Design of Electrotactile Stimulation to Represent
Distribution of Force Vectors
1

2

Katsunari Sato and Susumu Tachi
1

2

The University of Tokyo, Keio University

ABSTRACTTransmission of the distribution of force vectors on a fingertip
improves the stability of a given operation in a tele-operation
system. As such, we seek the representation of the distribution offorce vectors via an electrotactile display. The electrotactile
display directly activates nerve fibers connected tomechanoreceptors, thereby representing a tactile sensation. The
system has a simple structure, is easily controlled and, it has seen
a number of different applications. However, the majority of theconventional electrotactile displays only present a symbolic
sensation of contact. In this paper, we propose the design ofelectrotactile stimulation for the distribution of force vectors, on
the basis of the concept of “tactile primary colors”. Consideringthat a selective stimulation of mechanoreceptors enables therepresentation of the distribution of force vectors, we evaluate theproposed design in terms of the distribution, magnitude, anddirection of a force vector. The results demonstrate that the
proposed design is a viable approach to representing the
distribution of force vectors.
KEYWORDS: Electrotactile display, tactile primary colors,
distribution of force vectors.
INDEX TERMS: H.5.2 [INFORMATION INTERFACES ANDPRESENTATION]: User Interfaces—Haptic I/O; H.1.2[MODELS AND PRINCIPLES] User/Machine Systems—Human
information processing
1

INTRODUCTION

Transmission of haptic information between tele-operation
systems is essential for the manipulation of remote objects. In
particular, we believe that the distribution of three-dimensional
force vectors (the force that is applied both perpendicular and inparallel to the skin surface) upon a fingertip is essential for
dexterous operation. By determining the spatial distribution,
magnitude, and direction of motion of an object from thisinformation, the operator is able to firmly grasp the object andsuccessfully manipulate it. For example, force vectors can indicate
whether the grasping force is sufficient. They can also improve
the operator’s perception of the object by allowing him or her to
identify its shape and to determine its hardness.
To transmit data regarding the distribution of force vectors, wefirst require a sensor for their measurement, as well as a display torepresent the values. To date, many studies have been conductedon both haptic sensors and associated displays. Recently, afinger-shaped sensor that allows for the measurement of force
vector distribution has been developed [6] [20]. However, a haptic
display that is capable of representing this information has yet to
be developed because of the difficulty associated with its
implementation.
Many types of haptic displays have been developed in the past[2]. One such type is the mechanical haptic display, which
represents skin deformations by way mechanical actuators.
Actuation of the pins in an up-and-down motion simulates the
distribution of pressure [9] [13] [14] [16]. If we employ anactuator having three degrees of freedom (DOF) for the
movement of each pin, it might be possible to represent the
distribution of force vectors, though it is very difficult to form andcontrol a dense array of small actuators. In contrast, some displays
can capable of representing forces applied perpendicular or inparallel to the skin surface at a given point [7] [22] [25]. However,
it is also difficult for these displays to represent the distribution of
the force due to the size of the actuator. Another type of haptic
display, the electrotactile display, uses electrical stimulation to
fire nerve fibers that are connected to mechanoreceptors,
simulating a tactile sensation. This form of display is small, hashigh spatial resolution, and can be controlled easily as it does not
employ mechanical actuators. However, the majority ofelectrotactile displays show only symbolic contact information in
the form of on-off signals [4] [15] [18]. Recently, a few studies
have attempted to implement a method for simulating the degreeof pressure that has been applied perpendicular [19] [26] or inparallel [5] to the skin’s surface. A three-dimensional tactile
sensation, however, has yet to be implemented using theelectrotactile display.
We therefore propose a design employing electrotactile
stimulation that is capable of representing the distribution of force
vectors (Fig. 1). To achieve this, we develop the design based on
the concept of “tactile primary colors.” Kajimoto et al. [10]proposed that the electrotactile display is capable of simulating anatural tactile sensation via the selective stimulation of eachmechanoreceptor. Furthermore, they showed that the selective

1)

7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan,
Email: {Katsunari_Sato }@ipc.i.u-tokyo.ac.jp
2)4-1-1 Hiyoshi, Kohoku-ku, Yokohama-city, Kanagawa,
223-8526, Japan. Email: tachi@tachilab.org
IEEE Haptics Symposium 201025 - 26 March, Waltham, Massachusetts, USA
978-1-4244-6820-1/10/$26.00 ©2010 IEEE

Figure 1. Representing the distribution of force vectors on afingertip using the electrotactile display.

