Proposal for tactile sense presentation that combines
electrical and mechanical stimulus
Shinobu Kuroki* Hiroyuki Kajimoto† Hideaki Nii* Naoki Kawakami* Susumu Tachi**
Graduate School of Engineering, The University of Tokyo, JPN†
Department of Human Communication, The University of Electro-Communications, JPN
E-mail: {shinobu_kuroki, hideaki_nii}@ipc.i.u-tokyo.ac.jp,
kajimoto@hc.uec.ac.jp, {kawakami, tachi}@star.t.u-tokyo.ac.jp
Abstract
This study proposes a combination of electrical andmechanical stimulation as a foundational method torealize a tactile display with a high temporal andspatial resolution. We further confirm the interaction
between the electrical and mechanical stimulations.
The interaction (1) lowers tactile threshold and (2)reduces the so-called “electrical” sensation. The
results of this study may resolve certain practicaldifficulties with regard to electro-tactile displays.

1. Introduction
Several studies have reported on tactile displays thatuse mechanical stimulation. These studies proposed thereplication of (1) the surface property of the object or(2) skin deformation. More specifically, there are two
strategies with respect to the abovementioned
proposals—using vibration or exerting pressure. Manyof the former-approach-based tactile displays employ
the vertical vibration of a pin array to change thephysical amplitude and oscillating frequency on thesurface of the skin [1, 2].
Spatial and temporal frequencies are very importantparameters with regard to tactile displays. Certain
mechanical devices realize a high spatial resolution by
employing special types of actuators such as the shapememory-alloy (SMA) actuator; however, their
response times are typically relatively low. Others useda piezoelectric device as the actuator to solve theresponse time problem. As the actuator must be smallenough, many of these devices generate a large
amplitude through using mechanical resonance, whichleads to a band-limited temporal frequency.
On the other hand, certain studies proposed the useof electrical current as stimulus in order to directly
stimulate the sensory nerves [3, 4, 7]. One apparent
advantage of the electro-tactile displays is that they canachieve a high resolution both temporally andSecond Joint EuroHaptics Conference and Symposium on Haptic
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spatially; however, this is not true. Since the electricalstimulation cannot easily stimulate the receptors that
reside deep beneath the skin, it cannot stimulate thePacinian corpuscles (PC) that lie in deeper regions. As
a result, the high temporal frequency, which isnormally handled by the PC, cannot be easily
replicated by electrical stimulation [10]. In addition,
the regulation of current amplitude is also difficult.
Therefore, the so-called “electric” sensation during
stimulation hinders its practical usage.
We think that tactile displays should give richinformation with a simple structure. Based on thismotivation, despite the apparent disadvantage, we
believe that by employing an electric stimulus we canrealize a practical tactile display. In this paper, we
propose an electrovibration display that uses electricaland mechanical stimulations simultaneously. This
concept of using two complementary techniques helps
maintain the advantage of using an electric stimulus
efficiently.

2. Reasonable combination
By combining electrical and mechanicalstimulations, we anticipate two advantages as follows:
Optimal role allocation
One possible advantage is that the two types ofstimulations can stimulate different types of receptors;
this may lead to the realization of an optimal tactile
display design.
There exist different types of mechanical receptors
in the human skin—Merkel cells (SA1), Meissnercorpuscles (RA), and PC. Each of these provides adifferent spatiotemporal responses; the human tactile
sensation is a combination of the activities of thesereceptors (Fig. 1) [8, 9]. From Fig. 1, we can see thatthe human skin is incapable of sensing both thetemporally and spatially high frequency regions.
As discussed previously, creating tactile displaysthat support all spatiotemporal spaces using a single
type of actuator is quite difficult. However, humans
can perceive only a limited range of spatiotemporalspaces; we can therefore optimize the tactile display
design by combining electrical and mechanicalstimulations.
Spatial Frequency
RA
SAI
PC
Temporal Frequency
Fig. 1: Schematic illustration of thespatiotemporal roles of the mechanoreceptors(RA: Meissner corpuscle, SAI: Merkel cell, and
PC: Pacinian corpuscle).

