Advanced Robotics

ISSN: 0169-1864 (Print) 1568-5535 (Online) Journal homepage: http://www.tandfonline.com/loi/tadr20

Amplifying shear deformation of finger pad
increases tracing distances
Kenya Matsui, Shogo Okamoto & Yoji Yamada
To cite this article: Kenya Matsui, Shogo Okamoto & Yoji Yamada (2014) Amplifying shear
deformation of finger pad increases tracing distances, Advanced Robotics, 28:13, 883-893, DOI:
10.1080/01691864.2014.894939
To link to this article: http://dx.doi.org/10.1080/01691864.2014.894939

Published online: 06 Jun 2014.

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Date: 11 July 2016, At: 09:10

Advanced Robotics, 2014
Vol. 28, No. 13, 883–893, http://dx.doi.org/10.1080/01691864.2014.894939

FULL PAPER
Amplifying shear deformation of finger pad increases tracing distances
Kenya Matsui, Shogo Okamoto∗ and Yoji Yamada
Department of Mechanical Science and Engineering, Nagoya University, Nagoya, Japan(Received 21 February 2013; revised 7 August 2013 and 13 December 2013; accepted 17 December 2013)

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Human sensory inputs and motor outputs mutually affect one another. We pursue the idea that a tactile interface can
influence human motor outputs by intervening in sensory–motor relationships. This study focuses on the shear deformationof a finger pad while a person traces a line or circle. During these tracing movements, the finger pads were deformed usinga tactile interface. The tracing distances increased when the finger pad deformations were amplified by the tactile interface,which indicates that the intervention in the haptic sensorimotor loop affected the tracing movements. Elucidation of suchinteraction between the tracing movements and the shear deformations of finger pads enhances the understanding of
human-assistive haptic techniques.
Keywords: skin stretch; sensory–motor relationships; friction sensation

1. Introduction
Human sensory inputs and kinesthetic outputs mutuallyaffect one another.[1] In this study, we pursue the ideathat a tactile interface can influence human kinetic outputs
by intervening in sensory–motor relationships. Attempts todemonstrate this phenomenon have been made in the fieldof haptics with regard to prehension adjustment,[2,3] theeffects of sole sensation on body posture,[4–6] and thecontrol of walking cycles.[7] These studies showed that
effective stimulation of the human body through tactileinterfaces almost unconsciously affects its motor outputs.
Like these studies, we investigate how stimulation induces
motor outputs rather than whether such inductions are unconscious or not.
We investigate the sensory–motor relationships between
tracing tasks and finger pad deformations. A tracing movement is one in which a human traces characters or figures ona sheet of paper using a finger pad. The tracing movements
are intimately related to the cutaneous sensation of the finger
pad,[8] and the cutaneous deformations have been observed
and mathematically modeled.[9–12] Although the effects ofsuch deformations on perception have been studied,[13–15]
their effects on hand motions have hardly been reported thus
far. The objectives of this study are to intervene in the relationships between the shear deformation of the finger pad
and the tracing movements by using a tactile interface andto examine how the deformation influences the movements.
We use a tactile interface to apply a shear deformation to thefinger pad when a human traces a line or circle on a sheetof paper.

∗ Corresponding author. Email: okamoto-shogo@nagoya-u.jp© 2014 Taylor & Francis and The Robotics Society of Japan

We place emphasis on the sensory–motor relationshipsof tracing motions; however, tracing is intriguing from theperspective of interface design because it is linked withdragging with pointing devices. Tactile stimuli to the hand
are known to influence hand motions and the performanceof tasks that involve pointing interfaces. Campbell et al.
studied how vibrotactile stimulation of the finger through a
pointing stick improved performance on and reduced error
ratios of a steering task.[16] Akamatsu et al. found thatthe use of a vibrotactile-feedback mouse improved performance on a target-selection task.[17] Vibrotactile stimulation of the finger pad has been shown to improve tapping
performance on touch panels, which have recently becomecommon as the input interfaces of information terminals.[18,19] However, there have been few attempts to identify factors influencing tracing or dragging movements,
which are principal input motions for touch panels. To fillthis gap, the present article reports a fundamental investigation of the possibility that the application of shear deformation to the finger pad influences tracing movements.
In the authors’ earlier presentation, finger-tracing
movements were influenced using a tactile interface that
acted on skin stretch during tracing tasks.[20] Experiments
1 and 2 in the present article were grounded in the previous
report but with the precisely controlled conditions of force
and skin stretch measurements. This measurement allowed
us to conduct an experiment in which the magnitude of shear
deformations was controlled (Exp. 3). Such an experimentpotentially supports the sensory-motor loop of tracing movements and finger pad stretches by showing the correlated

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relationships between the inputs and outputs of the sensorymotor loop. The experimental procedures described in thepresent article were approved by the ethical committee of
the School of Engineering, Nagoya University.