121stimulation of two types of mechanoreceptors could be used to
produce a sensation of vibration or pressure. Our design ispredicated on the fact that it is possible for the distribution offorce vectors to be simulated via different combinations of theseselective stimuli.
In this study, we aim to transmit the spatial distribution,
magnitude, and direction of a force vector. In this paper, we
introduce the theories of electrotactile stimulation and tactile
primary colors. We then formulate the design of electrotactile
stimulation on the basis of the concept of tactile primary colors.
Finally, we assess the viability of the proposed design withrespect to representing the distribution of force vectors.
2

CONVENTIONAL STUDY OF ELECTROTACTILE STIMULATIONIn this chapter, we introduce a conventional study of theelectrotactile stimulus. We describe the theory and application of
the electrotactile display and subsequently, introduce the conceptof “tactile primary colors.”
2.1
Theory and Applications
The electrotactile display operates by activating the nerve fibersthat are connected to mechanoreceptors, thereby producing atactile sensation. The electrodes are placed on the skin surface anda flow of electrical currents is applied to the skin. It is possible toapply a consistent stimulation to the nerve fibers bearing theelectrodes by using two coaxial electrodes. Furthermore,arranging the pin-electrodes into a matrix pattern, theelectrotactile display is capable of producing two-dimensional
patterns [4].
The electrotactile display has a simple structure and is therefore
easily controlled. Because of these advantages, the electrotactile
display can be used not only in the contest of physiologicalstudies, but also in the context of technological applications. For
example, Kajimoto et al. [11] developed a haptic augmented
reality system, referred to as “SmartTouch.” The SmartTouch
enables the user to receive sensory information simulating thetouch of a surface via any modality, even for those surfaces thatwould not be touchable. Methil et al. [15] and Sato et al. [18]mounted the electrotactile display onto the control system of arobotic hand and achieved the transmission of spatially distributed
contact information. BrainPort (Wicab, Inc) and AuxDeco(EyePlusPlus, Inc.) are two systems that have been developed that
implement a visual-to-tactile conversion system for use by theblind or visually impaired. These systems convert a camera image
into patterns of electrotactile stimulation and subsequently deliver
them to the tongue or forehead of the user. These applications of
the electrotactile display present contact information in two
dimensions, but not capable of simulating natural or complex
tactile sensation. Recently, Sato et al. [19] and Yoshimoto et al.
[26] attempted to simulate the three-dimensional shape andtexture of the object, respectively, using an electrotactile display.
However, these studies proved incapable of dealing withhorizontal forces, i.e., forces applied parallel to the skin surface.
2.2
Tactile Primary Colors
To produce a natural tactile sensation using the electrotactile
display, Kajimoto et al. [10] proposed the idea of “tactile primarycolors,” an analogy to the three primary colors of vision. The idea
behind this approach is to selectively stimulate each type of
mechanoreceptor beneath the skin. By combining the selective
stimulation of different receptors, natural tactile sensations areperceived by the user. Kajimoto et al. used electrical current
issued from surface electrodes to generate an electric field within
the skin surface, inducing activity with specific nerve fibers.
There are four types of mechanoreceptors under the surface of122

