Biofeedback, Third Edition: A Practitioner's Guide

BIOFEEDBACK A Practitioner's Guide Third Edition

Edited by

Mark S. Schwartz Frank Andrasik

~ TH E GUILFORD PRESS

New York

London

© 2003 The Guilford Press A Division of Guilford Publications, Inc. 72 Spring Street, New York, NY 10012 www.guilford.com Figures 4.1 -4 .J2,1 4.Al, 14 .A2, 22.1, 22.6, 22.7, 22 .9, 22.10, 22.12, 22.13, 23.1-23.1 1, and 27.2- 27.4 are It> 2003 The Mayo Foundation. All rights reserved No part of this book may be repr()(luced, translated, stored in a retrieval system, or transmined, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from The Publisher. Primed in the United States of America This book is primed on acid-free paper. Last digit is prim number: 9 8 765 4 3 2 Li brary of

Congr~ ss

CaTaloging-in-PublicaTion DaTa

Biofeedback : a practitioner's guide I edited by Mark S. Schwartz, Frank Andrasi k.- 3rd cd . p. cm . Includes bibliographical references and index . I S I~N 1-57230-845-1 (hc : al k. paper) 1. Biofeedback fTaining. I. SchwarTZ, Mark S. (Mark Stephen ) II . Andrasik, Frank, 1949RC489.B5J S39 2003 615.8·5 1-dc21 2002154975

About the Editors

Mark S. Schwartz, Ph D, began his professional experience with biofeedback in 1974 at the Mayo Clinic in RochesTer, Mi nnesota . From 1978 through 1982, he chaired the Professiona l Affairs Committee (PAC) of the Biofeedback Society of America (BSA). During that time, the Committee developed the original Applications Standards and Guidelines for Providers of Biofeedback Services, published in 1982. T he Biofeedback Certification Institute of America (BClA), fOllllded in 1981, started from within the PAC. Dr. Schwartz chaired the BClA Board for several years and then chaired the early growth of the written examination. He also served as President of the BSA from M arch 198 7 to March 1988 . He has been on the staff of Mayo Clinics since 1967, including 21 years in Rochester, Minnesota, and nearly 16 years at the M ayo Cli1]ic in Jacksonville, Florida . He is a Sen ior Fellow of the Bi ofeedback Certification Institute of America. Frank Andrasik, PhD, began his professional career in the Deparrment of Psychology at the State University of New York in Albany. He later served as Associa te Director for The Pain Therapy Centers in Greenville, South Carolina . His current affiliation is Senior Research Scientist at the Institute for H uman and Machine Cognition and Professor of Psychology at the University of West Fl orida in Pensacola, Florida. He has also served the Association for Applied Psychophysiology alld Biofeedback (AA PB) in numerous capacities, in particular as Chair of the T ask Force o n Biofeedback Treatment of T ension Headache in 1984 , President from 1993 to 1994, and most recently and currently as Editor-in-Chief of the Association 's journa l, Af)f)lied PsychOf)hysiology and Biofeedback. He is also a recipient of the AA PB's Merit Award for Long·Term Research and/or Clinical Achievements, as well as the AA PB's Disti11 guished Scientist Award . H e is a Senior Fellow of the Biofeedback Certificatioll Institute of America .

iii

Contributors Sami R. Achcm, MD, Division of Gastroenterology and Hcpatology, .\IIayo Clinic Jacksonville, Jacksollville, Florida

Frank Andrasik, Ph D, Institute for Human • Ze< O

Powe< Line

FIGURE 4.3. Differenrial am plifier eliminating the eltttrical inTerference picked up by the body acting as an antenna.

nearly anywhere on the body, bUT it is shown ill Figure 4.2 between the twO active elecrrodes for the sake of il1ustra tion, and because it is a common arrangement. T he diffcrcmial amp lifier requires these two sources ill order to separate the EMG Cl1ergy from the extraneous cnergy. To sec why, we must remember that this extraneous energy is the hum or noise Trallsmirced through space from power lines, motors, and appliances that is picked up by the body acting as an antenna. M ost of this extraneous electrica l noise energy rises and falls rhythmically at 60 cycles per second . At any given moment, this energy is in exactly the same place in its rhythm ("in phase") at any point on the body and at any point that an electrode can be placed. H ence it is possible for the differential amplifier to continuously subtract the voltage at source 1 from that at source 2 . This cancels the noise voltage. O nly slightly simplified, th is is illustrated graphically in Figure 4.3; it is assumed that the muscle is at rest and giving off no EMC signals. The following steps explain Figure 4.3 : I. Electrical interference is received by the body acting as an antenna. 2. The interference is in the same place in its rhythm for both active electrodes. 3 . Therefore, the active inputs (from source 1 and source 2) of the differential amplifier "see" exactly the same interference signal at any given moment (imerference is in the "common mode"). 4 . Ikcause the output of the differential amplifier is proportional to the difference betwecn the signals at its two active inputs (from sources 1 and 2), 5. and the interference signals arc always identical (restatemem of point 3), 6 . then the output of the differential amplifier is zero for electrical interference.

But What about EMC Sigllals ? Suppose that mOtor neurons now signal the resting muscle to contract. Each electrode receives signals most strongly from the area of muscle directly belleath it. Since electrodes are spaced along the muscle, they each receive a different pattern of EMG signa ls. Here is an analogy: If two microphones were placed in a room full of speaking people, each Olle would pick up a different pattern of sounds, even if the overall loudness of sound in each micropho ne were the same. T herefore, at any given moment, the electrodes feed "differential" EMG signals superimposed 011 the previously discussed identical "common-mode" signals. Now, as the differential amplifier cominuously subtracts the signal at source 2 from that at source 1 (thus amplifying only differe nces between them), the common-mode noise signals will be callceled, while the differential EMC sigllals will always le:lVe a remainder to be amplified and ultimately displayed 011 a meter. T he operation of the differential EMG ampli fier with the desired EMC signals is ShOWll graphically in Figure 4.4 and is summarized below.

