Tactile Feeling Display using Functional Electrical
Stimulation
Hiroyuki KAJIMOTO, Naoki KAWAKAMI, Taro MAEDA and Susumu TACHI
Graduate School of Engineering, The University of Tokyo
7-3-1 Hongo Bunkyo-ku Tokyo, 113-8656 Japan{kaji, kawakami, maeda, tachi}@star.t.u-tokyo.ac.jp
Abstract
We proposed an electro-cutaneous display which stimulate
three types of mechanoreceptors in human skin separately. We call these stimuli “tactile primary colors.” Twomethods are suggested. One is to use array electrodesthat are properly weighted. The other is to use anodic
current stimulation as well as cathodic current. We also
developed experimental system and tested these methods. Qualitative results suggested that the designed stimuli
succeeded in activating desired mechanoreceptors’ axons.
Key words: tactile display, electrocutaneous, array electrodes, anodic stimulation, functional electrical stimulation

1. Introduction“Tactile feeling” is a sensation produced in your skin when
you touch or rub something. If we generate tactile feelingin VR(Virtual Reality) space, we can not only experience
more realistic presence, but also perform more diﬃcult
tasks.
Most previous works on tactile displays [5] are variations of pin-head type, or belt type displays, which movevertically or horizontally on the skin. They are mechanical devices and have some diﬃculties. They are huge,
diﬃcult to fabricate, and not able to produce all kinds ofcutaneous sensations.
Some other studies proposed to use electric current
as a stimulus. Though fabrication became much easier,most of them lacked principle and stayed ad-hoc system,
because they didn’t fully understand what was happening
inside skin.
Our ﬁrst step is to consider mechanisms of tactile sensation. There are four types of mechanoreceptors in human skin. Meissner corpuscle (RA), Merkel cell (SAI),
Ruﬃni ending (SAII) and Pacinian corpuscle (PC). As
population of SAII is small in human ﬁnger, we only consider RA, SAI and PC from now on. In principle, if one
can stimulate these mechanoreceptors separately, combinations of these stimuli should be able to produce any
tactile feeling.
With this in mind, we propose a new electro-cutaneous
display with three modes which stimulate RA, SAI andPC separately. We name these modes “tactile primarycolors” after three primary colors (RGB) which stimulate

diﬀerent types of cone cells in human retina.
Second step is to consider mechanisms of electricalstimulation. We control current source distributed onskin surface and activate innervating nerve ﬁber indirectly. Then, activation probability can be written as a function of current source distribution on the skin. This function will be a clue for designing stimulation modes.
Two methods are presented. One is to use arrayed
electrodes instead of single one. The other is to use anodic
current as a stimulation, as well as cathodic current.
We also developed experimental system and tested theabove methods.

2. Structure of human skinFig.1 is cross section of human glabrous skin(Reconstructedfrom [9],[14]). RA and PC are considered to perceive vibration, while SAI is said to perceive pressure. Sometimes
RA and PC are considered as velocity and acceleration
sensors while SAI is considered as position sensor[10]. Two characteristics are known.
• Each kind of mechanoreceptor resides in diﬀerent speciﬁc depth.
• Nerve axon which is connected to each kind of mechanoreceptor has diﬀerent speciﬁc diameter.

Groove(Fingerprint)

Sweat Duct
Skin Surface

Stratum Corneum
Epidermis
Dermal Papillae

RA

SAI

DermisAdhesive Ridge
Glandular Ridge

PC

Connective
TissueFig.1: Cross section of human skin. RA: Meissner
Corpuscle, SAI: Merkel Cell, PC: Pacinian Corpuscle

Table 1 is depth of mechanoreceptors and diameterof nerve ﬁber which are connected to them. Note that

Table 1: Depth of mechanoreceptors and diameter ofnerve axons in human fingerpad)
Meissner(RA)Merkel(SAI)
PC(Pacinian)

Depth (mm)
0.7
0.9
2.0∼

Skin Surface

Diameter(µm)[13]
3∼5
7∼12
5∼13

Current Source (point)