2. Experimental system
Figures 1 and 2 show the tactile interface developed toapply the shear deformation to the finger pad. The interface
used an acrylic plate placed beneath and affixed to thefinger pad with double-sided sticky tape. The plate was activated by two independently controlled DC motors (RE-10,
maxon motor, Switzerland, maximum torque 3.04 mN m)
to which strings were fastened. The interface applied theshear deformations along the X-axis. An antiskid sheath
restrained the fingertip in order to ensure that the acrylic
plate effectively deformed the finger pad. The shear deformations applied to the finger pad were measured using two
encoders (GP 10 K, maxon motor, Switzerland, 1024 ppr)
with a resolution of 2.9×10−3 mm by way of the quadruple
reduction gear and a pulley with a diameter of 3.8 mm. Theinterface hardly moved the finger, because the sum of theinternal forces applied by the interface to the finger was
zero. We verified that the drive of the interface shifted
the finger by only 0.3 mm when the finger was still. Thisvalue was negligible compared to that of the spontaneous
finger movement induced by the manipulation of the shear
deformation, which was approximately 8 mm on average(see Section 3.2). The participants wore the interface andtraced a line or circle on a flat panel. Furthermore, as shownin Figure 3, two load cells (Model 1004, Tedea Huntleigh,Canada) measured the finger pressing force along the Z-axis
with a force measurement resolution of 1.0 × 10−4 N.
Under natural tracing motions, the finger pad experiences
full slippage across the floor. The above tactile interface
did not provoke such slippage. One option to achieve fullslippage is to activate the floor itself, as was done in the
study of Terada et al. [8]. However, such a mechanism is
not appropriate for our purposes because it exerts forces tothe hands and directly influences their tracing motions.
Acamera (320×240 pixels, 30 fps, PlayStation Eye, Sony
Computer Entertainment Inc., Japan) measured the positionof a marker attached to the interface. The resolution of
measurement was 1.00 mm. We calculated the velocities ofthe fingers using a Savitzky–Golay filter of length five.[21]The computer controlled the motor torques of the interface
with a control cycle of 0.3 ms on the basis of the equations
presented in Section 3.1.3.

3. Experiment 1: linear trace
We show the effects of shear deformation applied to thefinger pad on the displacement and speed of the tracing
movements while the finger traces a 150 mm line drawn on
a sheet of paper.

Figure 1. Interface to apply shear deformation to the finger pad.

Figure 2. Photograph of the interface to apply shear deformation
to the finger pad.

Figure 3. Experimental setup.

3.1. Method
3.1.1. Participants
The participants (P1–P10) were 10 right-handed students
aged 20–24 from Nagoya University. All of them wereignorant of the objective of the experiment and none wereresearchers specializing in human interfaces. Because wewere interested in the haptic sensory–motor relationships,
we instructed participants to close their eyes during eachtrial so that they would not see their fingers, to reduce theAdvanced Robotics

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effects of visual feedback. Additionally, they were required
to listen to pink noise through headphones, intended toaurally block the sound of the finger sliding on the paper.

3.1.2. Task
The participants wore the tactile interface on the right index
finger. They traced the line with the index finger withoutvisual cues while following a metronome played through theheadphones with the pink noise, as accurately as possible.
It should be noted that our interests are in the comparison between control and test stimuli rather than in theaccuracy of tracing tasks. In fact, most participants could
not correctly follow the line length (see the Results section). The metronome was introduced to approximately control the tracing speeds of participants. This was because inpilot tests, we found that some participants traced a line at
irregularly slow speed without any rhythmic cues. A single
line trace was performed with the beat of a metronome set
at 0.75 Hz, and then a reciprocating motion was performed
with the second beat. This reciprocation was repeated threetimes in each trial. Before the experiment, participants practiced following the metronome for a few minutes until theyfelt familiar with the task. At the beginning of each trial,
they visually confirmed the position and length of the line
and then placed the finger on the far right of the line, which
had been designated as the starting position. During the trial,
they maintained the finger’s posture so that it would follow
the Y-axis.

