous studies (Sherrick
and Rogers 1966; Kirman 1974). The tactile motion waspresented ten times at 600 ms intervals. After a 10 s adaptation, a 2 s interval was given, and test stimulus was then presented once with one of nine ISOIs (¡120, ¡60, ¡30, ¡15,
0, 15, 30, 60, and 120 ms). Positive ISOI values mean
motions to the Wngertip (described as “Upward”), and negative values mean motions to the base of the Wnger (describedas “Downward”). Participants answered “Upward” or“Downward” with two alternative forced choices. This
experiment was performed under three experimental conditions (Upward adaptation, Downward adaptation, and No
adaptation). In the No adaptation condition, the participant
put his/her Wnger on the stationary contactors during thesame period as the adaptation duration (10 s). The three conditions were carried out in separate blocks. One block of the

Figure 2A shows the individual data of “Upward” response
rates. The horizontal and vertical axes represent the ISOIs of
the pins, and the rate of a participant’s response of “Upward”,
respectively. The blue triangles, green crosses and red circles
represent the averages of each of 20 trials in No, Upward andDownward adaptation conditions, respectively. A thin blueline, a dotted green line and a broken red line indicate theWtted line with cumulative normal distribution. In the data of
No adaptation (blue triangles), when the ISOI was 0, the rateof “Upward” answers was about 0.5, and when the ISOI of
120 or ¡120 ms was presented, all participants in all trials
answered “Upward” or “Downward”. This means all participants surely perceived and judged the motion direction. If
upward motion was presented in the adaptation phase (green
crosses), less “Upward” responses were obtained in the wide
range of ISOIs. On the other hand, in the downward condition(red circles), the rate of “Upward” responses increased. Thesetendencies were observed for all participants, except in the
downward adaptation condition of the participant AK.
The values of Point of Subjective Equality are calculated
for all adaptation conditions. The shifts of the PSEs in the
downward (upward) adaptation condition from the no adaptation condition are shown in Fig. 2B. The red (green) bars
represent PSEs of individual data and their average in the
downward (upward) condition. The error bars represent standard errors. The averaged PSEs are ¡17.3 ms in the downward adaptation condition, and +17.3 ms in the upward
adaptation condition. Considering relatively small standard
errors (5.0 ms in the downward condition, and 2.9 ms in the
upward condition), we can conclude that the tactile motion
adaptation inXuenced the perceived direction of the motion
presented in the test phase in such a way that the number of
responses corresponding to the opposite direction to theadapted motion direction systematically increased. Thisimplies that the tactile MAEs were reliably observed.
Long-distance motion and circular motion
Kirman (1974, 1975, 1983) had reported various kinds oftactile apparent motions including those produced bymoving lines, rotating lines, and expanding boundaries. In

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Fig. 2 Results of the experiment for vertical tactile motion.
Horizontal and vertical axes
represent ISOI (ms) and the rateof participant’s “Upward” response. A Individual data. B
Shift of PSE (error bars represent standard errors). C Result of
the experiment for long-distancemotion. D Result of the experiment for circular motion

Exp Brain Res (2007) 180:577–582

Downward Adaptation
No Adaptation
Upward Adaptation

A
Rate of "Upward"

1.00
0.75
0.50
0.25

YO

SH

0.00

KS

Rate of "Upward"

1.00
0.75
0.50
0.25

SK

0.00
-120

-60 -30-15 0 15 30 60

120

MN
-120

-60 -30-15 0 15 30 60

AK

120 -120-60 -30-15 0 15 30 60

120Inter Stimulus Onset Interval (ms)
Downward Adaptation
Upward Adaptation

B

SH
SK
YO
MN
KS
AK
Average-40 -30 -20 -10

0

10

20

30

40

Point of Subjective Equality (ms)C

Downward Adaptation
Upward Adaptation

1.00
25 mm

Rate of "Upward"

1.00

0.75
5 mm

0.25
0.00

0.50
0.25

Average-60

order to investigate the generality of the tactile MAEs, ournext investigation looked whether it could be induced bylong-distance and circular motions.
The same experimental procedures as in the case of thevertical motion without ISOI variations were used. In thecase of long-distance motion, the pins were verticallyarranged with 25 mm spatial intervals, as illustrated on theleft in Fig. 2C. Although the motion sensations were
weaker than those in the Wrst experiment with 5 mm spatial
interval, the participants perceived the sequential stimuli
more as motion than as spatially separated three stimuli.
Indeed, Kirman (1974) reported that, if stimuli were arranged

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Rate of "Clockwise"

0.75

0.50
25 mm

Anti-clockwise Adaptation
Clockwise Adaptation

D

0

60

0.00

-90

Average

-30

30

90

within 2 in. distance, tactile apparent motion could be perceived and the temporal relationship played major role.
The presented ISOIs were varied in ¡60, 0, and +60. The
results are shown on the right in Fig. 2C. The vertical andhorizontal axes represent the rate of the response“Upward” and the ISOIs, respectively. We carried out
upward and downward adaptation conditions in separate
blocks. In general, the participants gave more “Upward”
answers after the downward adaptation, and vise versa.
Clear divergence between the two adaptation conditions
indicates the occurrence of the tactile MAE by long-distance motion.