51

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"

\

I """. 'f'J\ Re'",ooce

I

S"..ce ~

fV\

.

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"'asic skin conductance currenr loop.

a voltage is applied to the elecTrodes, a circuit is formed , and an electric current will flow. The size of the current will depend (according to O hm's law) on the resisTance of the skin, which in turn depends on the number of sweat glands turned "on." Sec Figure 4.1 7 for an illustration. As more and more sweat glands turn "on," more and more conductive pathways switch into the circuit, and (since some current flows through each pathway) more and more total curren t flows . In this case, Ohm's law determines current flow, juSt as it docs ill temperature instruments. The difference is that the skin (instead of a temperature probe) acts as a variable resistor that regulates current flo w through the circuit. T he meter measures currellT flow in the circuit, and the reading is proportional to sweat gland activity. (Review this circuit in the section on temperature biofeedback instrumellTs by substituting ·'skin resistance" for '·probe resistance" in the explana tion of Ohm 's law.)

Sca les and Measurement: Resistance and Conductance At this point, I distinguish resistance from conductance and explain why conductance is the preferred measurement 11I1it. " Resistance" and "conductance" arc defined as reciprocals of each other, and they represent the same basic electrical property of materials. As discussed

I I . INSTRUMIO NTATION

72

earlier, the ohm is the unit of resisTance. The unit of conductance is the "mho" ("ohm" spelled backward); iT is defined as the reciprocal of resistance (i .e., 1 divided by resisTance) . Therefore, resistance is also the reciprocal of conductance (1 divided by conductance). These are two scales for measuring the same phenomenon (sec Table 4.4 ). A newer term for micromhos (one mi1lionth of a mho) is "microsicmcns." T his Term appears in recent textbooks and is synonymous with "micromhos." This chapter continues to usc "micromhos" because the older term is morc familiar to most readers. Al though rcsisrancc and conductance scales measure the same property, there is a good reason TO use the conductance measurement scale. Recall that as sweat glands turn ,; on," they add conductance pathways within the skin . T his means that conductance increases in a linear relationship to the number of activated sweat glands. Resistance, on the other hand, decreases in a nonlinear fashion as more and more sweat glands are activated. This IS shown graphically in Figure 4. [8. Th e linear relationship between sweat gland activity and skill conductal1ce is statistically preferable for scaling and quantification . This is why skin conductance is now the standard unit. T here are times (e .g., when one is using Ohm's law or testing electrodes) when it is more convenient to think in terms of resistance rather than conductance. Once the relationship between t hese twO scales is understood, sh ifting from one scale to the other presents no problem. Spea king o f scales all d measurement, note that skin conductance is l10t a di rect measure of sweat gland activity (i .e., how many are turned on). Rather, it is an indirect measure that, except for artifact, correlates highly with sweat gland activity. That is, conductance is an electrical conccpt, not a physiological concept; it is not a direct measure of how many sweat glands are III operanon. Because skin conductance results only when an electrica l voltage is imposed from outside, the measurement apparatus is inextricably tied into the skin conductance phenomena and contributes heavily to the observations . For the technically inclined, skin resistance or skin conductance biofeedback instruments are designed to be ohm ·measuring or mho· measuring meters.

TABI. E 4.4. Correspo n den ce between Conduct ,l nce and R es ist.,nce Resista ncc

Conduct"'KC Units M ho

Ohm

Micromho (millionth )

Megohm (million) Conversion formulas Resi stance" [{conductance

Conductance" Ifresislance Mho" !lohm Micromho " l{mcgo hm

Ohm = IImho Megohm = lfmicromho Sample correspondences micromho _ I megohm \0 micromhos _

.1 megohm

100 micTomhos - .01 l11egoh11l Range of skin conductance values Approx. 0.5 micromho to 50 micromhos

Approx. 0.02 megohm to 2 megohms

73

4. A I' r ime r o f Bio fe edba c k In st rum e nta ti o n

Skin Resistance (SR)

Skin

Conduc tance ( SC ) M ;n "'-_ _ _ _ _ _.,., Ma ~

Min Number of Sweat Glands Activated

M in

~-::::::====7.: Ma x

M in

Number of Sweat Gland s Activated

HGU RE 4.1 8. Comparison of skin conductance (left ) and resistance (right ) scales.

As such, they objectively measure whatever electrical equivalent network is presented to their inputs. They arc characteriled in parr by the means of applying electrical excitation to the skineither a steady-state voltage (DC) or an alternating voltage {AC)--and by their readout in either ohms or mhos. If a calibrated readout is prm,ided, calibration is usually done by presenting a known value or values o f simple electrical resistOrs and by verifying that the unit displays those values to within the specified accuracy of the instrument. T he problem is that skin presentS a far more complex electrical network than simple calibration resistors. Sweat glands are not uniformly distributed in skin tissue, so sensing sites and electrode surface areas affect readings. If DC current loops are used, electrode material may be very important, because volTage may accumulate at the skin-dectrode interfaces, which then act like tillY barreries and influence the readings. T his is called "electrode polarilation" and is discussed later. The use of silver/silver chloride electrodes will minimize but not eliminate this artifact. If AC currenr loops are used, polariwtio ll effects arc minimized, bur "reactive" components of the electrical equivalen t network of the skin will cause an apparent increase in skin conductance. (T hese and other artifacts arc discussed in a later section.) Finally, the electrical resistance of skin tisslle may vary with the magnitude o f the current in the current loop. In summary, biofeed back users should 110t assume that each other's or published qualltified SCA readings are actually comparable . Specifications o f the conditions outlined above (plus the techllicaI knowledge required to interpret the effects of these conditions) arc necessary in order to compare SCA readings fr om different contexts. Param eter s o f" seA