3.1.3. Stimuli
We randomly subjected the finger pads of the participants
to three shear deformation conditions. We did not give theparticipants any information about these stimuli. Ten trials
were performed for each stimulus. The conditions were as
follows:• amplify condition – the finger pad shear deformations were amplified along the X-axis (Figure 4(i));• reverse condition – the shear deformations werereversed along the X-axis (Figure 4(ii)); and• control condition – the shear deformations were notaltered.
We were essentially interested in the comparison between
the amplify and control conditions. However, we prepared areverse condition, which is implausible under natural tracing movements, to investigate the effects of the direction of
the applied shear deformation on tracing movements.

3.1.3.1. Amplify condition. When humans trace a line ona paper, the finger pad commonly undergoes shear deformations in the direction opposite to the velocity of finger.

Figure 4. Shear deformation of finger pad during linear tracingtask.

Figure 5. Shear force applied to the finger pad under amplify
condition.

Under amplify condition, the motor torque increased toamplify the shear deformation of the finger pad. Because
we approximated that the kinetic friction was constant in therelative motion, the force exerted to the plate by the motors
was either the maximum or zero. Only when the speed was
relatively small, the motor torques changed linearly in orderto ensure the smoothness of the changes in shear force.
When a finger was moved along the X-axis, as shown in
Figure 4, the tensions on the strings exerted by the two
motors were controlled in a feedforward manner such thatthe net shear force f s (vx ) applied to the finger pad becamef s (vx )⎧⎨0 if |vx | < 100 mm/s
= α(|vx | − 100) · sign(−vx ) if 100 mm/s ≤ |vx | < 250 mm/s⎩
f max · sign(−vx ) if 250 mm/s ≤ |vx |(1)

where α and f max were 1.33 × 10−2 N s/mm and 2.0 N,
respectively. sign(vx ) returns the sign of vx . We chose α andf max so that the deformation of the finger pad would feel natural. Note that f s reached − f max at vx = 250 mm/s. In orderto prevent frequent switch of the force directions applied tothe plate, we set an insensitive zone of ±100 mm/s. Figure 5
shows the profile of the applied shear force depending on
the finger velocity. The friction of the gear of each motor
was specified as 2.0 N and compensated for.

3.1.3.2. Reverse condition. This condition transposessign(−vx ) of (1) to sign(vx ). Under this condition, the finger

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Table 1. Averages of shear deformations of finger pad for eachcondition in Exp. 1.

Figure 6. Example of tracing movements of the finger.

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pad is deformed in the direction of finger movement, whichis inconsistent with our daily experiences.

3.1.3.3. Control condition. Under this condition, thetactile interface did not apply any shear deformation to
the finger pad. The motor outputs were controlled so thatthe normal force exerted on the finger pad along the Z-axis
became equal to that under both the amplify and reverse
conditions. The outputs from motors 0 and 1 were given by
f 0,1 (vx )⎧0 if | vx | < 100 mm/s
⎪
⎪
⎪
⎨ 1
∓ α(|vx | − 100) if 100 mm/s ≤ |vx | ≤ 250 mm/s
=
2
⎪
⎪
⎪
⎩∓1 f
max if 250 mm/s ≤ |vx |.
2(2)
3.2. Results
3.2.1. Tracing distances
We compared the three conditions in terms of the tracing
distances along the X-axis (Figure 6). Six strokes were observed in each trial. We used four strokes, excluding the firstand last, to statistically analyze tracing distances. Figure 7
shows the averages and standard deviations of the individual
tracing distances. P3 was removed from any statistics due
to his extraordinarily strong finger forces along the Z-axis(see also the paragraph on fingers’ pressing force). For afew trials, the interface did not function correctly due tothe friction of strings on pulleys. We excluded these trials
from the statistics. We applied a Steel–Dwass test on allparticipants’data to compare three conditions. The numbersof samples used for calculation were 360 (9 participants ×
10 trials × 4 strokes/trial) for the control condition, and 336
and 352 for the amplify and reverse conditions, respectively.
The comparison revealed that the amplify condition produced larger tracing distances than did the control condition(q(3, ∞) = 4.96, p = 2.1 × 10−6 ), with an average difference of 8.5 mm. The reverse condition also produced larger
tracing distances than did the control condition (q(3, ∞) =3.51, p = 1.3 × 10−3 ), with an average difference of
5.6 mm. These trends were observed for the eight of nine
participants, the exception being P6. In the preliminary