human skin: Meissner corpuscles (RA), Merkel cells (SAI),
Ruffini endings (SAII), and Pacinian corpuscles (PC). In
particular, Kajimoto et al. showed that they were able toselectively stimulate the nerve fibers connected to RA or SAI
receptors, which reside at shallow depths within the skin.
Figure 2 shows the neural responses of RA and SAI receptors
when a given force is applied to the skin. From prior physiologicalstudies, it has been revealed that RA receptors are activated when
the formation of the skin under pressure varies over time [24]. RA
receptors are the most sensitive to 30 Hz vibrations [8].
Furthermore, single-nerve stimulation of RA receptors produces avibratory sensation [23]. Therefore, RA receptors appear to be
critical to the perception of low frequency vibration. In contrast,
SAI receptors have been shown to respond to any given
deformation of the skin [24]. Specifically, Srinivasan et al.
showed that the firing rate of SAI receptors is proportional tostrain energy density (SED) [21]. The sensitivity of SAI receptors
to vibration is almost constant from 0 to 200 Hz [8]. The singlenerve stimulation of SAI receptors generates a pressure sensation[23]. Therefore, SAI receptors appear to be critical to theperception of pressure.
Kajimoto et al. used two coaxial electrodes for selective
stimulation. They found that when a central electrode functions as
an anode and electrical current flows from the central electrode,an acute vibratory sensation is elicited. On the contrary, when thecentral electrode operates as a cathode and current flows to it fromthe outer electrode, a pressure sensation is elicited. Based on theseobservations and the aforementioned characteristics ofmechanoreceptors, it is reasonable to assume that anodic andcathodic stimuli activate the nerve fibers connected to RA andSAI receptors, respectively. In fact, prior electro-physiologicalstudy [17] has provided evidence of this selective stimulation,
supporting our interpretation. In addition, mathematical analysis
of a nerve-fiber electrical model has revealed that anodic stimuli
efficiently activates nerve axons that run orthogonal to the skinsurface, while cathodic stimuli activate those that run in parallel.
These results agree quite well with the anatomical observation [3]Figure 2. Neural response of RA and SAI receptors when aforce is applied [24]. When a force is applied quickly, theresponse ratio of RA receptors increases. The response ratio of
SAI receptors increases at a rate corresponding to themagnitude of the force.

that nerves of RA receptors typically run perpendicular to the skinsurface, while SAI receptors tend to run in parallel.
Figure 3 summarizes the process of selective stimulation. The
anodic stimulation excites vertically oriented nerves, which aremainly connected to RA receptors, and produces a vibratory
sensation. On the other hand, the cathodic stimulation excites
horizontally oriented nerves, which are mainly connected to SAI
receptors, and produces a pressure sensation.
3

ELECTROTACTILE STIMULI TO REPRODUCE DISTRIBUTION
OF THREE-DIMENSIONAL FORCE SENSATIONOn the basis of the concept of tactile primary colors, we propose adesign employing the electrotactile stimulation for thetransmission and perception of the distribution of force vectors.
3.1
Electrotactile Display
The electrotactile display we use consists of a number of 32 pinelectrodes that are arranged as a grid, in order to present various
spatial distributions of force vectors (Fig. 4). In order to enable
the delivery of increased intensities of stimulation, the pinelectrodes are arranged in a hexagonal, close-packed formation. If
the distance between pin-electrodes is small, greater voltage isrequired to deliver a sufficient flow of electrical current aroundthe mechanoreceptors. Therefore, we set the diameter of the pinelectrode to 1.0 mm, and the distance between electrode centers
to 2.0 mm, respectively. The 31 pin-electrodes and single
surrounding electrode are employed as points of stimulation andground, respectively. These electrodes are arranged so as to cover
the entire surface of an adult’s fingertip.
To simulate the spatial distribution of the sensation, we
employed a method of time-division scanning (Fig. 5) [11]. We
set one of the pin-electrodes to be an anode (or a cathode) and thesurrounding pin-electrodes to act as the ground, for a 20 μs period.
The electric current flows from the central pin-electrode to thesurrounding pin-electrodes (or vice versa) and stimulates thenerve fibers. After a short period of time (100 μs), we apply
stimulation via the pin adjacent to the central pin. This 100 μs
interval is essential to achieve complete discharge of the pinelectrode while still preventing the intensity of stimulation frombeing too large. By periodically changing the pin-electrode that isselected for stimulation, we are able to produce electrotactile

Figure 3. Selective stimulation of electrotactile display using
anodic or cathodic electrodes [11].