Ranvier Nodes

Myelinated Nerve
Myelin Sheaths

depths are measured at human ﬁngerpad. It is diﬀerent
in other parts of the body.
Major characteristic of ﬁngerpad skin is the thicknessof stratum corneum (Fig.1). It is about 600µm in ﬁngerpad while 15µm in other part of the body [18],[19]. Depthof dermal papillae is about 700µm and we regard it as the
RA depth. We calculate SAI depth from glandular ridge
height (about 200µm in ﬁngerpad). PC resides just beneath the dermis. It’s depth is about 2 to 3 mm.
Mechanoreceptor axon is classiﬁed as Aβ type, whichis largest in all aﬀerent ﬁbers in ﬁnger. Note that RA axon (4µm) is thinner than other 2 receptor axons (10µm).
There are other aﬀerent ﬁbers such as pain, warmth, andcold. Their diameters are about 1µm. Because of this difference, electrical stimulation of Mechanoreceptor nerves
are much easier than that of other aﬀerent nerves. We
will re-examine this fact in section4.
2.1. Axonal pathway
As we will see in later sections, axonal orientation anddepth are the most important factors for electrical stimulation. Unfortunately, there are very few comparative
works around them and we need some speculations.
RA axon branches from the superﬁcial dermal nerves
and ascend to the epidermis. Therefore, RA axon in ﬁngerpad is perpendicular to the skin surface[14], especiallynear the surface.
Pathway of SAI nerve is not well known, though somedrawings showed that it is horizontally oriented [12]. We
temporary assume it to be horizontal. Most of PC axons
are also horizontally oriented.

Im (x, t) = −∂I(x, t)∂Vm (x, t)= Cm+ Gm Vm (x, t). (1)∂x∂tOutsideIm(x,t)Vm

Gm

Membrance

Cm

Vr

V(x,t)

G

InsideI(x,t)

Myelin Sheath
Node

Fig.2: Electrical stimulation of nerve axon from skinsurface. Cross section and equivalent circuit.

As internal current I(x, t) can be expressed by internalpotential gradient,I(x, t) = −G

∂V
.
∂x

(2)

Substituting Eq(2) into Eq(1),G∂Vm (x, t)∂2 V+ Gm Vm (x, t).
= Cm
∂x2
∂t

(3)

Though nonlinear property of ion channels (expressedby Gm ) plays fundamental role in ﬁring and propagationof signals[4], we assume nerve axon to be linear, time invariant system (Speciﬁcally, assume Gm to be constant).
Then we can do some analysis as follows.
From now on, diﬀerential is expressed by suﬃx. Expressing Vm by V − Ψ,

3. Principle of electrical nerve stimulation
In this section, we show principle of electrical stimulation.
Most derivations are from [16] and [17].
First, we model nerve axon (Fig. 2)[1].We take the xaxis in the direction of nerve ﬁber. Membrane is expressedby membrane capacitance (Cm ), membrane conductance(Gm ) and internal conductance(G). Let electric potential
outside and inside membrane be Ψ(x, t), V (x, t), potential
diﬀerence V − Ψ be Vm (x, t). What we can do is to applyΨ(x, t) and manipulate Vm (x, t) indirectly.
From Kirchhoﬀ’s law, membrane current density Im (x, t)
must be equal to the leak of internal current I(x, t) along
x direction. Im is also the sum of current ﬂowing Cm and
Gm . Hence,xΨ(x,t)


where λ =− λ2 Vxx + τ Vt + V = τ Ψt + Ψ(4)GGm , τ = Cm /Gm .

By taking the Fourier transform of Eq(4) with respect
to x, we get
λ2 ω 2 Ṽ + τ Ṽt + Ṽ = Ψ̃ + τ Ψ̃t(5)

where Ṽ and Ψ̃ are the Fourier transform of V and Ψ,
while ω is corresponding spacial frequency.
By taking Laplace transform of Eq(5) with respect totime t, we get¯(λ2 ω 2 + sτ + 1)Ṽ¯ = (1 + sτ )Ψ̃

(6)

¯
¯
where Ṽ and Ψ̃ are the Laplace transform of Ṽ and Ψ̃.
From Eq(6),Ṽ¯ (s, ω) =1 + sτ¯ ω).
Ψ̃(s,1 + sτ + λ2 ω 2(7)

Substituting Vm = V − Ψ , we get¯Ṽ m (s, ω) =−λ2 ω 2¯Ψ̃(s, ω).
1 + sτ + λ2 ω 2(8)This is the basic equation which express relationship between Vm and Ψ.
Assume Ψ(x, t) can be expressed by w(t)u(x). ThenΨ(x, t)¯Ψ̃(s, ω)=w(t)u(x)(9)=w̄(s)ũ(ω).