Participants

Amplify (mm)Reverse (mm)Control (mm)

P1
P2
P4
P5
P6
P7
P8
P9
P10
Mean ± S.D.

1.7
2.7
4.3
4.3
2.7
3.1
3.8
3.1
4.1
3.3 ± 0.9

1.2
2.5
4.4
4.9
2.9
3.4
3.5
3.0
4.0
3.3 ± 1.1

0.2
0.2
0.3
0.2
0.3
0.2
0.2
0.3
0.2
0.2 ± 0.1

experiment with different α and f max values being 5.75 ×
10−3 N s/mm and 1.07 N, we observed the same trends asin this study.[20]3.2.2. Peak tracing speeds
We also compared the three conditions in terms of the peaktracing speeds along the X-axis. We used the six peaks observed in each trial in the statistical analysis. Figure 8 shows
the averages and standard deviations of the peak tracing
speeds for each participant. For the control condition, 540
samples were analyzed (9 participants×10 trials×6 peaks/
trial). We used 498 and 512 samples for the amplify andreverse conditions, respectively. The peak tracing speeds
under the amplify condition did not significantly differ fromthose under the control condition (q(3, ∞) = 1.61, p =0.24). In contrast, the reverse condition produced larger
peak speeds than did the control condition (q(3, ∞) =3.72, p = 0.58 × 10−4 ). The difference in average speeds
between these two conditions was 9.5 mm/s.
3.2.3. Finger pressing force
Figure 9 shows the individual comparisons of the finger
pressing forces ( f z ) under each conditions. We calculated
the average of f z while the finger slid on the paper. Standard
deviations were among trials. As described already, one
participant, P3, showed irregularly large f z values and was
excluded from the analysis. Apparently, there were no common trends among the participants. The f z values of both theamplify and reverse conditions did not significantly differ
from those under the control condition. Stimuli conditions
exerted a nonsignificant influence on pressing force.
3.2.4. Finger pad deformation
We averaged the maximum deformations of finger pads
during a single trace. These values are the displacementsof the plate from the natural position. Table 1 shows thatthe average finger pad shear deformations along the X-axis

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Figure 7. Tracing distances in Exp. 1 (linear trace). ** and *** indicate significance level of 0.01 and 0.001, respectively.

Figure 8. Tracing speeds in Exp. 1 (linear trace). ** indicates significance level of 0.01. N.S. means not significant.

Figure 9. Individual finger pressure forces ( f z ) by type of stimulus in Exp. 1.

under the amplify, reverse, and control conditions were
3.3 ± 0.9 mm, 3.3 ± 1.1 mm, and 0.2 ± 0.1 mm, respectively. The variations in shear deformation were attributed
to the differences in skin stiffness and size among the individual fingers.

4. Experiment 2: circular trace
In Experiment 1, both amplifying and reversing shear deformations of the finger pad increased the tracing distances.
Nevertheless, the applied shear deformation and the finger
motion were along the same X-axis. Our question now is

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Figure 10. X -axis tracing distances in Exp. 2 (circular trace).

Figure 11. Y -axis tracing distances in Exp. 2 (circular trace).

whether the applied shear deformation influences finger
motion in a perpendicular direction or whether the X-axial
deformation selectively affects the X-axial components offinger motions. In order to investigate this question, we
asked the participants in Experiment 2 to trace a circle
that involved both X and Y-axis motions, while the finger
pad deformations were altered only along the X-axis. We
show that the applied shear deformation does not affectthe perpendicular finger motion when tracing a circle inExperiment 2.