Figure 4. Electrodes of electrotactile display.

stimulation at any point and in any distribution. When thescanning of the 31 pin-electrodes completes, the pole of theelectrostimulus is inverted, resulting in an alternation between
anodic and cathodic stimuli.
A pattern of stimulation is first generated on a personal
computer and then transmitted to the H8 microprocessor. Then,the microprocessor directs the electrical pulse applied via eachpin-electrode based on the received pattern of stimulation. The
rate of transmission for this process is 30 Hz, with a single
transmission containing up to seven anodic and cathodic
stimulation patterns. Therefore, the maximum frequency of anodic
or cathodic stimulation is 210 Hz. A given stimulation pattern
includes details of the pole setting, as well as the intensity andfrequency of the electrical current, at each pin-electrode. The
range and step size of the intensity of stimulation (i.e. theamplitude of electrical current) are 0.0 – 5.0 mA and 0.02 mA,
respectively.
3.2
Design of Electrotactile Stimuli
To reproduce the distribution of force vectors upon the skinsurface, we integrate the anodic and cathodic stimuli and apply
them to generate neural responses within the RA and SAI
receptors. In the following section, we introduce the design of theelectrotactile stimulation, providing details regarding its
application with the desired spatial distribution, magnitude, anddirection.

Figure 5. Time-division scanning for representation of the spatial
pattern of stimulation [11]. We shift the timing of pulses of eachelectrode to apply the electrotactile stimuli at any given pinelectrode.

1233.2.1
Spatial Distribution
The spatial distribution of force vectors is simply replicated via
the selection of appropriate points of stimulation. Stimulationfrom a center pin-electrode can simulate the contact between thecenter of the fingertip and an object’s edged (Fig. 6a). Stimulationfrom one of the many pin-electrodes can simulate the contact
between an area of the fingertip’s surface and the base of acylindrical object (Fig. 6b).
It should be noted that conventional study [11] has shown thathumans are able to perceive a point of stimulation sufficiently,
from an anodic stimulation, though not from a cathodic one. The
proposed method utilizes both anodic and cathodic stimuli, thus it
is possible that it may fail to properly simulate the spatial
distribution of force. In order to assess this, we will evaluate theperformance of the proposed method in this respect, in section 4.2.
3.2.2
Magnitude
We simulate the magnitude of the force vector via a combinationof vibration and pressure sensations, perceived as variations in thenature of skin deformations and the mere presence of deformation,
respectively.
The basic idea of our proposed design is to simulate the neural
response of RA and SAI receptors, as shown in Fig. 2, using
anodic and cathodic stimuli. Figure 7 shows the design of theelectrotactile stimulation that simulates the desired magnitude offorce. The frequency of the cathodic stimulation is increased
according to the desired magnitude of force and its variation overtime. The anodic stimulation is employed when the magnitude offorce is changing over time, where the speed of this change ismimicked by variation in the intensity of the anodic stimulation.
It should be noted that we adjust the intensity but not thefrequency of the anodic stimulation. While it is possible to change
the frequency of stimulation in order to replicate the desired
neural response ratio of the mechanoreceptors, however, in thecase of the selective stimulation of RA receptors,
electrophysiological experiments have shown that the perceptionof vibration changes with the frequency of stimulation [12]. As
such, it is not possible to adjust the frequency of the anodic
stimulation to manipulate the perceived magnitude of vibration. Itshould also be noted, however, that it was observed that the neural
response ratio of RA receptors appeared to increase with theintensity of the anodic stimulation. Therefore, we manipulate theintensity of stimulation to mimic changes in the magnitude offorce over time.

Figure 6. Example of the simulation of spatial distribution.
Contact with (a) an edged object and (b) the base of cylindricalobject. The dashed line represents contact area of contact withthe object.

124

Figure 7. Design of anodic and cathodic pulse to simulate thedesired magnitude of the force vector.

