Abstract -T h e determination of impedance is performed by means of the frequency domain analysis or the time dom a i n analysis. T h e latter has an advantage of being able to measure instantaneously the whole frequency characteristics of impedance. T h e method is hence powerful t o obtain the biological impedance which changes with time. An instantaneous measuring method by using the time domain analysis has been then developed. Fast Fourier Transformation(FFT) of indicia1 response f o r current to the skin can determine the palm impedance. This method can carry o u t the determination of the parameters of the palm skin impedance during a galvanic skin refEex(GSR), which is impossible by means of the frequency d o m a i n analysis

Hiromi Isshiki

Takao Nakamura

Yoshitake Yamamoto

Yoshitake Yamamoto &apos

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Engineering
Electrical Engineering fields
Okayama University Year 1996
Instantaneous measurement of electrical
parameters in a palm during
electrodermal activity
Yoshitake Yamamoto∗ Hiromi Isshiki†
Takao Nakamura‡
∗Okayama University
†Takuma National College of Technology
‡Okayama University
This paper is posted at eScholarship@OUDIR : Okayama University Digital Information
Repository.
http://escholarship.lib.okayama-u.ac.jp/electrical engineering/55
Instantaneous ,Measurement of Electrical Parameters 
in Palm During Electrodermal Activity 
Yoshitake YAMAMOTO ', Hiromi ISSHIKI and Takao NAKAMURA 
Department of Electrical and Electronic Engineering, Okayama University 
Takuma National College of Technology 
School of Health Sciences, Okayama University 
Tsushima-Naka 3-1-1, Okayama 700, Japan  
Koda 551, Takuma-cho, Mitoyo-gun, Kagawa 769-11, Japan 
Shikata-cho 2-5-1, Okayama 700, Japan 
Abstract - T h e  de terminat ion  of impedance is performed 
by  means  of the  f requency  domain  analysis or the t ime  do- 
m a i n  analysis.  T h e  latter has  an  advantage o f  being able to  
measure ins tantaneous ly  the  whole f requency  characteristics of 
impedance. T h e  method i s  hence powerful t o  obtain the  bio- 
logical impedance which changes wi th  t ime .  An ins tantaneous  
measur ing  method by  using the t ime  domain  analysis has  been 
then  developed. Fast Fourier Trans format ion (FFT)  of ind i -  
cia1 response f o r  current  to the  sk in  can  de termine  the palm 
impedance. Th i s  method can  carry o u t  the de terminat ion  o f  
the  parameters  of the  palm sk in  impedance during a galvanic 
sk in  refEex(GSR), which i s  impossible by  means  of the f requency  
d o m a i n  analysis.  
I .  INTRODUCTION 
T h e  determination of impedance is performed by means 
of the frequency domain analysis or the time domain 
analysis. In the frequency domain analysis, since mea- 
surements are carried out with changing a frequency, 
it is difficult to  obtain the whole frequency character- 
istics of impedance in a n  instant. In particular, skin 
impedance, which varies every moment, makes a restric- 
tion in measurement, and frequency characteristics of the 
skin impedance during a phasic electrodermal activity are 
very hard to  determine[l]. 
By contrast, the  time domain analysis has an ad- 
vantage of being able to  determine instantaneously the 
whole frequency characteristics of impedance[2]. This 
method which determines the impedance with the tran- 
sient response,for current to  the skin can follow the rapid 
impedance variations. We next consider GSR(ga1vanic 
skin reflex) which is a transient variation of electrodermal 
activity. Since GSR is a n  attractive phenomenon which is 
connected with many kinds of external stimuli, researches 
into the phenomenon has been carried out in many fields. 
However, the variation of the frequency characteristics for 
the skin impedance during a GSR did not complete en- 
tirely satisfactory measurement. We have therefore devel- 
oped a transient response measurement system which can 
determine impedance instantaneously by using the time 
domain analysis. This paper describes the  complete mea- 
surement system, and investigates the problem relating to  
linearity of skin in its application. An example which the 
variation of the skin impedance during a GSR is presented. 
11. MATERIALS A N D  METHODS 
A .  Prznczple of Determaning I m p e d a n c e  
Suppose that  skin is a linear system, Fourier transfor- 
mation of the current applied to  the skin i(t) and its volt- 
age response v ( t )  shown in Fig.1 are I ( j w )  and V ( j w ) ,  
respectively. Then the skin impedance Z ( j w )  is expressed 
by the following equation: 
When the current is unit impulse S ( t ) ,  
d skin 
Fig. 1.  Relationship between current i ( t )  and voltage v ( t )  of 
the skin. 
0-7803-2615-6/95/$4.00  1995 IEEE 602 
three electrode syStem 
elec.1 : current and potential electrode 
elec.2 : potential elecirode , elec.3 : ground electrode 
