Vibration Enhances Geometry Perception with Tactile Shape Displays
María Oyarzábal *Masashi Nakatani † Robert D. Howe ‡(*)Div. Ingenieria de Sistemas y Automática, ETSII, Universidad Politécnica de Madrid, Spain(†)Information Physics and Computing, The University of Tokyo, Japan(‡)Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
E-mail: maria@oyarzabal.com, nakatani@star.t.u-tokyo.ac.jp, howe@deas.harvard.edu
Abstract
Tactile displays can provide detailed spatial
information to the skin, but little is known about theeffects of vibrating displayed shapes. This study
examines passive touch perception of flat and indented
surfaces displayed on a 36 pin tactile display with
2 mm pin pitch. Subjects could not perceive a 0.1 mm
deep central indentation when it was presented
statically, but it was readily detected when the pattern
was vibrated at 5 Hz. A central raised bar wasincorrectly perceived as indented when the adjacentpins were vibrated, which is consistent with the“fishbone tactile illusion” (Nakatani et al., Proc.
EuroHaptics 2006). These results suggest that tactile
display devices can use vibrational stimulus toenhance perception of small differences in height.

1. Introduction
Tactile displays for conveying spatial information
to the skin have been under investigation for over 40
years [1], [2]. Initial interest was motivated by sensory
substitution devices for the visually impaired, whilemuch current work addresses applications in virtualenvironments. The most common design approach forthese displays uses arrays of pins that can be raised
against the user’s skin to approximate arbitrary shapes.
A wide variety of actuator technologies and design
configurations have been reported; see [3], [4] forrecent reviews of the state-of-the-art in tactile display
research.
Actuation approaches for pin-based tactile displays
may be broadly divided into two categories:
vibrotactile and static. Vibrotactile displays typicallyoperate at frequencies of 200-250 Hz; at thesefrequencies perceptual thresholds are lowest, which
permits the use of lower-cost and more compact
actuators. They have most commonly been used toconvey abstract forms such as letter shapes for sensory

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substitution applications, e.g. [1], [2]. Static displays,
on the other hand, can provide continuous skindeformation with amplitudes on the order of 1 mm or
more, and thus require higher actuator power levels.
These displays are often intended to simulate thesensation of contact with three-dimensional objects in
virtual environments, e.g. [5], [6].
Two recent psychophysical results raise newquestions about the role of frequency in tactile displayof spatial patterns. First, Bensmaia et al.[7] reportmeasurements of tactile acuity as a function of
stimulus vibration frequency. These measurements
usedtheconventional
gratingorientation
discrimination test, with the grating stimuli vibrated
normal to the finger tip surface at frequencies up to
80 Hz. The results showed that highest acuity occurs at
frequencies of 5-10 Hz, where it is about 30-40%
better than the static case and up to 300% better than at
higher frequencies.
The second result is the fishbone tactile illusion,
where a raised surface feature is perceived as indented[9]. The archetypal stimulus consists of a central bar
raised 0.1 mm above the surrounding surface withsimilar bars projecting to each side (Figure 1a). For
central bar widths up to a few mm, when the central
bar is stroked back-and-forth with the finger tip there isa strong illusory perception that the central bar isindented with respect to the lateral bars. In fact, for the
narrower widths, the raised central bar is perceived as
more indented than the true indented reference shape
shown in Figure 1b, where the central area is the sameheight as the flat surrounding surface.
This illusion occurs for a variety of patterns where
the central bar is surrounded by raised textures in placeof the lateral bars projecting to each side, including
textures with elements that are a small fraction of amm in size. Motion of the finger tip over the central
bar and adjacent texture is required in all cases. These
results suggest that vibrotactile stimulus surrounding

Figure 1. Fishbone illusion stimulus plates
in machined aluminum. Black denotes areas
raised
0.1 mmabove
white
areas.
(a) Fishbone pattern with raised central barthat is perceived as indented; (b) Reference
pattern with true indented central bar.
the smooth central raised bar is a factor in the creationof the illusion.
In this paper, we explore the role of low-frequency
vibration in perception of shapes using pin-based
tactile displays. We report on two psychophysicalexperiments in which subjects were presented with bar
patterns combining two values of height (flat vs.
0.1 mm) and frequency (static vs. 5 Hz) on a 36 pin
finger tip tactile display. The first experiment
investigated differences in perception of static andvibrating patterns, in both flat and indented shapes. A
second experiment with vibratory patterns determined
whether the fishbone illusion could be reproduced
using the tactile display with the finger static but withpin motion to simulate the vibrotactile stimulus
generated by stroking the actual fishbone pattern.
In the next section we describe the tactile display
device and the five patterns used in the experiments, aswell as the experimental setup. The following section
presents the results of both experiments, which
demonstrate that vibration enhances the perception ofindented patterns, for both physically indented shapes
and the fishbone illusion.