The hypothetical 20-second SCA record in Figure 4.19 yields three primary and two secondary parameters. Similar descriptiolls o f measurement and typical waveforms appea r in Sterll, Ray, and Quigley (2001 ). PrilJlary ParalJle[ers

seL or TOllie Level. SCL expressed ill micromhos represems a baseline o r reStillg level. Although this level may change, ill a resting, quiescellt perSOll it is likely to hover around a value identified as the TOnic level. SeL or tonic level is thought to be an index o f baselil]e level of sweat gland activity, an inferred indication of a relative level of sympathetic arousal. For example, conductance values above 5-10 micromhos arc thought to be relatively high, whereas those below 1 micromho are thought to be low. Remember that these estimates depend on a

I I . INSTRUMIO NTATION

number of other variables and should be taken only as a rule of thumb based on the use of JI8-inch dry electrodes on the volar surface of fingertips.

SCR or Phasic Changes . Phasic changes arc noticeable episodes of increased conductance caused by sympathetic arousal generated by a stimulus. For example, in the case of the stimulus introduced after 5 seconds, there is a 1- or 2-second delay and then an increase in conductance that peaks, levels out, and falls back to the baseline or tonic level. This is a phasic cha nge, and its magnitude (heigh t) is expressed as the number of micfOm has reached abo\'e baseline. The size of phasic changes is thought to be an indication of the degree of arousal caused by stimuli (e.g., a startle or orientation to novel imernal or exterIlal stimulus). SCR Half-Recovery Tilllc. ';SCR half-recovery time" is defined as the time elapsed from the peak of the phasic change to O//e-ha/f of the way back down to baseline. SCR halfrecovery time is thought to be an index of a person 's ability to ca lm down after a transitory excitation. It has been hypothesized that persons with chronic overarousal may have difficulty returning to relaxed baselines after even minor stimulation. Secondary Parame ters

"SC R latency" is defined as the Time from stimulus onset until the beginning of an SC R. "SCR rise time" is defilled as the time elapsed from the beginning of an SC R to its peak. These parameters have carried little significance in biofeedback, and therefore they arc not discussed in detail here. No rm ative Valuesfor the Parameters

The hypothetical SCA record in Figure 4.19 shows specific va lues for the parameters. These values arc actual mean va lues taken from normative samples of SCA records for tropical nonpatienrs, summarized in Venables and Christie (1980). However, these are not necessarily representati vc of values obraina ble in ordinary biofeedback practice . Since large individual differences in SCL and SCn. are common, readings far different from those cited in Figure 4 .19 should come as no surprise. Furthermore, potential sources of normative variation illclude differences between patient and non patient groups, the effects of medications on SCL and SCR, differing procedures for establishing baselines and especially SCRs, and the great differences in instruments and electrodes likely to be used. For a discussion of the effects of such variables as temperature, humidity, time of day, or season, sec Venables and Christie (1973, 1980). My advice to you, the reader, is this: To increase your confidence in norms, find or build nonna ti ve samples specific to the instruments you arc using and to The populations you are working with . At this time, there is no solid substitute for systematically accumulated experience with your own patient group, purposes, and equipment. This is not meant to be discouraging to the clinician or disparaging to the field; it is only a reflectioll of the presellt state of the art. Sca les and Measuremenl: The " Percenlage Inc rease" Sca le ror sen Amplilude Displaying SCR ampliTUde as an increase in the 111lmber of micromhos is llot the ollly altemative. SCR amplitude can also be expressed as a percentage change from the tonic level. For

4. A I'rime r o f Bio feedba c k In st rum e ntati o n

35

,

........ .................: SCR Ampl itude

Mlcromhos

30

7S

..................

seL r"-""-/-7-,/ ; Stimulus

;

5

20 Seconds

I

SeR

SCR SCR Recover y Latency Rise TI me 12

I

Time

!'IGURE 4.19. Parameters of skin conductance . (SCA values shown are taken from Vena bles & Christie, 1980. )