Fig. 2. Block diagram of measurement of electrical parameters in palm skin. 
and v ( t )  is equal t o  an impulse response h(t) .  Hence, the 
skin impedance Z ( j w )  can be determined by performing 
the Fourier transformation of the impulse response as fol- 
lowing equation: 
( 3 )  
m 
Z ( j w )  = 1 h(t)e-jwtdt. 
Using the discrete style, we can obtain the impedance as 
follows: 
N-1 
Z ( j 2 m A f )  = At h ( m A t ) e - j 2 " v  (4) 
m=O 
where At = sampling time, N = data  points and A f  = 
1 / (  N A t ) .  
However, it is difficult to  generate atn impulse signal 
and then obtain the impulse response. Consequently, we 
measured an indicial response since the impulse response 
h(t)  can be expressed by using the indicial response A ( t ) ,  
which is easily measured, as follows: 
( 5 )  
B. Measurement System 
Fig.2 is a block diagram of the complete system of mea- 
suring the transient response. The output step voltage 
from the generator is converted into a current of the same 
phase angle and waveform by a voltage to  current con- 
verter, and this current flows through the skin via elec- 
trode 1 and 3 .  The voltage drop, whiclh is a indicial re- 
sponse, of impedance of skin under the electrode 1 is am- 
plified by a differential amplifier. The response waveform 
which is quantized by A/D converter is inputted to  a per- 
sonal computer. The sampling interval is 50 ps or 200 
ps. The  da ta  are saved in a floppy disk and analyzed by 
a personal computer after measurement. The  sequence 
of the analysis by the computer are as follows : e( 1) 512 
sampling points aire picked up from quantized response 
waveform in flopp,y disk. Those data  are differentiated 
numeric ally. The numerical difference method is central 
differences. Those da ta  are treated as impulse response. 
e ( 2 )  The irnpulse responses are calculated by means of 
Fast Fcwier Transformation (FFT) .  The impedance of 
skin is determined. e ( 3 )  The parameters of Cole-Cole arc 
are calculatted using determining impedance. 
We adopted a three-electrode technique as an electrode 
system. An Ag-AgC1 electrode (Skin surface electrode, 
Nihon Kohden Co.Ltd, Japan) ,  which is an unpolarizable 
electrodle, lOmm in diameter, was used. The electrode 
paste was redux cream (Electrode electrolyte, Hewlett 
Packardl, 17 .S .A) 
111. R.ESULTS A N D  DISCUSSIONS 
A .  Nonlinear Electrical Property of the Skzn 
In transient response, the object must be a linear sys- 
tem. We therefore need to  investigate the linearity of the 
skin impedance, which essentially has a nonlinear system. 
603 
0 
n > 
> 
v 
-1 
0 0.1 0.2 0.3 0.4 0.5 0.6 
0- 
n > - -1 
> 
- 
c : r 
Fig. 3. Waveforms of repetitive indicial responses. (a) I = 
-lOpA/cmz; (b) I = -50,uA/cm2 
The linearity of the skin impedance in indicial response 
depends on flow current. Fig.3 shows the waveform of 
indicial response, when the rectangular current continu- 
ously flows t o  skin. The period is 1 s. The flowing period 
is 0.5 s. When current density is -10 ,uA/cm2 as shown 
Fig.3(a), peak-to-peak values are constant. The potential 
level (SPL) in the repetitive response do not change be- 
fore and after flow. On  the other hands, when flow current 
density is -50 pA/cm2 as shown Fig.S(b), The potential 
level of the repetitive response becomes gradually smaller. 
The skin impedance is not influenced by the countinous 
flow current -10 pA/cm2. Steady waveforms of indicial 
responses can be sufficiently obtained during several sec- 
onds of GSR appearance. 
Fig.4(a) shows indicial response of skin impedance on 
forearm using several kinds of current values. There is 
a linear relationship between flow current and voltage of 
response when the current is smaller than -10 pA/cm2. 
0 
> - -1 
> 
-2 
2 
n > 
- 1  
> 
- 3 0 p A  
J o I t ~ s l s  
0 0.01 0.02 0.03 0.04 
time (SI 
/ /  1 P 0 t=0.001 (s) 
0 t=0.002(s) 
0 10 20 30 40 50 
Fig. 4. Nonlinearity of indicial response of the skin. (a) re- 
sponses for various current values; (b) linearity of response 
The linear relationship disappears in current of larger 
value. Fig.4(b) shows the relationship between flow cur- 
rents and voltages which are the values a t  the time t = 
0.001s, 0.002s, 0.020s in each waveform of respose of Fig. 
4(a). If skinimpedance has a linear system, there is a lin- 
ear relationship between current and voltage using a pa- 
rameter of time t .  We accordingly find there is a linearity 
a t  -10 pA/cm2 and the nonlinearity appears above -20 
pA/cm2. Skin impedance is greatly influenced by mea- 
surement conditions. The current of small value should 
not be used in order to avoid the influences caused by 
noize from the standpoint of measurement. Then appro- 
604 
priate current value also changes in every cases. 
B. Accuracy of the Measurement Method 
In order to  investigate the accuracy of impedance mea- 
surement by a transient response method, RC parallel cir- 