2. Materials and Methods
2.1. Tactile Display and Stimuli
The tactile display employed here uses RC
servomotors to actuate a 6 x 6 array of pins (Figure 2)[10]. The pins are 1 mm in diameter with 2 mm center
spacing in a square grid. Nominal vertical range oftravel is 2 mm and height resolution is 0.1 mm. To
evaluate the performance of the tactile display we used
a CCD laser displacement sensor (LK-500, Keyence

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Figure 2. Tactile display device.
Corp., Osaka, Japan). Data from the sensor was
digitized at 1 kHz and low-pass filtered at 50 Hz.
Results showed that full commanded heights wereachieved for frequencies of 5 Hz. For full details of thedevice design and performance, see [10].
The vibratory pin motion in these experiments
consisted of a 5 Hz square wave. Adjacent vibrating
pins were 180 degrees out of phase to convey anoverall impression of vibration instead of asimultaneous vertical movement of all the pins.
Commanded heights were either zero or 0.1 mm forboth static and vibrating pins.
The experiments used the five spatial patterns shownin Figure 3, arranged in two groups corresponding to
the two experiments. The first experiment uses patterns
1 to 4, which differ in spatial arrangement (flat vs.
center indented) and frequency (static vs. vibrating).
Pattern 1 is a flat static pattern consisting of all 36 pins
0.1 mm above the flat plate surrounding the pins.
Pattern 2 presents a static indented shape, with theinternal columns of pins at 0.0 mm in height. Pattern 3
is a vibratory version of pattern 1; drive electronics
restrictions preclude simultaneous vibration of all 36
pins, so the two outer columns were static in the raised
position. This provided 9 mm of vibrating contact area,
which subtended the great majority of the contact areaof subjects’ finger tips. Pattern 4 represents a central
indentation with the surrounding pins vibrating.
The second experiment uses patterns 4 and 5 toinvestigate properties of the fishbone tactile illusion..
Both of them present vibration on the outer pins andstatic shapes pins on the inner pins; in pattern 4 theinner pins are zeroed in height, representing an
indented surface, while in pattern 5 they are raised to

a

Flat

Indented

Pattern 1

Pattern 2

Pattern 3

Pattern 4

StaticVibrating(5Hz)

subject would withdraw the finger from the device andstate whether the pattern felt flat or indented. Therewas unlimited time to make the choice. After 10
practice trials, each subject proceeded to complete 50
trials, so each pattern was tested 10 times. Subjects
had a 3-minute break after 25 trials, in which they wereasked to rub their finger on a flat surface in order toavoid adaptation. They could also rest at any time theyfelt it necessary. The order in which patterns were
presented to each subject was randomized, but all
subjects received the same ordering. Data for the two
experiments was collected during the same
experimental session. A typical session lasted about 20
minutes.

3. ResultsbCenter indented

Center Raised

Pattern 4

Pattern 5

Vibrating(5Hz)

Figure 3. Side view of one row of eachpattern. Dotted pins in patterns 3-5 show
vibration. (a) experiment 1; (b) experiment2.
0.1 mm, corresponding to the fishbone illusion
configuration.

2.2 Experimental Design and Procedure
Ten graduate students volunteered for theexperiments, five males and five females. All of them
defined themselves as right handed and did not reportany hand injury or disease.
The tactile display was fixed to a table and subjects
were asked to place their right index finger on the pins,
so that the finger did not move once it was placed.
They could, however, vary the applied finger tip force
for comfort and to best feel the displayed pattern. We
used a 0.15 mm thick latex rubber sheet as a spatial
low pass filter to prevent subjects from feeling theeffect of individual pins [11]. The stimulus was alreadypresent in the tactile display when subjects positioned
their finger, so the area of the rubber in direct contact
with the pins was painted black to provide a target for
finger placement and to minimize visual feedback. To
eliminate audio cues, subjects wore headphones
playing white noise in the frequency range of sounds
made by the tactile display.
During each trial, a pattern was presented to thesubject’s fingertip for 5 seconds. After that time, theSecond Joint EuroHaptics Conference and Symposium on Haptic
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Results from the experiments are presented in Figure
4. This graph shows the probability of classifying eachpattern as indented in both experiments. The meanvalues with corresponding standard deviations were0.04(0.05), 0.14(0.25), 0.34(0.37) and 0.91(0.11) for
the first four patterns, and 0.93(0.08) for the fifth one.
A one-factor within subject repeated measures
ANOVA was performed on the probability ofclassifying the patterns as indented, with factor being
the pattern (5 levels). Mauchly’s test indicated that theassumption of sphericity had been violated(χ2(9)=35.72, p<0.001); therefore degrees of freedom
were corrected using Greenhouse-Geisser estimates ofsphericity (ε=0.342). Results showed that the effect ofpatternwasstatisticallysignificant(F(1.37,12.32)=38.14, p< 0.001 ).
For the first experiment, we carried out several
contrasts to identify the pairs of patterns that weresignificantly different. These contrasts showed that,while there is no significant difference between static
patterns 1 and 2 (F(1,9)=1.8, p=0.213), the difference
between vibrating patterns 3 and 4 is significant(F(1,9)=25.19, p=0.001). The difference between flat
patterns 1 and 3 was also significant (F(1,9)=5.55,
p=0.043). The effect of subject’s gender was notreported as significant at p<0.05.
A two-factor within subject repeated measures
ANOVA was then performed to data from the first four
patterns, with factors being vibration (2 levels) andheight of the pins (2 levels). There was a significant
effect of vibration on the probability of classifying apattern as indented (F(1,8)=28.27, p=0.001); contrasts
revealed that the probability of classifying a pattern as
indented was significantly higher when the pins werevibrating. The effect of height was also significant on
the classification of patterns (F(1,8)=15.53, p=0.004),