3.2.3
Direction
When force is applied perpendicular to the surface of the fingertip
as a result of contact with a cylindrical object, the skin isdeformed symmetrically, conforming to the surface (Fig. 8a).
However, when the force is applied both perpendicularly and inparallel to the skin surface, the resulting deformation is not
symmetric (Fig. 8b). The concentration of stress on the surface
accrues in a manner corresponding to the direction of the force.
This concentration of stress appears to result in spatial spread andgradient in the response of mechanoreceptors. Therefore, wesuggest that manipulation of the magnitude of stimulation must be
performed in accordance with the desired concentration of stress,
necessary to reproduce the direction of the force vector. To adjust
the magnitude of stimulation, we employ the method described insubsection 3.1.2.
Figure 8 provides an example of the electrotactile stimulation
simulating the direction of an applied force. We simulated theskin deformation by employing the two-dimensional finger model,
which we developed in a prior study employing finite element
method (FEM) [19]. The physical properties of the finger model
are summarized in [19]. We applied two deformations to thefinger model; one delivering a 1.0 mm displacement,
perpendicular to the skin surface, and the other a 1.0 mm and 0.5
mm displacement, perpendicular and in parallel to the skin surface,respectively. The width of the displacement area was 4.0 mm. We
simulated the deformation between the epidermis and the dermis,
as the RA and SAI receptors lie between these layers of skin. The
results of the simulation, and the electrotactile stimulation basedon them, are shown in Fig. 8. Referring to the figure, the spatial
spread and gradient of the skin deformation can be observed.
Based on the observed patterns of skin deformation, it appears
that our design achieves the electrotactile stimulation necessary tosimulate the desired direction of force.
Our design of the electrotactile stimulation is based on priorphysiological studies that only address simple scenarios; force isonly applied from objects having simple shapes. To employ these
designs in a practical setting, it is first necessary for us to estimate
the response of mechanoreceptors when force is applied fromcontact with an object having a complex shape. We now consider
the use of FEM to simulate the SED within the skin surface, as
well as the response of the mechanoreceptors, which is necessaryFigure 8. Example of the simulation of the direction of a force
vector. Middle graphs depict the deformation of the skinsimulated using FEM.

because of the strong, observed relationship between SED and
activity of the mechanoreceptors [21].
4EXPERIMENT

We now evaluate the ability of our design to simulate the
distribution of force vectors resulting from contact with an object
surface.
4.1
Participants
The participants, two males and two females, were volunteers
aged 20 to 29. Before beginning the experiments, the participants
were briefed on their content. The experiment was conductedusing the electrotactile display detailed in section 3.1. Eachparticipant was instructed to place the tip of his or her index finger
on the electrotactile display. Within the electrotactile display, thethreshold amplitude of electrotactile stimulation varied with eachparticipant. In order to address this, pre-testing was performed inorder to identify the threshold amplitude for each user. During thecourse of the actual experiment, single threshold was applied to
all pin-electrodes.
In the evaluation of the proposed electrotactile stimulation, thequestion of “how participants perceive stimulation” is a difficult
one, due to the ambiguity of the question. Following theexperiment, the qualities of the different stimuli were evaluatedqualitatively, by asking the participants to comment on thesensations they experienced with the application of each.
4.2
Two-point Discrimination
First, we assess the spatial resolution of the proposed electrotactile
stimulation by employing an experiment incorporating two-point
discrimination, in which participants are presented with various
single or two-point stimuli. The position of the stimuli wasselected from among the five points identified in Fig. 9a. Three
distances of two-point stimuli were prepared, as shown in Fig. 9b.
Two patterns of stimulation were prepared, for each number anddistance of stimulation, in order to mitigate any bias in the effectthat might result from the use of a particular stimulation position.
The signals of the stimuli over time are depicted in Fig. 9c; risetime, stable time, and fall time were 200 ms, 1000 ms, and 200 ms,

Figure 9. (a) Electrodes, (b) stimulation pattern, and (c) signal
over time for the two-point discrimination experiment. Note: thepulse of the anodic and cathodic stimuli is an example ofstimulation.

respectively. The frequency of anodic stimulation was 30 Hz. The
frequency of cathodic stimulation, which represents the magnitudeof static force, was 60 Hz. The strength of the anodic stimulation
was determined on the basis of the threshold of the electrotactile
stimulation for each participant, identified during pre-testing.
Each participant experienced the sensation of simulated touch,delivered by randomly selected stimuli, and was subsequently
asked to state the number of stimuli perceived (one or two). Eachpattern was applied five times.
Figure 10 shows the result of all participants. The horizontal
axis represents the distance between the stimuli and the verticalaxis represents the ratio of perceived stimuli reported by the users.
Each dot represents the specific results obtained for a given
participant, while the bars represent their averages. The error bar
represents standard deviation (SD). Participants are able to

Figure 10. Result of two-point discrimination. 0 mm represents asingle-point stimulation.