cuit is measured. I t  is well known that  equivalent elctrical 
model of simple RC parallel circuit approximately repre- 
sents electrical characteristics. Considering the  property 
of real skin impedance, the value of circuit elements were 
given as 50 k 0  resistance and 0.022 pF capacitance. The 
indicial response of this model circuit by flow current -10 
,uA/cm2 was recorded. Sampling interval A t  is 50 pA,  
sampling number is 512. The  impedance was determined 
the method as previously stated. Fig.5 shows vector loci 
of theoretial values and measured values by transient re- 
sponse method. The  vector locus of measured value is well 
similar to  that  of the theoretial values. It shows arc locus 
of perfect semi-circle'. The  determination of impedance 
means the determination of arc locus. The measured value 
has some dispersion differences in frequency range above 1 
kHz. I t  is not serious problem because differences in high 
frequency range do not influence to  the determination of 
arc locus. I t  is difficult to  obtain accurate value in the 
area of high frequency by calculating FF'I'. But  consider- 
ing following two points : measured frequency points from 
0 Hz to  400 Hz plot about 3/4 arc and the vector locus 
of skin impedance pass the origin, the arc locus can be 
determined accurately in measurement conditions with a 
little errors in that  frequency range. The results of the 
measurement tell us that  the relative error of measured 
50r---~ 
40t 1 
- 1  I 
0 0  
0 
1 I I 
20 30 40 
resistance(k Q )  
Fig. 5. Measurement result of equivalent circuit. o : measured 
value; : theoretical value; Af = 39.1Hx 
I 40 
0 
resistance(k Q) 
Fig. 6. Skin impedance on the palm site 
value to  theoretical value is less than 2 %. The accuracy 
is enough to  measure impedance. 
Fig.6 shows the results of the measurement of skin 
impedance. The flow current is -10 pA/cm2,  indicial re- 
sponse is, measured with 50 ps sampling interval and 512 
sampling, point. Although there are differences in high fre- 
quency range compared with in low frequency range, the 
arc locus can be determined. 
C. Determination cif the impedance during a GSR 
GSR is ;a phenoinenon which the electrical property 
of skin varies instantaneously. The  duration time of 
variations is for 3--5 s. Hence it is difficult to  obtain 
the change crf the frequency characteristics of impedance 
by a frequency domain analysis. On the other hand, 
impedance can be measured by a transient response 
method. Impedance during GSRs appearance is analyzed 
by that .  
Fig.7 shows changes of impedance during GSRs appear- 
ance. The time elapses are in order tl1t2,t3,t4. Indicia1 
responses are measured with 200 ps sampling interval and 
512 sampling points. The  arc locus changes with passage 
of time. The. property of skin impedance is presented us- 
ing param&ers of Cole-Cole arc (& : impedance value 
at frequency 0 Hz, fm : frequency which shows reactance 
maximum, /3 : coefficient of arc's angle). Those parame- 
ters are calculated using each arc. We find that  p, which 
is about 0.86, hardly changes. The skin impedance during 
GSRs appearance, which has been made a guess with in- 
sufficient, results of measurement, is confirmed using tran- 
sient res:ponr;e methods. 
“ 8  5P--“ 
I I I I 1 -  I 
0 5 10 15 20 - 25 
resistance(k Q) 
(4 
h 
15 
a 0
K a 5 10 c--“*** ‘ 8  2 
5 
8 
8 
I I I L 
0 5 10 15 20 25 
resistance(k Q )  
(c) 
10 c m  
t 1 1 1 I -  8 
0 5 10 15 20 
resistance(kQ) 
(b) 
a, 
0 c m 
0 
(d 
Y 
F 
resistance(k 0 )  
(4 
IV. CONCLUSION 
A new method determining instantaneously the 
impedance which changes trasiently with time has been 
developed. The total frequency characteristics of skin 
impedance is determined using time domain analysis. The 
method has an advantage of instantaneous determination 
of impedance compared with conventional methods us- 
ing frequency domain analysis. The skin impedance dur- 
ing GSRs appearance, which is difficult to measure using 
a conventional frequency domain analysis, can be deter- 
mined using time domain analysis. 
5 
- 1  15 
REFERENCES 
[I] W.Boucsein, Electrodermal Act iv i ty ,  Plenum Press, New 
York, 1992. 
in Physical Techni ues in Biological Research, vo1.6, 
W.L.Nastuk,  Eds. fTew York: Academic Press, 1963, 
pp.323-407. 
[3] Y.Yamamoto, “Measurement and analysis of skin electri- 
cal impedance”, Acta  D e r m  Venereol, Supp1.185, pp.34- 
38, 1994. 
“Non-linear electrical 
goper t ies  of skin in the low frequency range, Med. & Bio.  
[2] K.P.Schwan, “Determination of b;olo&cal impedance”, 
[4] T.Yamamoto and Y.Yamamoto, 
ng. fd Comput., vo1.19, pp.302-310, 1981. 
606 


Instantaneous measurement of electrical parameters in a palm during electrodermal activity

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Instantaneous measurement of electrical parameters in a palm during electrodermal activity

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