The electro-tactile display should be able to control
the “temporally low but spatially high frequency”
regions, while the mechanical display can control the“temporally high but spatially low frequency” regions.
Decrease in the threshold value
Another possible advantage of simultaneouselectrical and mechanical stimulation is the local
interaction between these two types of stimuli. A
previous study on vibrotactile threshold introduced theconcept of FSR (functional stochastic resonance) thatuses a combination of electrical and mechanicalstimulations [6]. It was discussed that the vibrotactile
threshold can be decreased by adding an electrical“noise” stimulus of a suitable intensity. Therefore,
conversely, the electrical current threshold of electricalstimulation should reduce due to mechanicalstimulation
Considering these two possible advantages, an“optimal” electrical and mechanical tactile display
should comprise (1) dense electrodes and (2) sparseactuators. A dense display can be easily realized by
employing electrical stimulation. The sparse actuator
works both as a high-frequency stimulator andthreshold reducer for the electrical stimulation.
Scope of this paper
As mentioned above, there are two possible meritsof the combination of electrical and mechanicalstimulation. This paper mainly focuses on the latter ofthese two merits. The known signal was used in orderto obtain a more efficient result instead of a noise. We
conducted comprehensive measurements of the electrotactile threshold under the application of both types ofstimuli.

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From the viewpoint of these two stimulation
methods, we need to consider three parameters—
amplitude, frequency, and waveform. The influence of
amplitude must differ greatly in each subject. Aboutfrequency and waveform of electric stimulus,
optimization has already been performed by previous
work. Since we used a combination of the two stimuli,
a new parameter—“phase”—that arises between theelectrical and mechanical stimuli was introduced.
Therefore, we verified the influences of the following
three parameters: phase, waveform of mechanicalstimulus, and frequency of mechanical stimulus.

3. Experiments
3.1. Experiment with synchronized stimuli
In this subsection, it is necessary to generate amechanical stimulus waveform whose phase differsfrom that of the electrical stimulus waveform.
There are numerous mechanoreceptors scattered in
our fingers. The receptors in a particular region
activate when the skin deformation in that region
reaches a certain value. When a sinusoidal mechanicalstimulation is applied to the skin surface, themechanoreceptors activate with spatially different
timings because each point reaches the thresholddifferently (Fig. 2).
FingerSkin Deformation
Neural Spikes
Far

Near
Time
Stimulus Point

Fig. 2: The delay of nerve activity caused by
the distance from the point of impression of
the sinusoidal mechanical stimulus.
On the other hand, when an electrical stimulus isapplied, all the receptors that receive a sufficientcurrent fire together. Therefore, we used an impulse
wave for the mechanical stimulation so that the nerves
are activated at the same moment, even the numerous
tactile receptors spread spatially. This mechanicalstimulation was a kind of imitation of the electricalstimulation. Using this technique, we can observe theinteraction of two types of stimulation.
We generated two sequences with phase differencesof 0 and 180 degree (Fig. 3).

50 msvibration. This is because RA are expected to firepreferentially by anodic stimulation.
The mechanical pulse height was 0 through 10 µm.
For each pulse height, five sets of experiments wereconducted. The subject group comprised four people
between the age group of 23–31 years.

50 ms

50 ms

50 ms

25 ms

Time

12.5 ms

Fig. 3: Mechanical and electrical stimulation(left) with no phase difference, and (right) with
180degree phase difference.

3.1.3. Results. Fig. 5 shows the rate of decrease in theelectrical threshold of one subject. The horizontal axis
and vertical axes represent the amplitude of themechanical stimulus and the rate of an electric current
threshold value on the basis of the time of not adding
mechanical stimulus that the subject perceived 20Hz
vibration.

