Method and apparatus for monitoring skin potential response

Biofeedback training of a subject may be achieved by monitoring the skin potential response of the subject and by shaping the monitored response so that a feedback signal is generated only if the response exceeds pre-established upper and lower limits which may be selectively increased or decreased. Biofeedback apparatus for implementing this technique includes electrodes for sensing the skin potential, an amplifier for amplifying the signal and feeding the signal into a pair of variable comparators which provide output when the response exceeds preset upper and lower limits. The outputs, in turn, provide a feedback signal either audio or visual, or to serve as inputs to auxiliary instrumentation. No feedback signal is produced as long as the monitored signal is within the selected limits. The magnitude of the responses may be quantified to the extent that they exceed the preset levels. The audio feedback signals may be recorded on a common audio cassette recorder. The method and the apparatus are useful in a clinical verbal interview.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to biofeedback techniques and apparatus, 
and more particularly is directed towards a new and improved method and 
associated apparatus for monitoring skin potential response. 
2. Description of the Prior Art 
Conventional biofeedback techniques which are employed to monitor the 
degree of autonomic arousal do so by measuring changes in skin resistance 
of the subject. While the monitoring of changes in skin resistance 
provides useful information in various biofeedback procedures, it has a 
number of inherent drawbacks which limit its utility. For example, changes 
in skin resistance in response to stimulation generally are rather slow 
and a rlatively long period of time is involved between the initial 
stimulation and the return of the skin resistance to a normal condition 
after the stimulation has been removed. Furthermore, normal skin 
resistance tends to vary so that the base line resistance, or reference, 
will vary thereby introducing error into the system. Furthermore, the 
signal produced by measuring skin resistance changes in one direction only 
and thus carries relatively little information. 
Accordingly, it is an object of the present invention to provide 
improvements in biofeedback techniques and instrumentation. Another object 
of this invention is to provide a quicker and more accurate means and 
technique for measuring the degree of autonomic arousal in response to 
stimulation. A still further object of this invention is to provide a 
method and associated apparatus for monitoring skin potential response and 
providing control means for shaping the skin potential response whereby 
signals are produced only when the detected signal exceeds certain preset 
limits, which limits may be increased or decreased selectively. 
SUMMARY OF THE INVENTION 
This invention features the method and associated apparatus for monitoring 
the skin potential of a subject in response to stimulation and providing 
shaping control over the skin potential response waves, whereby a feedback 
signal is generated only for the peaks of the waves. The control is 
variable so that by sequentially reducing the magnitude of the level at 
which the wave peaks generate a signal, a lower base line level of skin 
potential response can be gradually achieved. The apparatus includes 
sensors applied to the skin of the patient, amplifying means for 
amplifying the skin potential, filtering means, logarithmic amplifying 
means and variable comparators adapted to provide an output signal only 
when the peaks of the input signals exceed certain predetermined levels 
which are controlled manually. Output means connected to the comparators 
provide a feedback signal to the subject when the wave peaks of the signal 
exceed the preset limits. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a view in perspective of a skin potential response apparatus made 
according to the invention, 
FIG. 2 is a block diagram of a skin potential response system made 
according to the invention, 
FIGS. 3(a), (b) and (c) are wave forms showing typical skin potential 
response signals, 
FIG. 4 is a schematic diagram showing a decreasing skin potential response 
wave and decreasing feedback peak levels, and, 
FIG. 5 is a circuit diagram of the system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings and to FIG. 1 in particular, the reference 
character 10 generally indicates a biofeedback apparatus for monitoring 
the skin potential response of a subject 12 by connecting electrodes 14 to 
the subject through a cord connection 16. Preferably the skin potential 
response is monitored between the palm of the hand and the back of the 
hand and/or the forearm of the dominant hand. The palm provides the active 
electrode site, while the back of the hand or the forearm provides the 
reference electrode site. 