4.1. Method
4.1.1. Task
Participants who were aurally blocked and blindfolded
traced a circle of 150 mm in diameter in accordance with thebeat of a metronome set at 0.5 Hz so as to trace the circle
once every 2 s. They traced the circle three times in eachtrial. At the beginning of each trial, they visually confirmed
the circle and then placed a finger at the bottom of the circle,
which had been designated as the starting position.

4.1.2. Participants and stimuli
The participants (P1–P10) were the same 10 students as inExperiment 1. The tactile interface subjected their finger

pads to the same three shear deformation conditions described in Section 3.1.3. Ten trials were performed for eachstimulus.
4.2. Results
The means and standard deviations of the finger pad shear
deformations along the X-axis under the amplify, reverse,and control conditions were 3.2 ± 0.9 mm, 3.0 ± 0.9 mm,
and 0.3 ± 0.2 mm, respectively.
4.2.1. X-axial tracing distance
We compared the three conditions in terms of the tracing
distances along the X-axis. Figure 10 shows the means andstandard deviations. We applied a Steel–Dwass test on thesevalues. We used 344, 360, and 332 samples in the calculations for the amplify, control, and reverse conditions,
respectively. The amplify condition significantly differedfrom the control condition (q(3, ∞) = 3.99, p = 1.9 ×
10−4 ). On average, the distances along the X-axis under theamplify condition were larger than those under the control
condition by 5.6 mm. As in the linear trace in Experiment
1, for most participants, the values under the amplify condition were larger than those in the other conditions. No
difference was seen between control and reverse conditions(q(3, ∞) = 0.63, p = 0.80).

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Figure 12. Tracing distances under the amplify, small-amplify, and control conditions in Exp. 3.

4.2.2. Y-axial tracing distance
Figure 11 shows the means and standard deviations of tracing distances along the Y-axis. Trancing distances did not
significantly differ across the three stimuli conditions(amplify-control: q(3, ∞) = 1.78, p = 0.18, reversecontrol: q(3, ∞) = 1.22, p = 0.44).

4.2.3. X and Y-axial peak tracing speed
The X and Y-axial peak tracing speeds did not significantly
differ across the three conditions. The means and standard
deviations of X-axial peak speeds were 262.0±48.3, 256.4±
50.6, and 265.5 ± 54.6 mm/s for the amplify, control andreverse conditions, respectively. Values tended to be higher
for the amplify and reverse conditions than the control condition. However, no significant differences were observed
between any conditions (amplify-control: q(3, ∞) = 1.65,
p = 0.22, reverse-control: q(3, ∞) = 1.91, p = 0.14).
The Y-axial speed values were even closer across the three
conditions: 212.6±48.3, 209.7±46.0, and 213.4±46.7 mm/s
for the amplify, control, and reverse conditions, respectively.
To summarize the results, the amplify condition influenced only X-axial displacements and did not affect Y-axial
movements. This condition also affected X-axial displacements in Experiment 1. These results suggest that X-axial
shear deformations are linked with only X-axial components
of movements. The impact of the reverse condition was mitigated in Experiment 2. Because the participants underwent
irregular finger deformations in the reverse condition thatthey never experience in daily lives, the impact of suchdeformations might be unstable.

5. Experiment 3: the effect of degree of shear
deformation
In Experiments 1 and 2, we confirmed that amplifying theshear deformation of the finger pad significantly increased
the tracing distances along the direction of applied deformation. In Experiment 3, in order to seek the relationshipbetween the sensory and kinetic information of the feedback loop, we investigated how the degree of deformationinfluences the degree of increase in the tracing movement.
In this experiment, we applied large or small deformations
to the finger pad during linear tracing tasks.

5.1. Method
5.1.1. Stimuli
We randomly subjected the finger pads of the participants
to three shear deformation conditions. Ten trials were performed for each stimulus. The conditions were as follows:• amplify condition – the finger pad shear deformations
were amplified along the X-axis with an f max of 2.0 N;• small amplify condition – the shear deformations wereslightly amplified along the X-axis with an f max of
1.12 N; this value would result in roughly half the skinstretch as under the amplify condition; and• control condition – the shear deformations were notaltered.
The amplify and control conditions were the same as thosein Experiments 1 and 2.

5.1.2. Participants and task
The participants (P2–P10) were the same nine students from
Nagoya University. They performed the same task as wasgiven in Experiment 1.