(10)

From now on, we consider the impulse input case, orw̄(s) = 1.
Substituting w̄(s) = 1 into Eq(8),Ṽ¯ m (s, ω) =−λ2 ω 2ũ(ω).
1 + sτ + λ2 ω 2(11)

By inverse Laplace transform,−λ2 ω2λ2 ω 2exp τ t ũ(ω).
(12)τ
As Vm is obviously monotonically decreasing alongtime, maximum value of Vm can be obtained by substituting t = 0.

Ṽm (t, ω) = −Ṽm (t = 0, ω) = −λ2 ω 2ũ(ω).
τ(13)

4.2. uxx (x)By inverse Fourier transform,λ2
uxx (x).
Vm (t = 0, x) =τwhere G and Cm are internal conductance and membrane
capacitance of axon with unit length.
Assume that the nerve is non-myelinated (or all part ofmembrane is uncovered). If axonal diameter is increased
by n times, G and Cm becomes n2 and n times bigger.
Therefore, activating function is multiplied by n.
However, most mechanoreceptor ﬁbers are myelinated (or most part is sealed and only “Ranvier node” isuncovered). In this case, when axonal diameter becomesn times bigger, Cm doesn’t change because gap length ofRanvier node is constant and distance between each nodeis proportional to diameter. G becomes n2 times bigger
and hence, activating function is multiplied by n2 .
When you compare myelinated and non-myelinatedﬁbers with the same diameters, activating function of
myelinated axon is always much bigger than that of nonmyelinated one, because Cm for non-myelinated axon istypically 101 to 103 times bigger.
As mentioned in section2., axonal diameters of mechanoreceptors are about 101 times bigger than other aﬀerent
ﬁbers (pain, warmth, cold, etc). Furthermore, mechanoreceptor axon is myelinated while other aﬀerents are nonmyelinated. In conclusion, activating function of mechanoreceptors are much bigger than that of other aﬀerents sothat we can selectively stimulate mechanoreceptor nerves
easily.

(14)

Right hand side of Eq(14) is called “activating function”(AF),or sometimes refered to as “stimulating function”[2],[3]. It
is the measure of maximum membrane voltage and membrane current when impulse external stimulation is applied, and hence, good criterion to decide whether considering nerve will ﬁre or not.
Following properties are observed.

Second part of activating function is second derivative ofelectric potential along axon. Electric potential is generated by input current from skin surface. Therefore, activating function should be expressed by current source
distribution on the surface.

I• It is proportional to uxx (x), which is second spatial
derivative of electric potential along ﬁber.
2• λτ is unique property of each nerve axon.
Note that assuming nerve will ﬁre at any point of axon, we can judge wether it will ﬁre or not by evaluating
maximum value of activating function along ﬁber. This
argument will be repeatedly used in later sections.

(-)

xρ :resistivity

R

i=

Depth
y

I
2πR
Nerve Axon

4. Activating function
In this section, we observe activating function more carefully by decomposing Eq(14) into two parts.
4.1.

λ2
τ

Fig.3: Current stimulation from skin surface. 2D,
single line electrode case2First part of activating function is λτ , which is uniqueproperty of each axon.
λ2
τ

=
=

Gm G
Cm Gm
G
Cm

(15)
(16)

First we consider single line electrode with cathodic
current I(Fig.3). For simplicity, we assume uniform inﬁnite space. Take x axis parallel to skin surface and y axis
in the direction of skin depth. Letting the electrode coordinate be origin, current density i at (x, y) is expressedas

i(x, y) =Hence,

I,
2πR

x2 + y2 is distance from electrode. Electricwhere R =potential Ψ at (x, y) isΨ(x, y)

 R

Ed

−

(18)

−

iρdR

(19)

I
ρdR
2πR
R

(20)

R
=

−

i=1

∝
∝

(24)

Ii

y2 − (x − xi )2,((x − xi )2 + y2 )2

(25)

I1

I2

I4

I3

I5

I6

d
X

∂ 2 Ψ(x, y)∂x2
2y − x2.
(x2 + y2 )2

x1

(22)

x2

x3

x4

x5

x6

Fig.5: 1-D arrayed electrodes(23)

5. Three stimulation modes, or “tactile primary colors”

stimulating function for horizontal axon, cathodic current
3.5

3

As discussed before, if we can stimulate SAI, RA andPC separately, combinations of these stimuli, or “tactile
primary colors” should be able to produce any cutaneous
sensation. We call each of these colors “RA mode”, “SAImode” and “PC mode”. From now on, utilizing activating
function introduced in section4.
, we show how to create suchmodes.

