Psychophysical evaluation of receptor selectivity in electro-tactile display
H. Kajimoto, N. Kawakami, and S. Tachi
The University of Tokyo, JAPAN
7-3-1 Bunkyo-ku Hongo, Tokyo, 113-8656, Japan
E-mail: {kaji, kawakami, tachi}@star.t.u-tokyo.ac.jp
Abstract
We have been developing an electro-tactile display topresent realistic tactile sensation for Virtual Realityand robot teleoperation. This paper shows
quantitative evidence that Meissner corpuscles are
selectively activated in our specific setup of electricalstimulation, dubbed “Meissner mode.” We also showthe fundamental limitation of electro-tactile displays,
which is caused by the physical incapacity toselectively stimulate Pacinian corpuscles, which
reside in the deeper tissue, because shallower parts
are inevitably co-activated.
Keywords: Virtual Reality, tactile display, electricalstimulation, Meissner corpuscle, Pacinian corpuscle

1 Introduction
1.1 Electro-tactile display and selective stimulation

The electro-tactile display is a type of tactile
display that uses an electric current as a stimulus. We
have been working on the electro-tactile display topresent realistic skin sensation for Virtual Reality androbot teleoperation[1][2].

Figure 1 Primary color approach. If each type ofreceptor is selectively stimulated separately, thestimuli could be treated as “primary colors,” andcombining these stimuli could produce any tactile
sensation.

Our approach is as follows. There are four typesof mechanical receptors in human skin, which convert
skin deformation into nerve activities: Meissnercorpuscles, Merkel cells, Pacinian corpuscles andRuffini endings. If we selectively stimulate each ofthese receptors separately, we could treat the stimuli
as “primary colors,” analogous to the three primarycolors in vision, and produce any tactile sensation by
combining them (Figure 1).
1.2 Meissner and Pacinian corpuscle

This paper deals with two types of receptors,
Meissner and Pacinian corpuscles. As both of them
respond to mechanical vibration, we believe that
selective stimulation of these receptors is
indispensable to present tactile texture.
Meissner corpuscles are located in the shallow
part of the skin (less than 1[mm]), and reacts to low
frequency vibration (20-70[Hz]), with a resonant
frequency of about 30[Hz]. Pacinian corpuscles arelocated in the deeper part of the skin (about 2[mm]),and reacts to high frequency vibration (100-300[Hz]),with a resonant frequency of about 250[Hz] (Figure 2).

Figure 2 Activation threshold of the Meissner andPacinian corpuscles when mechanical stimulation(sine wave) is applied on the skin surface[3].

We used electrical stimulation to generate tactile
sensation. An electric current pulse from surface
electrodes generates an electric field inside the skin,which induces nerve activity. Compared toconventional mechanical tactile displays, the electrotactile display can be manufactured very thin, durable
and free from mechanical resonance.
In our previous work, we showed theoreticallyand experimentally that selective stimulation of these
two receptors is possible by changing the current
polarity and electrode size [1].
We used normal concentric electrodes comprisedof an inner stimulating electrode and an outersurrounding electrode. If the inner diameter of theouter electrode is sufficiently small, an electric current
from the inner electrode to the outer electrode only
passes through the shallow part of the skin, which
activates shallow receptors.
In particular, Meissner corpuscles are more easily
activated by anodic stimulation (inner electrode works
as anode and outer electrode works as ground) thancathodic stimulation (Figure 3). We call this setup“Meissner mode”.
On the contrary, if the inner diameter of the outerelectrode is large, the electric current reaches deeper
tissue, which also activates deeper receptors such as
Pacinian corpuscles. We call this setup “Pacinianmode.”

Figure 3 Selective stimulation with an electric currentpulse. (Left) Meissner mode. Positive pulse is applied
from the central part of small concentric electrodes.
(Right) Pacinian mode. Negative pulse from central
electrode reaches to deeper tissue.
The perceived sensation (vibratory sensation) for
each mode has been qualitatively explained inprevious works by the role of each type of receptors[1].
However, this paper is the first quantitative validationof the selective stimulation of the Meissner andPacinian corpuscles in electro-tactile display, verified
by psychophysical experiment.

2 Experiment
2.1 System configuration

We constructed a mechanical and electrical stimulator
as shown in Figure 4. An electrode (stainless steel,1.0[mm] in diameter) is mounted on a voice coil,which stimulates the finger skin both electrically andmechanically. Electrode motion is measured by aphoto reflector.