Probaility of classifying pattern as indented

a

b
1.00

1.00

0.90

0.90

0.80

0.80

0.70

0.70

0.60

0.60

0.50

0.50

0.40

0.40

0.30

0.30

0.20

0.20

0.10

0.10

0.00

0.00

2.Indented

1.FlatSTATIC

3.Flat4.Indented

4.Indented

5.Center Raised

VIBRATING

VIBRATING

Figure 4. Results from (a) experiment 1 and (b) experiment 2. For each pattern, dots
represent data from one subject and bars represent the mean of the ten subjects.

Indented
1.0

Flat
4

Probability of classifying pattern as indented

with indented patterns having a significantly higher
probability of being classified as indented.
Most interesting results come from the significant
interaction effect between height and frequency(F(1,8)=22.31, p=0.001). This indicates that frequency
had different effect on subjects’ perception dependingon the indentation of the pattern. The interaction graph(Figure 5), shows that vibration raised the probabilityof feeling a pattern as indented more in indented
patterns than in flat patterns. For flat patterns, theeffect that adding vibration had on the probability ofidentifying the patterns as indented was very small,whereas for indented patterns this effect raised theprobability of classifying the pattern as indented from
0.14(0.08) to 0.91(0.03).
For the second experiment, we analyzed data fromthe one-factor within subjects repeated measures
ANOVA. Contrasts revealed that there was no
significant difference between patterns four and five(F(1,9)=1.00, p =0.34).

0.8

0.6

*

*

0.4
3

0.2

*

2
1
0.0

VibratingStaticFrequency

Figure 5. Height and Frequency interaction graphfor the first study. Number refer to patterns. *denotes statistically significant
difference(p<0.05).

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4. Discussion
4.1 Experiment 1
The first experiment investigated the role ofvibration in distinguishing indented patterns of 0.1 mm
height by passive touch. Results showed that subjects
were not able to perceive the indentation with a static
tactile display (pattern 2). As expected, adding
vibration (pattern 4) increased the probability ofperceiving the indentation to near unity.
This is likely due to the small height of theindentation and the lack of stimuli from active touch.
Bensmaia et al. [7] showed that tactile acuity
thresholds are better if patterns are vibrated at 5-10 Hz
than for static presentation. Hollins and Risner [8]found that under passive touch the discrimination of
sandpaper surfaces was difficult for fine grits (under
0.015 mm particle size) but readily accomplished forcoarser grades (over 0.141 mm). When the sandpaper
surfaces were drawn over subjects’ fingers, the finer
grits were also easily distinguished. Together with theresults of the present study, this suggests that adding
vibration to tactile rendering algorithms can enhance
the perception of fine surface features with tactile
displays.
It is interesting to note that the vibrating flat pattern(pattern 3) was classified as indented in about a third of
the trials. Contrasts showed that there was a significant
difference between the flat static pattern (pattern 1) andthis pattern. One possible explanation is that vibratory
stimulus was not the type of information subjects
expected, and some confused vibration withindentation. We also note that the edge pins in pattern
3 were fixed due to hardware limitations, which may
be a confounding factor.
Nevertheless, the fact that the probability ofidentifying pattern 3 as indented is still well below
chance, suggests that vibration on its own, with nospecific pattern, is not enough for strong identificationof an indented shape. Even with vibratory stimulus, astrong perception of indentation requires a contrasting
central element such as patterns 4 or 5.
Surprisingly, while some subjects identified the flat
vibrating pattern as indented, others informallyreported feeling it as rounded. Both the indented androunded responses to the flat vibrating surface
demonstrate that high-performance tactile displays
have the capability to deliver stimuli with nocounterpart in the physical stimuli experienced ineveryday manual tasks. This has the potential fordelivering confusing sensations if deficient rendering
algorithms are employed. Conversely, it may be

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possible to create new sensations that cannot be
experienced in the physical world, opening new
possibilities for communications and aesthetics.