3.1.1. Apparatus. We constructed an experimental
system, as shown in Fig. 4. A stainless steel pin of
diameter 1mm was placed in the center of analuminum board with a hole of diameter 6 mm. The
pin was vertically driven by a linear actuator throughthe hole in the board. The actuator can output 800N.
During electrical stimulation we used anodic
stimulation where the pin and the aluminum board
served as anode and ground, respectively. The subjects
placed their right-hand index fingers on the pin.
FingerFig. 5: Threshold of the electrical stimulation
GroundStimulating
Electrode
6.0

LinearActuator

1.0

Fig. 4: System configuration: top (left) andside (right) views.
3.1.2. Experiment procedure. The mechanicalstimulus was the pulse wave of 40 pps (pulses per sec),
while the electrical stimulation was set to 20 pps. In
one set of the experiment, the mechanical pulse height
was set to certain value. We ask the subjects to adjust
the amplitude of electrical pulse so that they can sense
20Hz vibration (method of adjustment).
As the subjects can easily distinguish between 40Hz
and 20Hz, cognizing 20Hz means that the elicited
sensation was not solely derived from mechanicalstimulation, but also from electrical stimulation. The
pulse frequencies were chosen because thesefrequencies are the frequency range of the RA, which
resides in the shallow part of the skin and perceives

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The graph shows typical V-shape. Here, two points
warrant discussion: First, the threshold decreased withthe addition of a mechanical stimulus with an adequate
amplitude. Second, the threshold increased on adding astronger mechanical stimulus. The reslt shows that atleast a certain interaction exists between electrical andmechanical stimuli.

3.2. Experiment with de-synchronized stimuli
To see if the time delay between mechanical andelectrical stimulation affects the threshold, we
conducted the following experiment. In thisexperiment, we used a sequence with a time delay
between electrical and mechanical stimulations. The
delay was fixed to 12.5ms (i.e. 180 degree phasedifference for 40Hz stimulation)

3.2.1. Results. Fig. 6 shows results of one subject,comparing with the previous experiment. The dataobtained from the second experiment exhibited thesame trends as those obtained from the previous one.
No significant difference was observed between the
delayed (green line) and no-delayed (red line) sequence
results.

FingerStimulating
Electrode
Array
LinearActuator

2.5

1.25

Fig. 7: System configuration: top (left) andside (right) views.
3.3.2. Experiment procedure. A 40Hz sine wave wasapplied as the mechanical stimulus and the electricalstimulation was set to 20pps. The remaining conditions
were the same as that in the previous experiments.

Fig. 6: Threshold values of the electricalstimulation: delayed (green line) and no-delay(red line) sequences.

3.3. Experiment with sinusoidal wave
Previous experiments with a pin electrode indicatedthat the timing of the nerve firing does not induce
significant problem. Based on this result, we decided touse a sine wave as the mechanical stimulus. Our future
goal is to combine "a simple vibrator which stimulates
large area" and "a high density matrix electrode" (Fig.
7). This is considered to be a solution toward realizing
an optimal system. Furthermore, this improvement
facilitates the easy development of the device.
3.3.1. Apparatus. The electrodes were arranged atintervals of 2.5 mm each in a 3 × 5 matrix. This
electrode plate was mounted on and driven by a linear
actuator. In the following experiment, one electrode of
the array was used, and the actuator used was the sameas that used in the pre-experiment. The mechanicalpulse height was changed to 0 through 70 µm because
recognition of sine wave vibration is difficultcompared with impulse wave.
Since the threshold value was dramatically changed
if the subject's finger moved, we fixed the subject's
finger to the electrode plate.

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3.3.3. Results. Just like the previous experiments, theresults of the 3rd experiment showed V-shape graph,indicating that the interaction between mechanical andelectrical stimulation occurred as we expected.

3.4. Experiment with high frequency
In all the previous experiments, mechanicalstimulation was 40 times per second, while electricalstimulation was 20 times per second. To see if thematching of the frequency between the mechanical andelectrical stimulation is the necessary condition for theinteraction, we changed the mechanical vibration
frequency to 240Hz.
In the first three experiments, frequency of
mechanical vibration was the resonance frequency of
RA. As the electrical pulse also stimulates receptors
which reside in the shallow part of the skin including
RA, we can say that both in the mechanical andelectrical stimulation, RA were stimulated. On theother hand, frequency of vibration used in thisexperiment is resonance frequency of PC. PC is notconsidered to be stimulated by the electricalstimulation (because it is in a deeper region of the skin).
Therefore, in this experiment, mechanical andelectrical stimulation stimulates different types ofreceptors. An advantage of this technique is that,theoretically, there is no cross talk between theelectrical and the mechanical stimuli since it is thethreshold value that we are observing.
The mechanical pulse height was 0 through 30 µm,
because recognition of high frequency wave is easierthan low frequency wave. The remaining conditions
are the same as that in the previous experiments.