example, an SCR consisti llg of a I-micromho change from 3 to 4 micromhos is expressed as a 33% change. Th is has the effect of "relativizillg" the SCR to the baseline from which it occurs. With this method, a change from 6 to 8 micromhos is also a 33% change, and so is a ehange from 1.5 to 2 micromhos. The rationale for this sca le is the assumption that a given increase in autonomic arousa l leads to a given percemage increase in conductance over the baseline level, alld that this holds for all baseline levels. The following hypothetical examples and the electrical model of the skill illustrate this. Imagine that 200 sweat glallds are tUflled on, givillg an SC L of 2 micromhos. Now a stimulus comes along that turns on an additional 100 sweat glands, thus leadillg to a l-micromho or a 50% increase. Now imagine another case in which there are 600 sweat glands turned on for an SC L of 6 micromhos. According to the percentage model, a stimulus with the same arousing properties as in the first case will lead again to a 50% increase in conductance by turning on an additional 300 sweat glands, for a 3-micrornho increase in conductance. The assumption here is that changes in arousal are better gauged as percentage increases in conductance over existing baselines than as absolute increases in conductance with no regard to initial baselines. This is analogous in the economic domain to expressing a year's growth in the gross domestic prod uct as a percentage increase over the previous year's level, rather than as an increase in the number of dollars. Loudness perception also provides an analogy: Achieving a given increase in perceived loudness takes a larger absolute increase in loudness a bove a noisy background level than above a quiet background level. If an SCR is some sort of "orienting response," it is plausible that to be psychophysiologically -'noticeable," a stimulus must lead to a significant increase in conductance relative to existing baseline arousal-parallel to what oceurs in loudness perception. Pitch perception supplies a third analogy. The difference in pitch berwccn rhe nOte C and the note A above it sounds the same in any octave. (It is the musical interval of a sixth. ) The difference berween middle C (256 hem) and the A above ir (440 henz) is 184 henz, a 72% increase in frequency. The difference between the next C (5 12 hern) and the next A (880 hertz) is 368 henz, bur it is also a 72% increase in frequellCY. In rhis case, the percentage increase in frequency, rather than the number of vibrations per second, leads TO the perception of equal increases in pitch. The absolute-micromho increase scale for SCR amplitude rests on an assumption opposire to that of the percen rage increase scale : Namely, a micromho increase in conductance indicates a given increment in arousal, no matter where it is observed on the continuum of

I I . INSTRUMIO NTATION

76

2

.5

3

4

5

10

20

30

40 50

Mic fomhos

HGURE 4.20. Logarirhmic scale for SeA values.

possible initial baselines-a fixed increment of arousal, regardless of initial baseline. This assumption is also plausible. There arc, to my knowledge, 110 published data or definitive conceptual arguments to supporr Of disconfirm eiTher of the assumptions prescmcd above. Each of these scales has plausibility and appeal, and it is apparently yet to be discovered whether either has distincr practical advantages or greater psychophysiologica l appropriateness. H owever, I prefer the assumptions supporting the usc of the percentage increase scale for SCR amplitude. This is because the method of relating the magllitude of changes to initial baselines is appropriate and useful in perceptual contexts that to me are analogous to SCR. In addition, my informal obsen'atiolls suggest that persons with low SCL baselines often show fewer micromhos of SCR than persons with average SCL baselines. For me, intrinsic plausibility and these ill formal observations tip the balance toward the percemage increase scale for SCR amplitude. However, at very h igh SCLs, the percentage increase scale probably loses appropriateness, because most o f t he available swear glands are already turned on to make the high SC Ls. Conveniem scaling follows from the percentage increase scale assumption . If the skin conductance continuum is plotted along a line, a logarithmic scale conveniently contains all possible SCL \'alues while retain ing a useful degree of resolution for SCRs all along the line. This scale is illustrated in Figure 4.20. It has the advantage of providing adequate resolution at the low end while avoiding excessive resolution at the high end. Recall that the percentage increase scale supposes that the difference between I and 2 micromhos is more sigllifieant than the difference between 10 alld 11 micromhos, and is equivalent to the difference between 10 and 20 micromhos. On the logarithmic scale, equal distances along the line represent equal percentage changes. That is, the distance from I to 2 is the same as that from 10 to 20; both arc 100% changes. This means that an SCR amplitude of any given percentage is represented by the same distance along the lille, regardless of illitial baseline.

Skin Conductance Hecord Interpretation The three primary parameters discussed earlier help professionals describe actua l skin conductance records alld extract data from them. gut because records usually comain compounded changes in both responses and levels, interpretation is often required to specify values for the parameters. gelow are paradigmatic descriptions of complex skin conducrallce records and inTerpretive hypotheses.

Upward Tonic Level Shift

The sample record in Figu re 4.2 1 revea Is a phasic change away from the begi 1111ing tonic level and incomplete return to t hat level. Think of this as an SC R that did not recover and led to a

4. A I'rime r o f Bio feedba c k In strum e ntati o n

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J2 t----~ o

2

1 Minutes l'IGlIlt E4.2 1. Upward ronic level shift.

4

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seR

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Superimpo sed

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0 Minules H GUR" 4.21 . Downward ronic level shift.

new and higher tonic level from which subsequent phasic changes depart. A hypothesis is that whatever arousal led to the phasic change did not completely "wear off," thereby leaving the person with a new and elevated tonic level. Increase in conductance may be slow like "drift," rather than rapid like a typical SCR. Do wn ward Tonic Level Shift

The arousal leading to the new or elevated tonic level discussed above may in time "wear o ff" or be "rela xed away," leading to a downward trend in skin conductance. As shown in Figure 4.22, this record has downwa rd slope to it, although SC Rs may be superimposed. In this way, a new lower tonic level may eventually be reached. SWirstcppina

With multiple excitatory stimuli , especially for persons who show high-magnitude phasic changes and slow recovery time, a phenomenon called "stairstepping" may occur . As shown in Figure 4.23, this results when an excitatory stimulus occurs befo re the phasic changes from previous stimuli have had time to return to rhe prior tonic level . The SeA may thell "srairstep" higher and higher. T his sta irstep ping process could theoretically be implicated in developmellt of overarousal. Figure 4.24 illustrates how individuals who show lower magnitude phasic changes and more rapid return to baseline arc less susceptible to stairstepping from repeated stimulation.

I I . INSTR UMIO NTATION

78

;......... . Level 4 ....... ~ ......•... Level 3 /"-~·'···········-·i········;·· ....... l e vel 2 •••••• • •••••• -1' ••••••• ; •••••••••

o

2 Stimuli 3

Levell

4

2

Minutes

1:IGURE 4.13. Stairstepping.

o

Stimuli 2

3

4

2

Minutes !'IG UR E 4.24. Rapid return ro baseline, reducing stairstepping.