5.2. Results
Table 2 shows the means and standard deviations of the finger pad shear deformations along the X-axis under the amplify and small amplify conditions: 3.3 ± 0.4 mm and 2.0 ±0.5 mm, respectively. We compared tracing distances along
the X-axis among the three conditions. Figure 12 shows
the mean and standard deviation of the tracing distancesfor each participant. We used 320 samples (8 participants ×890

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Table 2. Shear deformation of finger pad in Exp. 3.
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P2
P4
P5
P6
P7
P8
P9
P10
Mean ± S.D.

3.1
3.6
3.9
2.8
3.0
3.1
3.9
3.1
3.3 ± 0.4

2.5
2.5
1.0
2.3
2.5
2.0
1.7
1.6
2.0 ± 0.5

0.3
0.2
0.2
0.3
0.3
0.2
0.2
0.3
0.3 ± 0.1

10 trials × 4 strokes/trial) from the control condition and
304 each for the amplify and small-amplify conditions.
The Steel–Dwass test showed that tracing distances under
the amplify condition were significantly larger than thoseunder the control condition (q(3, ∞) = 5.08, p = 1.2 ×
10−6 ), with the average difference being 8.3 mm. The smallamplify condition produced larger tracing distances thancontrol condition (q(3, inf ) = 3.95, p = 2.3 × 10−4 )with the average difference being 6.1 mm. Tracing distances
did not significantly differ between the amplify and smallamplify conditions. The tracing distances were largest under
the amplify condition, followed by the small-amplify condition, and smallest under the control condition. However,owing to the lack of statistical evidence, no clear relationships such as proportional one were found between the
degree of amplified shear deformations and the increases
in tracing distances.

6. Discussion: sensory–motor relationships underlying
experiment6.1. Prospective feedback model of sensory–motor
relationships in tracing movements
Figure 13 shows the conjectured sensory–motor relationship between the tracing movements and the shear deformation of the finger pad. A human inputs a target planar
trajectory xd (t) into an internal model to output the tracing
movement through his/her musculoskeletal system. We describe this operation as ẍ(t) = I [xd (t), μk (t)]. As shownin Figure 14(i), by the Amontons–Coulomb law, the shear
force on the finger pad, fs (t) is given byfs (t) = −sign[ ẋ(t)]μk f n(3)

where μk is the coefficient of kinetic friction between thefinger and the floor, while f n is the finger pressing force on
the plane. A human does not change f n depending on the applied deformation of the finger pad as shown in Experiment
1. The shear deformation, ds (t), is described byds (t) = S[ ẋ(t)] =fs (t)
sign[ ẋ(t)]μk f n
=−
kf
kf

(4)

where k f is the stiffness of a human finger pad and approximated to be isotropic, although it is known as an increasing
function of applied pressure.[22,23] Using ds (t), the humanestimates the friction coefficient of the surface [14,24] andthe force applied to the finger pads.[25–27] For tracing
movements intimately related to kinetic friction, an increasein shear deformation affects the kinetic friction perceived
by humans.[14] We express this estimation process asµˆk (t) = E[ds (t)](5)

where µˆk (t) is the estimated coefficient of kinetic friction
between the finger and the floor. We speculate that humans
feed this perceived friction back to their internal models, andthat ẍ(t) is then determined to realize the desired trajectoryof the tracing.

6.2. Effect of additional shear deformation of finger pad
on tracing movements
As shown in Figure 13, we applied the shear deformation
ds (t) to the finger pad of the participant using the tactile interface. Then, the deformation increased to ds (t) +ds (t), as shown in Figure 14(ii). In this case, the perceived
coefficient of friction increases toµˆk (t) + µˆk (t) = E[ds (t) + ds (t)].