2.5

depth(mm)y2 − (x − x )2dx .
((x − x )2 + y2 )2where Ii the current from ith electrode, M the number ofelectrodes and xi the coordinate of ith electrode.

where ρ is resistivity.
Assuming that electric potential of inﬁnite distance is0, we get−ρI log(R)Ψ(x, y) =.
(21)
2π
Now we will calculate activating function (AF). If theaxon lies in x direction,AFMAF (x, y) ∝


=

I(x)

AF (x, y) ∝For arrayed electrodes case(Fig.5),


=



(17)

2

1.5

1

0.5

5.1. RA mode by anodic current
0
-3

-2

-1

0
horizontal displacement(mm)

1

2

3

Fig.4: Activating function for horizontal axon. Cathodic (-) single current case.

First we consider RA mode, which stimulate RA axon uniquely. The fact that direction of RA axon is perpendicular to skin surface changes the activating function dramatically (Fig.6).

Single Electrode(Anodic Current Source)

Fig.4 is the plot of Eq(23). Following two things areobserved.
• Activating function takes the maximum value whenx = 0. It is directly under electrode. As AFx=0 ∝1/y2 , it decreases in inverse proportion to square ofaxon depth y. It means shallower part of nerve iseasier to stimulate.
• This is the activating function for horizontal axon, cathodic single current case. Therefore, if current becomes anodic(+), plot will be reversed and activating
function will have minus value. This is the reason why
most previous works on functional electrical stimulation used cathodic current as stimulus.
Eq(23) is for the case when current source distributionis Iδ(x). In general, it is distributed on the skin surface.

Skin Surface
RA

Stimulating Function
SAI,PC

Fig.6: Cathodic current stimulation. Activating function of horizontal axon is positive while that of vertical axon is negative.

As discussed in section4.
, activating function is second
spatial derivative of electric potential along ﬁber. Therefore, AF of horizontally oriented axon is d2 V /dx2 whilethat of vertical axon is d2 V /dy2 . From Gauss’s law,
d2 V
d2 V
+
=0
2
dx
dy 2

(26)

SingleElectrode

Current Direction

Skin Surface
Stimulating Function
SAIPC

for space where no electric charge ﬂows out. Therefore,
d2 V
d2 V
=
−
dy 2
dx2

(27)

always holds true. Hence ordinary cathodic current can’t
stimulate vertically oriented nerve axon such as RA, because activating function takes negative value.
Now we will use anodic (positive) current as stimulus. Then electric potential is reversed so that activating
function is reversed (Fig.7). In this case only verticallyoriented nerve axon is stimulated while horizontally oriented axon is suppressed. This is the principle of RA
mode.

Fig.8: Merkel and Pacini stimulation by single electrode.

Adding anodic current around central cathodic current, attenuation of activating function becomes faster sothat we can stably stimulate only SAI (Fig.9). This is
the principle of SAI mode. Note that in this case, “array” is not used to express pixels (which is the case ofmany previous electrocutaneous displays), but as a kindof beamformer which form required activating function atrequired depth.

Single Electrode(Cathodic Current Source)

ArrayedElectrodes

Skin Surface
RA
SAI

Stimulating Function
SAI,PCPC

Fig.9: SAI stimulation by arrayed electrodes.

5.3. PC mode by weighted array electrodes

Fig.7: Anodic current stimulation. Activating function is reversed and only vertical axon is stimulated.

Kaczmarek[6] has reported that in his experiment, heobserved that anodic pulse, as well as cathodic one seemed
to evoke some kind of sensation. Though he was not awareat that time, we suppose he was stimulating RA axon.
5.2. SAI mode by weighted array electrodes
Second, we consider SAI mode, which stimulate SAI afferent ﬁber uniquely. SAI and PC ﬁbers are horizontallyoriented. Thought RA axon is also horizontal in deep dermal region (Fig.1), we don’t need to consider it because
diameter of RA axon ﬁber is about half of that of PC andSAI(Table1) and hence, activating function is one-fourthat the same depth (as we discussed in section5.
). We utilize
the fact that SAI axon resides in shallower part than PC
axon.
Fig.8 is activating function of SAI and PC when single
cathodic current is used. As SAI is shallower than PC,
activating function of SAI is bigger than PC so that we
can stimulate SAI separately. Furthermore, If we use arrayed electrodes, We can make activating function of SAI
much bigger while suppressing that of PC.