Nonresponsive Pa ttern

A "nonresponsive pattern" is an unusual! y flat conductance level (sec Figure 4.25), which docs Jlot respond to typically arousing stimuli even when there is a rcason to believe that arousal or emotion is or should be present. A hypothesis for this pattern, when eXTreme, is inappropriate detachrncTlt, ovcrconrrol, or helplessness rather than relaxation (Toomin & Toomin, 1975), Optimal Skin Conductan ce Pattern s

Skin conductance is linked TO arousal, but optimal SCA patterns arc not necessari ly the lowest or flatte~n patterns. Th is is because persistent mill irna l aro usal, overcontrol, illattentioll, or flattened affect is not usually considered hea lthy or adaptive. Th ere is a time for minimi,ing arousal during deep rela xation, in wh ich a steady, low level of skill conductallce may be desired, but uniformly invariant or flat levels are not necessari ly desirable . For example, encounterillg a novel stimulus calls for recognizing and treating it appropriately. H a bitual blunti ng of the arousal associated with orientation or action is nor thought to be healthy or adapti ve. However, a fter a person orients to the 1l0Vel stimulus and takes appropriate action, arousal should drop to baseline levels, avoiding unnecessary arousal or wasted energy. It is possible for a person to react tOO vigorously to novel stimuli, so that the reaction is out of proportion. In this case, the person is treating stimuli as more alarming, dallgerous, or excitillg them warranted, and is paying a price in energy and physical tension . SCA is not something to be minimized but something to be optimized, and this requires judgmem aboUT what is appropria te for a given person in a given circumStallce. At this time, no one claims to know optimum tonic levels and SCRs, or to be able to show that there is any

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nGUR E 4.15. Nonrcsponsivc parrcrn.

such thing as specifiable optimums . What is clear is that it is possible to have overreaction and underreaction, and that this holds for both the tonic levels and phasic changes. Qu ick return to baseline after an SCR may be consistently desirable except when it is part o f an underresponsive pa nern . Because of large individual differences in SCA patterns and the lack of nonnative data under various standard paradigms of stimulation and measurement, it is difficult to specify elear and widely accepted procedures for relaxation training with SCA. Useful SCA biofeedback requires experience and judgmenr on the part of the clinician. The best way to acquire the "feel" of how SCA works under various conditions is to observe it within and between individuals, especially oneself. Those who work regularly with SeA are often quick to point out its ambiguities and uncerraillTies, bur, undiscouraged, arc also eager to discuss its unique responsiveness to transitory emotional states and thoughts. Its appa rent complexity and ambiguity may cOllceal a wealth of valuable psychological as well as physiological information to those who have The patience to learn and further describe its patterns .

Operation of th c Skin Conducta n cc Instrumc nt Most Basic Constant DC Voltaae Scheme

Figure 4.26 shows the lIIo st basic SCA monitoring scheme . A constant voltage is impressed across the twO electrodes. T he variable resistance of the ski ll leads to a variable current through the circuit. A current ampli fier monitors this current, and, through proper scaling, drives a

Meter

+

(

v

+

I Current Amp.

Cons tant Voltage

Electrode Skin

HGURE 4.26. Most basic SeA monitoring scheme.

I I . INSTR UMIO NTATION

80

meter that reads out in micromhos. [n this most basic form, it is similar to Temperature inStruments as shown in Figure 4.1 5 . H owever, to be pracTical, it mUST be refined. Adjustable Vie wing " Wi ndo w"

SCL baselines arc spread over a wide range, yet it is impOrt3m to distinguish small SCRs {e.g., a 5% change from ally Sell from all possible baselines. If the en tire range of possible SeA values were made to fit O i l a meter face, SCL values would show, bur an SCR would barely deflect the needle. Figure 4 .27 ill ustrates how a 5% SC R from a I-micro mho seL would be barely discernible. It would take a much larger SCR TO move the lleedle enough to accurately gauge SCR amplitude and recovery time. T his is the familiar issue of "resolution," discussed earlier in connection wi th temperature biofeedback instruments. A digital meter overcomes resolution problems simply by having enough digits (e.g., tenths or even hundredths). However, a digital meter is l10t suitable for observillg SCRs, because changing digits during an SCR are hard to read . In contrast, the swing of a meter needle or light bar up and then back down is much more meal1i ngful for SCRs. A common solution to this problem is to usc an analog meter for SCR display, hut to restrict its range to form a "viewing window" that looks on oilly a portion of the SCA eOlltinuum. O f course, this "window" must be movable to any part of the SCA continuum, so an SCR can be monitored regardless o f illitial baseline SCL. T his is ShOWll graphically in Figure 4.28 . The "window," which looks upon a small portion of the SCA range, is expanded 01 1 the entire meter scale. T his way, small SCRs resu lt in significant meter deflections. T he center zero point on the meter in the figure represents the center of the window. The meter scale is calibrated so that the extent o f the needle swing indicates the percentage change from a starting baseline. In the illustration, the meter is calibrated for a +50 % or - 50% change. The user operates a calibrated control that moves the window up and down the SCA continuum until the SCL o f the person being monitored "comes into view." If this level is approached from the left, the needle will remain off scale to the right unti l the window moves over the SCL. Once the SCL is in view, the needle falls back to the left as the window moves toward the eemer zero point. With the needle at zero, the window is centered over the person's SCL. T he SCL value is read off a digitally calibrated control that moves the window up and down the SCA continuum . When an SC R occurs, the meter needle moves upward. At maximum deflection, the needle points to the percentage change. T he digital control remains in place during SCRs, so it "remembers" the starting SCL baseline. If a person's SCL changes a lot or "drifts," the

,

2

345

Micromhos

o HGtm E 4.27. t ack of resolution when entire SeA range is squeezed onto 3 meter face.