The skin potential response, in practice, is a minute voltage potential, 
and, depending upon the intensity of the applied stimulation, the response 
may occur in any of the three forms shown in FIGS. 3(a), (b) and (c). In 
FIG. 3(a) there is shown a negative wave form which is produced by mild 
stimulation; FIG. 3 (b) shows a biphasic wave resulting from an 
intermediate level of stimulation, while FIG. 3(c) shows a positive wave 
form resulting from an intense stimulation. The skin potential response 
has been found to reflect the degree of autonomic arousal in a subject and 
provides significant information with respect to the subject beyond that 
available to the monitoring of changes in skin resistance. 
It has been found that the amplitude and the frequency of the wave form 
generated by skin potential response is related to sympathetic arousal. 
The positive wave of the skin potential response is believed to be an 
indicator of the defense reflex, while the negative wave is an indicator 
of the orienting relfex. The positive wave has been found to increase 
monotonically with the intensity of sound and electric shock applied to 
the subject, there being a direct relation between the stimulus and the 
amplitude of the wave. The positive wave persists under conditions in 
which aversive stimulation is anticipated. In addition, the high frequency 
of skin potential responses associated with stressful conditions disappear 
when the external stresses are removed. 
The skin potential response signal is sinusoidal, as will be seen in FIG. 
3, and has a base line of zero to which it always returns when recorded 
with AC (time constant). This characteristic is extremely useful insofar 
as with a zero base line the signal always returns to the same point, 
thereby eliminating a source of error which is common to a skin resistance 
monitoring system or when skin potential is monitored with DC (direct 
current). Insofar as the signal goes both up and down, two polarities are 
available to provide more information, and since no current is applied to 
the skin, instrumentation is simplified. Skin potential response provides 
a measurement and permits the training of direct autonomic responses 
rather than an indirect somatic muscular index relaxation, as is the case 
with an EMG feedback. Further, the skin potential response is measured in 
voltages much larger than with EMG, being on the order of one thousandth 
of a volt rather than a millionth of a volt, whereby the skin potential 
response signal is much less susceptible to electrical and motion 
interference as is the case with an EMG system. 
The skin potential response signals detected by the electrodes 14 are fed 
first into an input amplifier 20 which amplifies the signal and delivers 
it to a filter 22. 
Preferably the input amplifier 20 is an amplifier having an input range of 
.+-. 0.3 to .+-. 30 millivolts. The filter 22 serves to separate the 
monitored skin potential response signal from background noise and is 
provided with filter controls 24 in the form of switches and, typically, 
the system may have two filter settings 0.2Hz and 1Hz, operated by the 
push buttons 26 and 28 on the front of the cabinet. In addition to the 
filter setting switches 26 and 28, the front panel of the unit also 
includes a push button switch 30 which turns the system on and off, a push 
button switch 32 providing a battery checking function, three push buttons 
34, 36 and 38 for changing the sensitivity of the system through ranges, 
for example, .+-. 30, .+-. 3 and .+-. 0.3 mV. A final push button switch 
40 is connected to the filter circuit and functions as an initial filter 
setting. 
The control panel also includes a pseudo-logarithmic meter 42 providing a 
visual display of the monitored signal, a volume control knob 44 
controlling a loudspeaker 46, which preferably is built into the cabinet, 
and a shaping control knob 48, the function of which will be described 
more fully below. The control panel also includes a pair of indicator 
lights 50 and 52 which, preferably, are red and green light emitting 
diodes providing a visual feedback indication whenever the skin potential 
response goes above or below variable levels set into the system by the 
shaping conrol knob 48. 
The output of the filter 22 is then fed to a pseudo-logarithmic amplifier 
56 which allows greater amplification of the signal near the baseline, 
thereby providing a wider range over the control of the signal. The 
logarithmic amplifier provides an output to the meter 42 and detector 60 
as well as through an optical isolator 54 to a terminal 58. The terminal 
58 may be employed to operate various auxiliary equipment, such as 
recorders, or the like, while the detector 60 provides the shaping control 
over the skin potential response of the subject. 