3.4.1. Results. Just like the previous experiments, theresults of the 4th experiment showed V-shape graph,indicating that the interaction between mechanical andelectrical stimulation occurred even if the frequency of
mechanical stimulation is much higher than theelectrical stimulation.

4. Discussion
We have measured interaction between mechanicaland electrical stimulation in four different conditions
as follows.
1.
Mechanical stimulus was 40pps impulse andelectrical stimulus was 20pps, electrical pulse
was synchronized with mechanical impulse.
2.
Electrical pulse was delayed 12.5ms withmechanical impulse.
3.
Mechanical sine wave was used. Contactor
was changed from pin to plate.
4.
Mechanical stimulus was 240Hz sine wave.
In all the experiments, we saw that there werecertain interactions between mechanical and electricalstimulation. To quantitatively compare these four
experiments, minimum current normalized with current
without mechanical stimulation for each experiment iscalculated (Fig. 8). The horizontal and vertical axes
represent the experiment numbers as described above
and the rates of a threshold value, respectively. The
circles, triangles, squares and stars represent the
average of each of 5 trials by four subjects.

Fig. 8: Results of the four experiments(Experiment number is described in the text)
Explore the interaction levels
Let us consider how the interaction between theelectrical and mechanical stimuli works. The humantactile signal-processing system is roughly divided into
two layers. One is a receptor level and the other is anerve center level.

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Our first hypothesis is that the interaction occurs in
the receptor level. We hypothesized that the nerve that
does not activate under the application of asubthreshold electrical stimulus can be activated bysubthreshold mechanical stimulation. We call it, “aperipheral level interaction hypothesis”. Thishypothesis assumes that the interaction occurs in thesame mechanoreceptor, or in the same nerve fiber (Fig.
9). As addition of membrane potentials caused bymechanical and electrical stimulation is the key point,
temporal synchronization is essentially important inthis hypothesis.

+E

Fig. 9: Peripheral level interaction hypothesis
However, in the second experiment, it turned outthat time delay of electrical stimulation does not
significantly change the result. The "peripheral levelinteraction hypothesis" was therefore rejected, which
insisted that nerve firing will arise as a result of themechanical and electrical stimuli added to the samemechanoreceptor.
It is an interesting fact that the threshold ofelectrical stimulation decreases by applying
mechanical stimulation even when the phase difference= 180 degree (i.e. time delay was 12.5ms). It implies
that the mechanical stimulus applied before a fixedinterval (assumed to be sufficiently long) from theelectrical stimulation lowered the threshold. From thisresult the existence of the integration process with thetemporal resolution larger than 12.5ms is suggested.
Thus far, no study has reported on the role of group
stimulus in human perception; however, theircontribution may be considered in this case.
Our second hypothesis is the “center levelinteraction hypothesis”. Several different receptors
activities can be considered as a group stimulus. Whenthe number of active receptors reaches threshold, we
perceive sensation. In this hypothesis, both mechanicaland electrical stimulation activated some receptors, but
the number did not reach the threshold. When both areapplied, sum of the active receptors reached threshold.
As the second hypothesis assumes integration process
in our brain, our next interest is what level of thecentral nerve process is related to this interaction
phenomenon. It is well known that at the entrance of
our somatic sensory system (i.e. Brodmann’s area 3a),each region of the cortex receives inputs from single
type of mechanoreceptors. In the higher level (i.e.
Brodmann’s area 1 and 2), information from different
types of receptors are integrated.
In the 4th experiment, theoretically, mechanical andelectrical stimulation stimulates different types ofreceptors. The result of that experiment showed apossibility that the interaction worked even in this case.
For the first time, it was shown that the “decrease in
the threshold value” occurs universally, irrespective ofthe type of receptor.
Reduction in the so-called electrical sensation
During the experiment, another interesting sideeffect was observed. It is a reduction of so-called“electric feeling”. In other words, by adding
mechanical stimulation, the sensation by electricalstimulation became somewhat more “natural”. As this“electric feeling” is one of the major drawbacks ofelectrical stimulation, we should dig the phenomenon
deeper in our next study.