125

identify the presence of two electrotactile stimuli more than 75%of the time, when these stimuli were more than 4 mm apart.
Employing ANOVA, we determined that significant differences inresults are produced by changes in the distances of the stimuli(F(3, 15) = 7.9, p < 0.01). In particular, the results of multiple
comparisons demonstrate that there are significant differences
between the separations of 0 mm and 4 mm (t(15) = 4.1, p < 0.01)
and again between separations of 0 mm and 6 mm (t(14) = 3.8, p< 0.01).
During the experiment, some participants stated that they
perceived two different types of single-point stimuli: large andsmall. In addition, some participants stated that they felt
differences in the stimuli, depending on their position.
4.3
Magnitude Discrimination
Next, we examine the effectiveness of the proposed design ofelectrotactile stimulation in simulating a given magnitude of force.
We presented participants with the electrotactile stimulation thatsimulates contact with a cylindrical object, as shown in Fig. 6b.
The performance of our approach to electrotactile stimulation was
evaluated with three different magnitudes of force: small, medium,and large. To simulate the magnitude of force, we employedcathodic stimuli with frequencies of 30 Hz, 60 Hz, and 90 Hz. The
intensity of anodic stimulation was determined based on thethreshold of electrotactile stimuli for each participant, identified
during pre-testing, as well as the magnitude of stimulation. The
frequency of anodic stimulation was 30 Hz. The resulting signalof stimulation over time is shown in Fig. 9c. The interval between
the applications of stimuli was 500 ms. Participants were asked totouch two stimuli, selected at random, and to subsequently state
which stimulus was perceived to be of greater magnitude. Eachpossible combination of stimuli was evaluated 10 times.
Figure 11 shows the observed results of all participants. The
horizontal axis represents the combination of magnitude ofstimulation, while the vertical axis represents the percentage ofcorrect answer. The dots represent the actual results, and the bars
represent their average. Error bars represent the SD. Participants
were able to recognize the difference in magnitude between
stimulation 80 % of the time. Using ANOVA, we determined thatno significant difference was obtained between the various
magnitudes of stimulation (F(2, 11) = 1.7, p = 0.26).
A number of participants stated that they were able to perceivea natural sensation of force applied perpendicular to the skinsurface during the experiment, though other participants perceived
a constant vibration when the large magnitude of stimulation wasapplied. The participants also noted that differences were

Figure 11. Result of experiment involving discrimination
magnitudes of stimulation.

126

perceived between the various stimuli, depending on their position.
4.4
Directional Discrimination
Finally, we examine the ability of the proposed design ofelectrotactile stimulation to replicate the direction of force. We
exposed participants to the electrotactile stimulation thatsimulating contact with a cylindrical object as shown in Fig. 6b.
We prepared stimuli replicating three different directions of force.
The directions used were ulnar (left), normal, and radial (right),
with respect to the skin surface. The magnitude of force wassimulated by the application of cathodic stimulation with afrequency of 60 Hz. To represent the concentration of stress, as
shown in Fig. 8, we employed a frequency of 90 Hz. The
frequency of anodic stimulation was 30 Hz. The intensity ofanodic stimulation was determined on the basis of the threshold ofelectrotactile stimuli for each participant, as determined during
pre-testing. The resultant signal of stimuli over time was shown in
Fig. 9c. Participants were asked to touch randomly selected
stimulation and to then state the perceived direction of force. Eachdirection was evaluated 10 times.
Figure 12 shows the results for all participants. The horizontal
axis represents the direction of stimulation, while the vertical axis
represents proportion of correct answers. The dots represent theactual results for each participant, while the bars represent theiraverages. The error bar represents SD. Each participant was able
to identify the correct direction of stimulation, more 70 % of thetime. Using ANOVA, we determined that there is no significant
difference in results obtained between the different directions ofstimulation (F(2, 11) = 0.4, p = 0.69).
Participants commented that they were able to recognize thedirection of force from the spatial spread of the stimulation andthe difference in the intensity of stimulation. Some participantsalso stated that they perceived the movement of the stimuli.
4.5
Discussion
From the experiment involving two-point discrimination, we areable to confirm that participants are capable of discriminating
between two stimuli when they are separated by more than 4.0
mm. As this result is obtained when applying anodic stimulation[11], the proposed design of electrotactile stimulation therefore
appears to have no effect on the spatial resolution of theelectrotactile display. With regard to the mechanical tactile
display, the threshold of two-point discrimination isapproximately 1.5 mm [1]. This value is implemented by setting
the distance between pin-electrodes to 0.75 mm. Furthermore, itappears that participants are capable of recognizing the difference