81

4. A I' r ime r o f Bio fe edba c k In st rum e nta ti o n

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Micromho s

.,

Windo w

/

....... ,··· ...•. 5 ... '. .........

'0

....................

....

o

SCR in Perc en t Change

o

FlG UHE 4.211. Movable viewing window.

user moves the window along the SCA comilluum with the digital control. T he SCL is kept ';in view," and the digital control indicates the new baseline. If a computer is used, equivalem functiolls can be programmed to occur automatically. To reduce how often the window must be moved to keep the SCL reading on sca le, some illstrumems have adjustable window "widths" or choice of resolutioll. A \'ery wide window width (e.g ., 100% change) will cover more of the SeA continuum and hence will require readjustmem less often durillg periods ofSCL drift or for very large SCRs. Wider windows also reduce resolution. That is, small SCRs will be less pronounced on the meter scale. When a very stable SCL with very small SCRs is being observed, switching to a narrow window width (e.g., ±I O%) expands small changes on the meter face, thus increasing visual resolution of the response.

Electrical Operation of the .Movable Vie lVinIJ Windo lV Figure 4.29 shows a sk in conductance instrumen t with a movable viewing window. The constant-voltage source feeds twO current loops. First is the familiar loop through the electrodes and skin, described earlier. T he second loop is identical, except that a calibrated variable resistance (or varia ble conductance) takes the place of the electrodes and skin . In each loop, a currellr-ro-voltagc amplifier produces an output voltage proportional to the current through its loop_ Whenever current flow through the loops is equal, the outputs of the amplifiers are equal. Whenever the current flow through the loops is unequal, the OutputS of the loop ampli fiers are unequal in proporTion to the difference in loop current flow. A meter measures the difference between the twO loop amplifier Outputs . This meter has a needle that is normally at rest in the center of the scale, p ointing to zero. When current Through the loops is equal, the outputS of the amplifiers are equal, and the needle remains at rest in the center, pointing to zero_ When current through the loops is unequal, The meTer needle will swing left or right, depending 011 which loop has the greater current. T he extenr of the deflectioll illdicates the magnitude of the difference in current through the loops and reads out in percentage. The meter measures only differences 111 current flow ill the twO loops . T he reader may notice the similarity between this circuit and the differential amplifier discussed in the EMG section.

I I . INSTR U M IO NTATION

82

r

Curren! Loop 2

Calibrated Potentiometer

lor Sel

•

'(

,,,

Meter

SCR

Constant Voltage

Current

loop'

Electr ode

"-

Skin

./

I'lG UIt E 4.29. Skin conduCTance device with movable viewing window.

The user adjusts the control (the variable conductance) so that the meter balances at its zero poim. This means that currcllT through the loops is equal. This, in turn, means that the micromho value set 011 the control equals the person's SCL. Now suppose an SC R comes along. Skin conductance increases, but the calibrated comro1 remains at the same position (i.e., at a value that equa ls the prc\·jous SeL). Because skin conductance has JUST increased over the prC\'iOllS level, current now through the loops is unequal, and the meter deflects to the right in proportion to SCR magnitude. The needle points to the percent increase above the SCI. shown on the calibrated control. As recovery from the SCR takes place, the needle moves back toward its center balance point, where loop currents arc equal. This is how the viewillg window is electrically moved along the SCA collti11llUm to accommodate any person's baseline SCL. Changing window width is accomplished electrically by changing the "gain" or sensitivity of the currem-to-voltage amplifiers, so that a given di fference in loop current leads to greater or lesser meter deflections. This is why the window width switch is usually ca lled a "sensiti\'ity comrol," though " resolution control" would be a better term for it. Audio feedback follows SCR, shifting along with the viewing window. The pre\'ious section illustrates the basic design. Filtering or smoothing can be used to minimize the need to manually readjust the position of the viewing window as SCA drifts to a much differem baseline SC L. (Smoothing and filtering have been discussed in the section on EMG.) Simple SCR DeY;res

Simple SCR devices usc a manual, noncalibratcd baselinc adjustment and feed back SCR with an audio tone or nonca librated meter scale. These devices quantify neither SC L nor SCR and arc morc susceptible to artifact than full-sized instruments. Even so, they arc very convenient and provide very interesting and useful information to a person about patterns of SC R. For example, the risc and fall of an audio tone commun icates a great deal about the person's responsivity in actual situations, even when quantified SCL or SCR is absent. These devices have distinct ad\'antages when it comes to ambulatory use in real life. Pocket·sized miniaturization, dry finger electrodes, and an earplug for private feedback permit a person to wear the unit convelliemly while walking, talking, driving, phoning, writing, thinking, reading, or carrying out other real activities. This provides insight into patterns of responsiviry in