(6)

As described above, the participants feed µˆk (t) + µˆk (t)back to their internal models that determine their generative forces. Due to this underlying mechanism, the tracing
movements are influenced by the tactile interface. Experiments 1, 2, and 3 showed that ds (t) influenced x(t) in
the linear and circular tracing tasks, respectively. However,Experiment 3 did not fully support such feedback systembecause the incremental changes in ds (t) and x(t) were
not statistically accompanied with each other.
The above explanation is still just one possibility withwhich the experimental results are not inconsistent. This
article does not completely vindicate the underlying mechanism and does not rule out other possibilities. The explanation does not cover some issues.
First, the participants could have known the increases intheir hand motions through the proprioception. Such perceived increases potentially functioned to curtail the hand
movements because participants were instructed to sustain
the tracing distances. Nonetheless, the distances were eventually facilitated by the dominant effects of the increased
skin stretches under the present series of experiments. Suchconflicts or integration of the effects of cutaneous and proprioceptive sensations have attracted the interest of someresearchers.[8,28–30]
Second, Figure 13 suggests that human tracing velocity
and acceleration are also potentially influenced byds (t). If the effects on the hand velocities had been observed, our hypothetical model would have been further
consolidated. However, these effects were not statisticallyDownloaded by [Nagoya University] at 09:10 11 July 2016

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Figure 13. Prospective relationships between shear deformation of the finger pad and tracing movements. Solid lines represent a closed
loop with no presentation of shear deformation applied. Dashed lines represent a closed loop with additional shear deformation ds (t)applied using the tactile interface.

Figure 14. Shear deformation of finger pad by using tactile interface and perceived friction coefficient.

validated, mainly because of the fluctuations in hand speeds;in contrast, differences in less-fluctuated hand displacements could be validated statistically.
Furthermore, the mechanism mentioned above involves
a hypothesis that humans feed the perceptually increased
friction coefficient back to their internal models. In order tovalidate this hypothesis, we need to observe status variablesin the internal model of generative force, which is generally
very difficult to achieve.
Finally, the effects of the reverse condition are not explained by this mechanism. Provided that reverse stimuli
should be perceptually different from control stimuli,[13,31,32] we feel inclined to consider a model that explicitly involves the direction of shear deformations. However,
the effects of the reverse condition on tracing movements
are not conclusive. In Experiment 1, the reverse condition increased tracing distances and speeds, whereas its
impact was reduced in Experiment 2. Hence, it is somewhat
risky to make conclusions about the influences of reverse
conditions.

7. Conclusion
In the present study, we investigated the effects of finger pad deformations on tracing movements. These finger
pad deformations and tracing movements form a sensory–
motor relationship in which sensory information andkinetic outputs are intimately connected to one another. We
intervened in this relationship using a tactile interface that
produced shear deformations of the finger pad. When theshear deformations were amplified, the tracing distances
increased by approximately 8 mm on average for a traced
line of 150 mm length. Moreover, such effect is sensitive
to their directional information. The amplification of shear
deformation effectively influenced the tracing motions when
their directions were the same. These experimental results
suggest that the information on finger pad deformations is
fed back to the generator of kinetic outputs and composes
the sensory–motor relationship. Well-designed skin stimuli
possibly result in stronger effects on tracing movements.
Finding such conditions and understanding the underlying

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K. Matsui et al.

mechanism will lead to the development of human-assistive
haptic technology.
Acknowledgements
We thank Prof. Susumu Hara and Dr Yasuhiro Akiyama for theirhelpful comments and advice.

Funding
This study was in part supported by MEXT KAKENHI[22800030].

Downloaded by [Nagoya University] at 09:10 11 July 2016

Notes on contributors
Kenya Matsui is a mechanical engineer who
received BS and MS degrees in engineeringfrom Nagoya University in 2011 and 2013,
respectively. Since 2013, he has been working
for Mitsubishi Electric Corporation. His study
interests include human–computer interfaces.

Shogo Okamoto received a BS degree inengineering from Kobe University in 2005,
and MS and PhD degrees in information
sciences in 2007 and 2010, respectively, fromthe Graduate School of Information Sciences,
Tohoku University. Since 2010, he has been an
assistant professor at the Graduate School ofEngineering, Nagoya University. His researchinterests include haptic interfaces and humanassistive technology.
Yoji Yamada received a PhD degree fromthe Tokyo Institute of Technology, Japan, in
1990. He has been an associate professor at
the Toyota Technological Institute, Nagoya,Japan since 1983. In 2004, he joined the
National Institute of Advanced Industrial andScience Technology (AIST), as a group leaderof the Safety Intelligence Research Group at
the Intelligent Systems Research Institute. In
2008, he moved to the Graduate School of Engineering, Nagoya
University, as a professor.

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