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In the case of circular motion, the pins were arranged ina square with 5 mm spatial intervals as illustrated on theleft in Fig. 2D, and they were sequentially vibrated clockwise or anti-clockwise. All participants perceived circularmotion with smooth direction changes, rather than squaremotion with sharp changes. This indicates that the tactile
motion system was functioning here in such a way to interpolate the trajectory of the circular motion. The presented
ISOIs were varied in ¡90, ¡30, +30, and +90. The results
are shown on the right in Fig. 2D. Positive values in thehorizontal axis mean clockwise motion, and vice versa.
Basically, when the ISOI increased, more “Clockwise”
responses were obtained. More “Clockwise” responses
were obtained in the anti-clockwise adaptation condition
than in the clockwise adaptation condition, suggesting thatthe tactile MAE can also be observed by circular motion.
These results indicate that the tactile MAEs can berobustly observed for the tactile motions of various types.

Discussion
We demonstrated that the tactile MAEs can be observed
when the sequential pin stimulation was used for both adaptation and test stimuli. How can we explain this diVerence
between earlier studies that did not report robust tactile
MAEs, and our experiments? One reason may be that vibration generally makes our stimulus more eVective in activating the relevant receptors and neurons (cf. Vierck Jr andJones 1970), but we further consider it critical that we carefully chose stimulus parameters considering the selectivity ofthe mechanoreceptors to vibration frequency. Tactile
mechanoreceptors, which would be stimulated in the rotating
cylinder, the OPTACON and the pin stimuli (our) experiments, are shown in Table 1. The upper rows represent thestatuses in the adaptation phase and the lower rows represent
those in the test phase. This summary table suggests that, inthe previous studies, the diVerent mechanoreceptors might be
activated in their adaptation and test phases. On the other
hand, it is likely that the same mechanoreceptor (RA) was
stimulated in both the adaptation and test phases in our
experiment, and this may be why we could observe strong
tactile MAEs. In addition, it was observed in our pilot study
that when the participant lifted his/her Wnger oV the stimulator
Table 1 Tactile mechanoreceptors that would be stimulated in rotating cylinder, OPTACON, and our experiments (Pins)
CylinderSA in adapt

V

RA in adapt

VVV

SA in test

VVV

RA in test

OPTACON

Pins

VVV

VVV

and held it in the air after the adaptation of the successive pin
stimulation, as in the Lerner and Craig study, the tactile
MAEs were not perceived. This observation also supports
our claim that the activation of the same type of mechanoreceptors in both adapting and testing phases is essential in production of robust aftereVects. Examining the temporalfrequency tuning of the tactile MAEs would be a promising
future experiment for the purpose of estimating the quantitative characteristics of the underlying neural mechanism, andtesting the validity of our hypothesis.
The neural mechanisms for tactile motion processing are
not as well understood as those for visual motion. The primary somatosensory cortex S-1 contains four areas, Brodmann’s area 3a, 3b, 1, and 2. Area 3b and 1 are
predominantly engaged in the analysis of cutaneous information from the mechanoreceptors in the skin (Dykes et al.
1980; Phillips et al. 1988). Most neurons in both areas
respond to moving stimuli, and two types of neurons havebeen identiWed (Hyvarinen and Poranen 1978; Constanzo andGardner 1980; Gardner and Constanzo 1980; Essick andWhitsel 1985; Warren et al. 1986; Ruiz et al. 1995): themotion sensitive, responding to motion in all directions,shown to a higher extent to be located in area 3b (Warren
et al. 1986) and the direction sensitive, showing a clear preference for movement in one direction and to be locatedmostly in area 1. Considering the occurrence of the tactile
MAEs for long-distance motion, which is beyond limited
size of the receptive Weld in area 3b, it can be speculated thatarea 1 or/and higher areas can be involved with this phenomenon. Moreover, recent functional imaging studies showedthat the human putative MT/V5, which has been considered
to be responsible for the “visual” motion perception and
MAE, is activated by the tactile motion stimuli (e.g., Hagen
et al. 2002). An intriguing open question is how MT/V5 contributes to the tactile motion processing and its aftereVect.
Although the somatosensory information processing andtheir corresponding areas in the brain are intensively examined in non-human primates (e.g., Hyvarinen and Poranen
1978; Iwamura et al. 1994) and recent human (Bodegard
et al. 2000) studies, few physiological studies have been
performed on how the tactile motion can be processed inconjunction with kinesthetic information such as the Wnger
positions and arm posture. Our development of a protocolthat reliably produces the tactile MAEs will provide a useful psychophysical probe into the neural mechanisms ofsuch tactile motion processing.

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