Last one is PC mode. As PC resides deeper than SAI,
and axon diameter of PC is about the same as SAI, we
can’t stimulate PC separately because activating functionof PC has smaller maximum value than that of SAI.
Proof of the above argument is as follows.
∂ 2 AF∂ 2 AF
+
2
∂x
∂y2

∝
=
=
=

∂2 ∂2 V∂2 ∂2 V
+ 2
2
2
∂x ∂x
∂y ∂x22
2∂2 ∂2 V
∂ ∂ V
+
2
2
∂x ∂x
∂x2 ∂y2
2
2
∂2 V
∂ ∂ V
(
+
)
∂x2 ∂x2
∂y2
0

(28)
(29)
(30)
(31)

where AF represents activating function for horizontallyoriented axon. Last equation is obtained by using Gauss’slaw (Eq(26)). Therefore, activating function itself is harmonic function. Let depth of SAI and PC be y1 and y2 .
y1 > y2 because PC resides deeper than SAI. If you set
y = y1 as a boundary and consider y <= y1 region, “principle of maximum value” for harmonic function states that
Harmonic function is a function which satisfy Laplace
equation.

ArrayedElectrodes

V

Pulse Signal

t

+150V

Weight Signals

the activating function takes maximum and minimum value only on the boundary. Therefore, activating functionof SAI takes larger maximum value than that of PC.
However, we can stimulate PC, at least as good as
SAI by using arrayed electrodes.
Contrary to SAI mode, we add cathodic current stimulation around central cathodic current, so that electrodesize becomes virtually bigger. Then, attenuation of activating function becomes slower and PC will have as big
activating function as SAI (Fig.10). This is the principleof PC mode.

+-

1mm pitch
10mm width-150V

Analog Multiplier

V-I conversion unit

Fig.11: System configuration.

Current when weight=1.0

Adjustable(0~1kHz)

SAI

200µs fixed(Adjustable)

PC
Time

Equal Areas

Fig.10: Pacini stimulation by arrayed electrodes.

In Pacinian mode, Merkel is also stimulated. However, in some physiological papers, it was shown that
SAI always ﬁres when PC ﬁres, at least to the extent that frequency of mechanical stimulation is less than
200Hz[12]. Therefore, for the present, we think it can becalled Pacinian mode.

6. Design and fabrication
We constructed experimental system shown in Fig. 11.
Multiplying 1 channel high speed bi-phasic signal by eightchannel low speed weight signals, we got synchronizedeight waves. V-I converters were used to change themto current stimuli. Subject put his ﬁnger on arrayed electrodes while he wore metal ring which was connected to
ground.
Arrays were composed of eight line electrodes whichwere equally spaced. Distance between each electrode is
1mm and electrode size is 0.5mm × 10mm.
For safety, some restrictions were imposed. First, maximum current which ﬂows from one electrode was limited
to 2mA. Second, sum of the eight weights was set to 0 sothat current was conﬁned locally. Third, time average ofbi-phasic pulse was also set to 0 so that electric charge
didn’t accumulate on the skin.
Fig. 12 is the waveform of high speed bi-phasic signal. Though it seems negative (cathodic) pulse, you canchange it to positive (anodic) pulse by multiplying negative weight. Main pulse duration was ﬁxed to 200 µs
tentatively.

Adjustable(0~2mA)

Fig.12: Waveform of input pulse.

• Sum of array weights is 0, as discussed before.
Optimization problems are as follows.
For RA mode:maxx,y (AFP C , AFSAI )subject towi = 0w
maxx,y (AFRA)(32)
where AFRA, AFSAI and AFP C are activating functionsof RA, SAI and PC, while w
 is the weighting vector to
obtain.
Numerator and denominator of Eq(32) are maximum
values of PC, SAI and RA activating function along axon.
x and y take the coordinates where axons exist. Eq(32)tries to suppress maximum values of AF for PC and SAI
while preserving that of RA, and hence stimulate only RA
ﬁber.
In the same manner, for SAI mode:
min

min
w

maxx,y (AFP C , AFRA)
maxx,y (AFSAI )(33)

maxx,y (AFRA , AFSAI )maxx,y (AFP C )(34)

and for PC mode:minw

are the optimization problems. Solving these problems
numerically, we obtain optimized array weights.