Figure 4 Mechanical and electrical stimulator. An
inner electrode is mounted on a voice coil.
Mechanical vibration is observed by the photoreflector, which is attached on the coil.
2.2 Frequency discrimination with mechanical sinewave

First, we measured the frequency discriminationthreshold of mechanical sine wave vibration with themethod of limits. Four participants, three males andone female between 25 and 35 years old, were asked
to place their finger on the electrode.
Standard and comparison stimuli with a vibration
amplitude of 0.1[mm] were presented for 1.0[s], andthe participants were asked which stimulus frequency
was higher (Figure 5). The standard frequencies were15, 30, 60 and 120[Hz]. The comparison frequency
started from either 0.5× or 2.0× the standard frequency,
and gradually approached to the standard frequency aslong as the participant answered correctly.
The first comparison frequency in which theparticipant answered incorrectly was considered thefrequency discrimination threshold.
A typical result is shown in Figure 6. The
horizontal axis is the standard frequency, and thevertical axis is the ratio of discrimination thresholdfrequency to standard frequency. We see from thefigure that for all frequencies, upper and lower
discrimination thresholds are within 0.8 to 1.2, whichmeans that the participant could discriminate afrequency difference of about 20%.

Figure 5 Standard and comparison mechanicalsine wave with amplitude of 0.1[mm] was applied
on a finger-pad for 1.0[s], and participants wereasked which stimulus was higher in frequency.

Comparison Frequency / Standard Frequency

2 .2
21 .8
1 .6
1 .4
1 .2
10 .8
0 .60 .4

L ow e r D iffe re n c e T h re sh old0 .2

U p p e r D iffe re n c e T h re sh old

0020406080100
S tan d ard F re q u e n c y(H z)

120

140

Figure 6 Upper and lower difference thresholds ofmechanical vibratory stimulation (sine wave),obtained by the method of limits. The horizontal
axis is the standard frequency, and the vertical axis
is the ratio of discrimination threshold frequency
to standard frequency.

The experiment set-up was as follows: the central
electrode was 1.0[mm] in diameter, the inner diameterof the outer electrode was 6.0[mm] and the stimulation
was anodic pulses (Meissner mode) at frequencies of15, 30, 45, 60 and 120[Hz]. The pulse amplitude was2.4[mA] and pulse width was 0.2[ms]. The result isshown in Figure 8.
At 15[Hz] and 30[Hz], the results are quite thesame as the mechanical vibration. However, at 45[Hz],the upper threshold suddenly reached to about 2,
which means that the participants could not interpret
90[Hz] stimulation to be higher than 45[Hz].
This puzzling result is explained by assuming that
Meissner corpuscles are selectively stimulated. In
mechanical stimulation, 15[Hz] and 30[Hz] are thefrequencies where only Meissner, not Pacinian
corpuscles are activated (Figure 2). Therefore, there isno difference between mechanical and electricalstimulation from the receptor’s viewpoint. However,from 70[Hz] or more, Pacinian corpuscles are also
activated in mechanical stimulation. At that frequency,
it is quite natural to conclude that we discern
frequency by observing the activity ratio of both theMeissner and Pacinian corpuscles.
This reveals the contrast between electrical andmechanical stimulation. In electrical stimulation mode,
as we mentioned before, only Meissner corpuscles areactivated even at high frequency. This situation should
never happen in mechanical stimulation. Hence, theparticipants could not discern the frequency of thestimulation.

2.3 Frequency discrimination in Meissner mode

Figure 7 Standard and comparison electricalstimuli were presented for 1.0[s]. Pulse amplitude
was 2.4[mA] and pulse width was 0.2[ms].
Previous work showed that in mechanicalvibration, nerves that are connected to the receptors
are electrically activated at the same frequency as thevibration[4]. Therefore, conversely, we supposed that
if electrical pulses are applied, participants should
perceive the pulse train as mechanical vibration withthe same frequency, which should result in the same
frequency discrimination threshold.

Comparison Frequency / Standard Frequency

The same test was conducted for electricalstimulation. We used the Meissner mode (Figure 7).