The detector 60 is in the form of a pair of comparators 62 and 64, one for 
the positive portion of the monitored signal and the other for the 
negative portion thereof. The shaping control knob 48 is connected to the 
detector by means of which the reference levels for the comparators 62 and 
64 may be selectively increased or decreased at the same time. The 
comparators 62 and 64 are set by means of the control 48 to generate an 
output signal only when the amplitude of the monitored signal goes above 
or below levels set by the shaping control 48. As long as the monitored 
signal is within the limits set by the shaping control, the comparators do 
not produce an output and there will be no feedback signal to the subject. 
However, if the monitored signal goes above or below the level set by the 
shaping control, a feedback signal is generated so as to illuminate the 
light emitting diodes 50 and 52 for the visual indicators as well as to 
operate the audio output comprised of the speaker 46 driven by an audio 
amplifier 66 and a voltage controlled oscillator 68. The audio portion of 
the system generates a decreasing tone pitch when the negative wave of the 
signal falls below the level set by the shaping control and produces an 
increasing tone pitch when the positive wave exceeds the level set by the 
shaping control. 
The feedback signal may also be fed through a mixer 72 for recording the 
audio feedback output, and in this fashion record the iteraction of the 
subject, the technician, and the responses produced. By recording the 
results of the system, it is possible to measure the magnitude of 
responses and an ordinary tape recorder may be used for this purpose. 
Operation of the system is, in part, illustrated by reference to FIG. 4. In 
FIG. 4 the monitored skin potential response signal is represented by a 
wave form 74 alternating above and below a zero base line 76. Upper and 
lower limit lines 78 and 80, respectively, represent different settings by 
the shaping control 48 which vary the reference signals to the comparators 
62 and 64 in the detector 60. As long as the peaks of the wave form 74 
remain within the limit lines 78 and 80 there is no feedback signal since 
the comparators are not actuated and the loudspeaker 46 remains silent and 
the lights 50 and 52 remain off. However, if the signal 74 goes above the 
limit line 78 or below the limit line 80, the comparators 62 and 64 will 
be actuated to generate a feedback signal both audio and visual. 
Initially, the wave form may be relatively large in amplitude and 
biphasic, indicating both negative and positive response to stimulation. 
The subject may learn to relax by lowering the magnitude of the wave 74 
through manipulation of the shaping control which, as shown in FIG. 4, may 
be done by gradually reducing the levels at which the peaks of the wave 
form produce a feedback signal. Thus, in FIG. 4, the limit lines 78' and 
80' indicate a reduced setting of the shaping control, thereby reducing 
the envelope within which the wave form may oscillate without producing a 
feedback signal. By sequentially reducing the envelope, the range may be 
reduced to audibly feed back skin potential response peaks of lesser 
magnitude. In this way reinforcement of successive approximations to the 
desired low arousal level is made possible. Learning criteria for each 
individual may be adjusted to both the degree of arousal and the speed of 
learning. The method and apparatus utilizing the skin potential response 
of the subject in a shaping control program may be used to measure anxiety 
responses to verbal stimuli and to monitor trained relaxation, as well as 
to provide exact measurements of the signal. Using a tape recorder in 
conjunction with such a program, a permanent record can be made of the 
verbal response and the simultaneous skin potential response and audio 
response. The technique measures and permits training of direct autonomic 
response rather than an indirect somatic muscular index of relaxation and 
will yield a more general systemic measurement rather than specific muscle 
areas thought to reflect an overall body-wide condition. The skin 
potential response technique measures voltage in much greater levels than 
other similar systems which thus makes the system much less susceptible to 
interference. 
In addition to the foregoing, the system may be modified to generate a 
feedback signal to the subject whenever the skin potential signal fails to 
exceed the reference signals (i.e., stays within reference limits) by 
means of allowing the subject to reduce his muscle tension which may be 
recorded and fed back to the subject by means of an EMG system. Such a 
technique is useful in training total somatic relaxation by first 
establishing autonomic quiescence then allowing muscle relaxation 
training. (Making muscle relaxation training contingent upon autonomic 
quiescence, i.e., low magnitude skin potential response).