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active siTUations that are not obtainable in the clinic setting. It is a \'ery good way for a person (including the therapist!) to discover his or her own patterns of responsivity. In any application, the therapist involved must provide adequate instruction in the use and limitations of the device and in the interpretation of results. Artifact There arc many ways to process and display SCA, and there is no firm consensus on the most appropriate way to do it. Historically, the most common method is probably the one shown in Figure 4.29, with a manual calibrated baseline comrol alld all analog zero cemer meter for SCR. Simple SCR devices certainly have a useful place in clinical biofeedback, even though quamitative measuremellts are usa lly 1l0t possible. The following poims aboUT artifact arc important for all SCA devices. Electrode Size. Different-sized electrodes lead to different readings. A larger electrode covers more skin and therefore places more sweat glands in the current loop. T his leads to a higher SCL than docs a smaller electrode that places fewer sweat glands in the loop. T herefore, electrode size must be stalldardized ill order to assure comparability of quamified SCL readings. Movemellt. Because electrode size affecrs SCA, anything that alters the effective contact area of an electrodc also alters SCA . Fingcr or hand movement causes variations in contact pressure. The electrode may lift slightly and diminish the comact area, or press harder agaillst the skin and increase the contact arca . T hese effects arc more pronounced for dry electrodes thall for precious-metal electrodes with electrode gel. The practitioner should encourage the monitored person to minimize hand movements and arrange the electrodes and cables for a reasonably stable position. When hand movemellt cannot be avoided (e.g., monitoring while the person is doing something with both hands), corresponding sites on the toes could be used . This would require exploring new norms for SCL and SCR on those sites. Fortunately, movement artifact is usually easy to spot, because the resultant parterns arc often abrupt and uncharacteristic of true SCA patterns, and because movement can often be observed.

SkiJl COllditioll. Skin condition can affect conductance readings. For example, if a person has a skin abrasion or a fresh cur through the high-resistance skin surface, a highconductance path may be established from the electrode to deeper layers of the skin and lead to an increased SCL If a person has developed a callus, the high·resistance surface layer increases in thickness and dryness, leading to a much lower SCL and diminished SCR amplitude. Venables and Christie ( 1980) nOte that SCL falls markedly after a washing with soap and water, as residual salt is removed. Because salt builds up over time siTlce the last wash, they recommend that persons begill sessions with freshly washed hands. It is not clear how important this is to clillica l biofeedback, but it is clear that this stalldardizing procedure is not universally followed . Room Tem(Jerature. There is some evidence (Venables & Christie, 1980) that SCA is affected when individuals feci cold, and that warmer·than-usua l office conditions appear to produce what they call more "normal" responsivity. It is also plausible that the temperatureregulating function of sweating in an overly warm room leads to increases in SC L that arc not psychophysiologically significam. Electrode Polarization Potel/rials alld Electrode Desigll. T he exosomatic method illvolves the passage of current through the skin via surface electrodes. Polarization potentia ls develop

I I . INSTRUMIO NTATI ON

84

at the skin-clccrrodc interface as DC passes, and the polarization effect builds up over time. The size of polarization pOTential is variable and unknowil . EDA units have hisTorically varied widely in their susceptibility to the effects of electrode polarization. But in general, this is probably lIor a major problem, especially with DC instruments that apply very small electrical currents to the skin. Nevertheless, biofeedback clinicians who are interested in EDA and the devices that have been employed ro assess it over the years should probably be aware of the issues concerning electrode polarization and methods that have been used to minimize it. A somewhat technical discussion of this foll o ws. Dry electrodes are often used for EDA. They are made from various ma terials, induding lead, zinc, chrome, sta ill less steel, gold, or sil ver-coated fuzz, alld are often secu red by Velcro straps that conveniently adjust to different finger sizes . T hey are simpler, cheaper, and more cOllveniellt than silver/silver chloride electrodes, especia ll y in clinical practice. However, when used with DC EDA equipment, the simple dry electrodes suffer from polarization potentials to various degrees. When polarized, the skin--clectrode interface is like a tiny battery charged by the passing currem. Polarizatioll voltage is thereby added to (or subtracted from) the constant voltage applied by the instrument . Because the polarization potential (voltage) is variable, the voltage in the current loop is no longer constanr. Therefore, what appear to be changes in SCL may be due in part to variable electrode polarization potentials. Drift in skin conductance level due to the buildup of polarization potenrial causes artifact, bur this effect may not be all that significant, for practical purposes . Ncvertheless, silver/silver chloride electrodes have sometimes been used, because they develop minimal polarization potemials and therefore add minimal polarization artifact. But they arc more expensive and less convenient than dry electrodes, and the gel used with silver/silver chloride electrodes may prolollg the recovery phase of SCRs. A similar prolongation may occur in very humid climaTes even when dry electrodes are used. Artifacmal prolongation of SCR recovery could lead to resu Its mistakenly in terpreted as "stairstepping."

Use of A C to collfro/ electrode po/ariUltioll artifact. Instrument designs have been evolved to circumvent the effecTS of polarization potentials.The mOst obvious way to minimize electrode polarization arrifaa in EDA equipment is to usc an AC voltage source rather than the DC voltage source described earlier. Using AC in the current loop helps in two ways. First, the constantly reversing polarity of AC first charges and then discharges the electrodc-skin imerface "battery," thus reducing the buildup of polarization voltage. Second, any remaining polarization voltage is blocked by capaci tors in the current loop. Capacitors permit only the AC to pass . T his is illustrated in Figure 4. 30. A "capacitor" is an el ectronic component with conductive plates separated by an insulating membrane. Alternating attraction and repulsion of charges across the insulating membrane Output

r Elec tr ode

v

AC Current Amp~tie r

Const ant AC Voltage

6 --

Capacitors Sk in

HGURE 4.30. A C currcm loop with DC-blocking capacitors.