2 .2
21 .8
1 .6
1 .4
1 .2
10 .8
0 .60 .4

L o w e r D iffe re n c e T h re s h o ld

0 .2

U p p e r D iffe re n c e T h re s h o ld

0020406080
100
S ta n d a rd F re q u e n c y (H z )

120

140

Figure 8 Upper and lower difference thresholds ofelectrical stimulation. Positive pulse with 2.4[mA]
amplitude, 0.2[ms] width was applied on thefinger-pad through the concentric electrode with
1.0[mm] and 6.0[mm] inner and outer diameter.
2.4 Verification of the hypothesis by deep tissue
stimulation

If the above-mentioned rationale is true, thefrequency discrimination threshold should decrease if
we also stimulate Pacinian corpuscles. This isachieved in Pacinian mode, which was performed byusing the inner electrode with 3.0[mm] in diameterand outer electrode with 10.0[mm] in inner diameter.
We stimulated using cathodic current pulses. The
result is shown in Figure 9. The discriminationthreshold approached to 1 at a higher frequency,
which supported our hypothesis.
Comparison Frequency / Standard Frequency

2.22
1.8
1.6
1.4
1.2
10.8
0.6
0.4

Lower Difference Threshold
Upper Difference Threshold

0.2
0020

4060
80100
Standard Frequency (Hz)120140Figure 9 Upper and lower difference thresholds ofdeeper part electrical stimulation. Negative pulse
with 2.4[mA] amplitude, 0.2[ms] width wasapplied on the finger-pad by the large concentric
electrode with 3.0[mm] and 10.0[mm] inner andouter diameter.
However, at 120[Hz], the upper threshold reachedabout 2, which means the participant could notinterpret 240[Hz] pulses to be higher than 120[Hz]pulses.
This result is also explained by comparing
mechanical and electrical stimulation. In mechanicalstimulation, only Pacinian corpuscles are activated at200[Hz] or more (Figure 2). However, in electricalstimulation, Meissner corpuscles are also activated in
Pacinian mode. Therefore, our brain mistakes thevibration frequency as being within the PacinianMeissner co-activation range, which is lower than200[Hz] (Figure 2), resulting in a misinterpretation of
the stimulation frequency.

3 Conclusions
In this paper, we showed quantitative evidence
that Meissner corpuscles are selectively activated inour specific setup of electrical stimulation, dubbed“Meissner mode.”
We also showed the fundamental limitation ofelectro-tactile displays, which is caused by the factthat we could not selectively stimulate Pacinian
corpuscles, which reside in the deeper tissue, because
the shallow part is inevitably co-activated. This
limitation was observed as the limitation of frequency
perception, in which pulses with more than 200[Hz]were not interpreted correctly. This limitation wasalready reported in the 1970’s[5], although there has
never been an explanation based on receptor activity.
To overcome this limitation, we are considering a
hybrid method of applying mechanical and electricalstimulation simultaneously. Mechanical stimulation
aims to selectively stimulate Pacinian corpuscles,
while electrical stimulation targets Meissnercorpuscles. Selective stimulation of Paciniancorpuscles with a mechanical vibrator is not sodifficult, because Pacinian corpuscles has the highest
resonant frequency (about 250[Hz]) of all receptors.
At the same time, the density of Pacinian corpuscles inthe skin is relatively low. Therefore, the onlyrequirement for Pacinian selective stimulation is tofabricate a sparse mechanical vibrator matrix andvibrate at high frequency. On the contrary, theMeissner corpuscle is densely populated (1[mm-2]),and the fabrication of an appropriately dense display
with electrodes is far more practical than using
mechanical stimulators.

4 Acknowledgments
This work was supported by the Japan Society for thePromotion of Science, Grant-in-Aid for Scientific
Research (A) 15200007.

5 References[1] Kajimoto, Kawakami, Maeda, Tachi, "TactileFeeling Display using Functional Electrical
Stimulation, " in The Ninth International Conferenceon Artificial reality and Telexistence (ICAT'99), 1999.
[2] Kajimoto,Inami,Kawakami,Tachi,"SmartTouch: A new skin layer to touch the nontouchable," in Conference Abstracts and Applicationsof SIGGRAPH, CD-ROM, 2003.
[3] Iggo, “Sensory receptors in the skin of mammals
and their sensory functions,” Rev. Neurol. (Paris),vol.141, pp.599-613, 1985.
[4] Vallbo and Johansson, “Properties of cutaneous
mechanoreceptors in the human hand related to touchsensation,” Human Neurobiology, vol.3, pp.3-14
Springer- Verlag 1984.
[5] Szeto, Lyman, Prior, “Electrocutaneous Pulse
Rate and Pulse Width Psychometric Functions forSensory Communications,” Human Factors, vol.21,
pp-241-249, 1979.