Figure 12. Result of experiment involving directional
discrimination.

between single-point and 2 mm separated two-point stimuli.
Based on this, we suggest that the proposed design effectively
represents the spatial difference (size) of a contact area via thedifference in distance between pin-electrodes.
The results of the experiment involving discrimination between
intensities demonstrate that the proposed method is capable ofeffectively representing variations in the magnitude of force
vectors. Furthermore, participants are able to feel pressure that is
applied in a direction perpendicular to the skin surface. However,high intensities of cathodic stimulation appear to cause sensationsof vibration, in addition to the sensation of pressure. In order tosimulate a natural tactile sensation, it is necessary for us to firstcarefully determine the stimulation threshold. In this experiment,
we compared only three magnitudes of force. We suggest that, inthe future, this work would benefit from an examination of thecomplete resolution of magnitude that the proposed design iscapable of representing.
The results of the experiment involving the discrimination offorce direction demonstrate that the proposed method is capableof simulating this effectively. However, participants appeared todiscriminate between the applied directions, but instead by the
movement of the locale of stimulation. We infer that the sensationof vibration (i.e. the response of RA receptors) plays an importantrole in the sensation of movement. As such, we must carefully
design the anodic stimulation.
Based on the results of the three experiments detailed herein,
we have confirmed that it is possible to represent the distributionof force vectors using the proposed design. However, it should be
noted that these experiments are relatively trivial. In order to
employ the proposed design in a practical setting, it would benecessary to simulate the response of mechanoreceptors and torepresent nerve activities strictly. It appears to be possible tosimulate the response of mechanoreceptors based on the SED
within the skin surface, employing a FEM model of the fingertip[21].
The experiments demonstrate that participants tend to perceive
differences in intensity of stimulation depending on its position.
The instability of the intensity of stimulation may cause unnaturalelectrical sensations. The reason for this seems to be that the
impedance of the skin varies with to the position. In theseexperiments, we have identified a single threshold of stimulation
for all pin-electrodes. In order to address this issue, we must shift
the threshold of stimulation according to the position of the user’s
finger, in order to reduce the aforementioned electric sensation. In
this experiment, we qualitatively evaluated the sensation resulting
from the proposed approach to electrotactile stimuli. After further
improvement of the electrotactile stimuli, we will proceed toevaluate the resulting sensations by quantitative means.
5

CONCLUSION

In this study, we have attempted to represent the distribution offorce vectors using an electrotactile display. We have proposed adesign of electrotactile stimulation based on the concept of tactile
primary colors. We simulated the peripheral response of RA andSAI receptors using anodic and cathodic stimuli, respectively. The
experimental results showed that the proposed design was feasible
for use in the representation of the distribution of force vectors.
The current design, however, retains a number of problems that
impede its feasibility for actual use. Therefore, we propose two
improvements to our model. First, we suggest simulating the
response of mechanoreceptors strictly, using a FEM. Second, we
modify the electrotactile stimulation so that we are able toperceive a natural tactile sensation, rather than an electricalsensation. We believe that the improved design would be feasible

for use in transmitting the distribution of force vectors andenabling stable interaction with an object.
ACKNOWLEDGEMENT
This research was supported by CREST and Grant-in-Aid for
JSPS Fellows (20·10009).
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