6.1. Obtaining array weights
In section 5., we saw the principles of three stimulation
modes. To obtain actual array weights, we must solve optimization problems. Tentative restrictions are as follows.
• Distance between each array is ﬁxed.

7. Experimental result and discussion
During experiment, some qualitative phenomena are found.
They seem to support that each mode is stimulating desired mechenoreceptor’s ﬁbers.

7.1. Sensation shift phenomenon in PC mode
In PC mode, current was set to zero at ﬁrst and gradually increased by subject himself. One interesting phenomenon was observed. The subject felt vibration, andthe point of sensation was not just below the electrodes
but always 1-3 mm shifted to ﬁngertip direction (Fig. 13).
It convinces us of the fact that the current doesn’t stimulate mechanoreceptor itself, but stimulates axon (which
was connected to the receptor), which coincides with ourprevious assumption.

Electrodes
Sensation Point

Nerve Bundle

Corpuscle

Spike Elicited Point
Bottom View

case, we didn’t change stimulating frequency while moving ﬁnger. Therefore, subject’s brain guessed that it was
soft material. This is the reason why subject felt “softmaterial” feeling.

StaticChange Pressure

Knife-edge feeling

Elastic rod feelingFig.14: SAI mode experiment. Subject felt knife-edge
at first, but when he slightly moves his finger up anddown, it changed dramatically to elastic rod thoughcontacting area didn’t change.

Side View

Fig.13: Sensation shift phenomenon. Though nerveaxon is stimulated, evoked sensation point is 1-3 mm
shifted to fingertip direction

7.2. Pressure sensation in SAI mode
In SAI mode, when current was gradually increased from0, sensation changed as follows.
• At 0.2mA (cathodic current of central electrode), very
tiny tremble sensation occurred.
• At 0.4mA, subject felt pressure in the shape of central electrode. As we used line electrodes, it was liketouching knife-edge.
• At 0.6mA, new sensation of vibration was evoked, as
well as pressure.
Stimulating frequency was always 200Hz. It is reasonable
that SAI mode evoked pressure sensation, because many
previous works [7],[8],[11],[12] suggested that Merkel cell
senses pressure.
Vibratory sensation evoked after pressure might havebeen caused by PC stimulation, because SAI and PC are
the same cathodic stimulation.
7.3. Sensation of soft material in SAI mode
As mentioned before, when pressure sensation was stably presented in SAI mode, subject felt knife-edge shapedpressure. Then, he was asked to move his ﬁnger up anddown slightly while contact area didn’t change so much.
At this instance, feeling of knife-edge suddenly changed
to soft elastic rod (Fig.14).
It is explained as follows. Many works on nerve information coding suggested that Merkel cell changes pressure
to ﬁring frequency [12]. Now, assume if one touches hard
object. Then one’s ﬁnger movement directly aﬀects pressure inside skin, so that ﬁring frequency changes. On theother hand, if one touches soft material, though one’s ﬁnger moves up and down, ﬁnger pressure doesn’t change somuch and hence, ﬁring frequency doesn’t change. In our

7.4. Vibratory sensation in RA mode
In RA mode, subject felt vibration and that sensation
was quite stable. If the pulse frequency was less than 100
Hz, generated sensation was quite the same as touchingvibrating speaker cone. However, if the frequency was
higher than 200Hz, subject received strange feeling which
was quite uncomfortable.
It may be explained as follows. We were stimulating
RA axon. RA is known to respond to low-frequency mechanical vibration (20-70Hz) [12] and at that range, ﬁring
frequency is the same as vibration frequency. Therefore
in our case, subject felt vibratory sensation of the same
frequency as electrical stimulation. However, stimulationof RA by more than 200Hz was unnatural situation, whichlead to uncomfortable feeling.
We stated in section 2.1., that direction of SAI axon
is not exactly known but temporary assumed to be horizontal. If it is vertically oriented, it should also ﬁre in
RA mode, because RA mode stimulate vertically oriented
axons. However, in this experiment, the subject only felt
vibration and pressure sensation was not generated. This
fact seems to support our assumption. The other possibility is that pressure sensation might have been masked
by vibratory sensation. In either case, we can say thatRA stimulation mode is made.