7. Conclusion
The objective of this study is to present an easy
approach to realize a spatially and temporally highgrade tactile perception device. In this paper, wedemonstrated the effectiveness and impact of the
combination of electro-tactile stimulation andmechanical stimulation. As a result, some of theshortcomings posed by the usage of only electricalstimulus can be overcome.
“A peripheral level interaction hypothesis” wasrejected in this paper; however, there exists
physiological evidence suggesting that both electricaland mechanical stimulations actually activate the RA
respectively. Furthermore, we need to reconsider thereason as to why interaction didn't occur clearly at the
peripheral level.
We fabricated an electrode array mounted on alinear actuator; it exhibited the same tendency as thatof a pin electrode. Furthermore, the possibility of theinteraction between different-species receptors wasshown. These two results can help realize an optimal
system that employs a combination of "high temporal
resolution mechanical stimulation" and "high spatial
resolution electrical stimulation".
Following the addition of mechanical stimulation,
further studies investigating the efficiency limit ofelectrical stimulation are required. The investigation of
the types of tactile stimuli that can be sensed by
employing a method to control the different
spatiotemporal spaces will yield interesting results.

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References[1] D. Stein. The optacon: Past, present, and future.
In http://www.nfb.org/bm/bm98/bm980506.htm.
[2] C. W. Wagner, S. J. Lederman, R. D. Howe: Design and
Performance-of-a-Tactile-Shape-Display-Using-RCServomotors, Haptics-e: The Electronic Journal Of
Haptics Research, Vol. 3, No. 4, 2004.
[3] K. A. Kaczmarek, M. E. Tyler, and P. Bach y Rita:
Electrotactile haptic display on the fingertips,
Preliminary resuls. In Proc. 16th Annu. Int. Conf. IEEE
Eng. Med. Biol. Soc, pp. 940-941, 1994.
[4] H. Kajimoto, N. Kawakami, T. Maeda, S. Tachi: The
electric tactile sense display which stimulates acutaneous sensation nerve alternatively, the Institute ofElectronics, Information and Communication Engineers
paper magazine, Vol. J84-D-II, No.1,pp.120-128,2001.
[5] K. A. Richardson, T. T. Imhoff, P. Grigg, J. J. Collins:
Using electrical noise to enhance the ability oh humans
todetect subthreshold mechanical cutaneous stimuli:
Chaos:An Interdisciplinary Journal of Nonlinear
Science, Vol 8, Issue 3, pp.599-603, 1998.
[6] J. J. Collins, T. T. Imhoff, P. Grigg: Noise-mediatedenhancements and decrements in human tactile
sensation. Phys. Rev. E 56: 923-926, 1997.
[7] S. Tachi, K. Tanie, K. Komoriya and M. Abe:
Electrocutaneous Communication in a Guide Dog Robot(MELDOG), IEEE Trans. Biomed. Eng., vol.BME-32,
no. 7, Jul., pp.461-469, 1985.
[8] J. Freeman, “A model accounting for effects of vibratory
amplitude on responses of cutaneous mechanoreceptors
in macaque monkey,” J.Physiol., vol.323, pp. 43-64,
1982[9] A. B. Vallbo and R. S. Johansson, “Properties of
cutaneous mechanoreceptors in the human hand related
to touch sensation,” Human Neurobiology, vol.3, pp.3—
14, 1984.
[10] H. Kajimoto, N. Kawakami, S. Tachi: Psychophysicalevaluation of receptor selectivity in electro-tactile
display, ISMCR, Madrid, Spain, 2003.