4.2 Experiment 2
The purpose of our second experiment was to findthe effect of raising the static center in a vibrating
pattern (Pattern 5), in analogy with the fishbone tactile
illusion. Contrasts showed there was no significant
difference between Pattern 4 and Pattern 5. Bothpatterns were perceived as indented, no matter if thepins in the center were 0 or 0.1 mm high. This meansthat as long as adjacent region of contact area wasvibrated, the central area of the pattern would beperceived as indented.
It is clear from this result that it is the difference instimulus between the central and lateral regions thatcreates the sensation of indentation and not adifference of height. This matches the conclusion fromprevious studies of the fishbone illusion [9]. One
hypothesis to explain this illusion begins with theobservation that the copious tactile stimulus in thelateral region greatly exceeds the stimulus in thesmooth, static central region. This relative absence ofstimulus is then equated with the absence of a surface,which is a description of an indented surface. Thus thestimulus contrast is equated with a geometric contrast.
The present study shows that this illusion can bereadily created through vibration of the adjacent region
while the finger is static. Thus active motion of thefinger is not needed to invoke the illusion, whichremoves proprioception as contributing factor. Thisimplies that vibration serves the same role in the tactile
display version of the illusion as motion plays in theactive version.

4.3 Conclusions and Future Work
Results from both experiments are limited to thesimple indented shape, and data concerning Pattern 3suggests that the effects of vibration could be different
when analyzing other shapes. From these experiments
we know the effect of vibration at 5 Hz on a 0.1 mm
indentation that is 4 mm wide, and considerable work
will be required to develop a more thoroughunderstanding of height, frequency and width in theuse of tactile displays.
Our experiments have shown that, for small heightvariances, a difference in height is not essential toproduce the indented sensation. This result suggeststhat developers of tactile displays and tactile rendering
algorithms can take advantage of using vibrational

stimulus intensity difference in place of differences inheight.

References[1] J.G. Linvill, and J.C. Bliss, “A Direct Translation
Reading Aid for the Blind”, in Proc. of IEEE, New York,
1966, vol. 54, pp. 40-51.
[2] J. C Bliss, “Dynamic tactile displays in man-machine
systems”, IEEE Transactions on Man-Machine Systems,
New York, 1970, vol. 11(l), pp. 58-65.
[3] M. Benali-Khoudja, M. Hafez, J.M. Alexandre and A.
Kheddar. “Tactile Interfaces: A State of the Art Survey”,
in Proc. of ISR: International Symposium on Robotics,
Paris, 2004.
[4] M. Jungmann and H.F. Schlaak, “MiniaturisedElectrostatic Tactile Display with High Structural
Compliance”, in Proc. of Eurohaptics, Edinburgh, 2002.
[5] S.J. Lederman and R.L. Klatzky, “Sensing and
Displaying Spatially Distributed Fingertip Forces in
Haptic Interfaces for Teleoperator and Virtual
Environment Systems”, Presence, MIT Press,
Cambridge MA, 1999, vol. 8, pp. 86-103.
[6] C.R Wagner, R.L Feller, D.P Perrin, O. Clatz, R.D.
Howe, H. Delingette, and N. Ayache, “Integrating
Tactile and Force Feedback with Finite Element
Models”, in Proc. of IEEE Intl Conf Robotics & Autom,
Barcelona, 2005, pp. 3942 – 3947.
[7] S.J. Bensmaia, J.C. Craig, and K.O. Johnson. “Temporal
Factors in Tactile Spatial Acuity: Evidence for RA
Interference in Fine Spatial Processing”, Journal of
Neurophysiology, American Physiological Soc.,
Bethesda MD, 2006, vol. 95, pp. 1771-1782.
[8] M. Hollins and S. R Risner, “Evidence for the Duplex
Theory of Tactile Texture Perception”, Perception &Psychophysics, Psychonomic Soc Inc, Austin TX, 2000,
Vol. 62(4), pp. 695-705.
[9] M. Nakatani, R.D. Howe, S. Tachi, “The Fishbone
Tactile Illusion”, in Proc. of EuroHaptics, Paris, 2006,
pp. 69-73.
[10] C.R. Wagner, S.L. Lederman, R.D. Howe,“Design and Performance of a Tactile Shape Display
Using RC Servomotors”, Haptics-e: The Electronic
Journal of Haptics Research (www.haptics-e.org), vol.
3(4), 2004.
[11] J.M. Lee, C.R. Wagner., S.J. Lederman, R.D. Howe,“Choosing a Spatial Low Pass Filter for Pin ActuatedTactile Displays”, In Proc. of 11th Intl Symp Haptic
Interfaces for Virtual Environments and Teleoperator
Systems, Los Angeles, 2003.

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