8. Conclusion
We proposed a new electro-cutaneous display withthree modes which stimulate RA, SAI and PC separately.
We introduced “activating function” as a measure for nerve activation. Using this function, we designed current
source distribution on skin surface for three modes.
For RA mode, we used anodic current to stimulate
only vertically oriented axon. For SAI and PC modes, weused array electrodes which are properly weighted.
We also constructed experimental system. In the experiment we found three phenomena. Sensation shift inPC mode, Pressure feeling in SAI mode and Vibratory
feeling in RA mode. These phenomena suggested thatour designed “primary colors” succeeded in stimulating
desired mechanoreceptors.

References
1. D. R. McNeal, “Analysis of a Model for Excitationof Myelinated Nerve,” IEEE Trans. Biomed. Eng.,
vol.BME-23, No.4, pp.329-337, Jul 1976.
2. F. Rattay, “Electrical Nerve Stimulation,” SpringerVerlag 1990
3. F. Rattay, “Modeling Axon Membranes for Functional Electrical Stimulation,” IEEE Trans. Biomed. Eng., vol.40 No.12, pp.1201-1209, Dec 1993
4. B. Frankenhaeuser and A. F. Huxley, “The Action
Potential in the Myelinated Nerve Fibre of Xenopus
Laevis as Computed on the basis of Voltage Clamp
Data,” J.Physiol. 171, pp.302-315 1964
5. K. A. Kaczmarek, “Electrotactile and Vibrotactile
Displays for Sensory Substitution Systems,” IEEE
Trans. Biomed, Vol.38, No.1, pp.1-16 Jan 1991
6. K. A. Kaczmarek, Mitchell E. Tyler, and Paul Bachy-Rita, “Electrotactile Haptic Display on the Fingertips: Preliminary Results,” Proc. 16th Int. Conf.
IEEE Eng. Med & Bio Soc, (Baltimore), pp.940-941
Nov 1994
7. J. Ochoa and E. Torebjork, “Sensations Evoked by Intraneural Microstimulation of Single Mechanoreceptor Units Innervating the Human Hand,” J.Physiol.
342, pp.633-654 1983
8. A. B. Vallbo, “Sensations evoked from the glabrousskin of the human hand by electrical stimulation ofunitary mechanoseisitive aﬀerents,” Brain Research,
215, pp.359-363 1981
9. A. B. Vallbo, “Properties of cutaneous mechanoreceptors in the human hand related to touch sensation,” Human Neurobiology, 3, pp.3-14 SpringerVerlag 1984
10. P. R. Burgess and E. R. Perl, “Cutaneous Mechanoreceptors and Nociceptors,” in “Somatosensory System,” Springer-Verlag 1973
11. A. Iggo, “The Structure and Function of a Slowly
Adapting Touch Corpuscle in Hairy Skin,” J.Physiol,
200, pp.763-796, 1969
12. A. Iggo, “Sensory Receptors in The Skin of Mammals
and Their Sensory Functions,” Rev. Neurol. (Paris),
141, 10, pp.599-613 1985
13. J. Zelena, “Nerves and Mechanoreceptors,” Chapman& Hall, 1994
14. Z.Halata, “The Mechanoreceptors of the Mammalian
Skin Ultrastructure and Morphological Classiﬁcation,” Springer-Verlag 1975

15. I. Nilsson and C. H. Berthold, “Axon classes and internodal growth in the ventral spinal root L7 of adultand deevloping cats,” J. Anat., 156, pp.71-96 1988
16. J. T. Rubinstein and F. A. Spelman, “Analytical Theory for Extracellular Electrical Stimulation of Nerve
With Focal Electrodes, I Passive Unmyelinated Axon,” Biophys.J. Vol.54, pp.975-981 Dec 1988
17. J. T. Rubinstein, “Analytical Theory for Extracellular Electrical Stimulation of Nerve With Focal
Electrodes, II Passive Myelinated Axon,” Biophys.J.
Vol.60, pp.538-555 Sep 1991
18. K. A. Holbrook and G. F. Odlond, “Regional Diﬀerences in the Thickness of the Human Stratum
Corneum - an Ultrastructural Analysis,” J. Invest.
Dermatol., 62, pp.415-422 1974
19. W. Montagna and W. C. Lobitz, “The Epidermis,”
Academic Press 1964