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8S

permits AC to pass through a capacitor. No current actually flows through the membrane. T his can be illustratcd by analogy. Imagine a fluid-filled cylinder fitted with an clastic diaphragm in the middle and an opening at each end (figure 4.3 1). If the fluid pressure at open ing A is greater than at B, then fluid flows imo the cylinder at A and bulges the membrane, forcing fluid OUT through opening I). If the p ressure diminishes, the membrane begins to move back to its original position as fluid retutlls to the cylinder through opening B and the same amount of fluid leaves through opening A. If pressu re at A con ti nues to drop, then more fluid leaves through opening A; the membrane bulges to the left, dra wing fluid imo the cylinder through opening B. So long as there is a cycle of pressure changes at A, Then there will be a corresponding cycle of changes at B. T his is analogous to how a capacitor passes AC. H owever, suppose that a modest unchanging pressure is introduced at A. The membrane bulges a little and then StOps. Fluid flows from A only while the pressure is building and the membrane is in motion. After it stops in a bulged position, no more fluid moves at B. This is analogous to how a capacitor blocks DC. This analogy shows how a capacitor blocks small residual DC polarization potentials in an AC loop while passing the AC unrestricted. This does remm'e polarization potemial artifact. However, it generates another kind of artifact, which is probably more of a problem . It TUrns out that the sk ill itself forms a capacitor. Although this faCt is of no cOllsequence when a constant DC voltage is used, as in the designs described earlier, it does become a significant factor whell AC is used. Figure 4.32 shows the location of this natura l capacitor. Skin capacitance forms a second ';reactive" pathway for AC in the current loop. If AC is applied ro the skin, a portion of the currem flows as usua l through the diminished resistance of the activated sweat glands, but some additional current flows through the skin capacitance. T his means that total currem flow is greater (and the readings show greater SC l ). This effect can be quite pronounced and cannot be neglected. A note for the technically inclined is that an AC measuremem in which skin capacitance contributes to the reading should be called "skin impedance" (analogous to resistance) or "skin admittance" (analogous to conductance). To make matters worse, impedance or admittance varies with the frequency of the AC, because higher frequencies pass through a capacitor more easily thall lower frequencies . The reader need not worry, because there are still other ways to minimize polarization arti fact withoUT gelleratingeapacitance artjfaets. Explanation of these systems is beyond the scope of this chapter. Such sysTems have been commercially available and minimize these artifacts even when simple dry electrodes are used. A wise buyer who is very concerned about repeatable quantified SCl data inquires about how these artifacts arc handled or avoided. As for the miniatu re SCR devices described earlier, their lack of quantification makes polarization artifact much less relevant than for instruments capable of quantification. Skin capacitance artifact is generally not an iSsue wirn the miniature devices, because they almost always use simple DC loops.

Safety Electrical safety precautions for SCA devices are the same as for EM G devices . Both arc electrically connected to The person via electrodes; therefore, the same stringent standards for

I'IGUI\!: 4. 31.

Fluid analogy to thc capacitor.

I I . INSTR U M IO NTATION

86 Output

v

AC ClI'rent Amplifier

Constant AC Voltage Electr o~e

-----L=r=c--~7Co-C_--~-=--~--~-"~~---- Sk~ Skil Resistance (or Conductance)

Ski'l Capacitance

FIGU RE 4.31. AC current loop and skin capacitance.

design, manufacTure, installation, and maintenance should be followed for SeA and EMC devices, and for the emire installatiOll of which any of these instrumcllts afC a part. The passage of DC from an electrode to the skin over a prolonged time may lead to the formation of chemical by-producTs on the skill if the voltage drop across the skin exceeds about 3 DC volts, such as might be encountered in "toy" or very early EDA gizmos (Leeming, Ray, & Howland, 1970). T his effecr is normally negligible, bur if the current passed is high enough and is passed long enough, then skin irritation could develop. This effect is unlikely to occur in modern skin conductance instruments, bur very old units, those that were made as novelties or toys, or those that have devcloped leakage currents may be more likely to create this effect. As a rule of thumb, a device that passes current o f 10 microamperes or less per square cell timeter of electrode area in its currellt loop, and applies under 3 DC volts to the skin, will not lead to the accumulation o f irritatillg chemicals 011 the skin .

ACKNOWLJ:DGMENTS My Ihanks go 10 the late Wallace A. Peek, the late Roland E. ,\olohr, and John B. Picchiol1ino, who have l Cled so gcnerously as my engineering menlOrS. I give special dunks to John B. Picchiollino, whose suggcstiuns for Ihis chapter in its first edition marked a long and much ·a ppreciated history of helpfulncss with biofeedback projects. A speci,,1 thanks must "ho go 10 Ma rk S. Schwan z, without whose enthusiasm the first edition' s cha pter and subscquent rC\';s;ons would doubtless have remained on my list of things 10 do someday.

IUFER);NC[S Boucscin, W. (1992). Eleclroderm,II.1C1;"iry. New York; Plenum Press. Brown, B. (1 974 ). New mimi, ".,w body, Newdireclim1S for the "'i",l. New York, Harper & Row. Cacioppo, J. T., T assinary, L. G., & BerntsOn, G. G. (&l5. ). (2000). U,uufbook of psychophysiology (2nd cd.). New York: Cambridge Uni"ersity Press. Dawson, M. E., Schell, A. M. , & Filion, L. (2000 ). The electrodermal system. In J. T. Cacioppo, L. G. Tassinarr, & G. G. Berntson (Eds. ), H~",lbook of ps)"ciJophysiology (2nd ed. ). New York: Cambridge University Press. Jennings, J. R., Tahmonsh, A. J., & Redmond, D. O. (1980). Non-invaS;"e measurement of periphera l " ascular aCli,·iry . In I. Marlin & P. H. Venables jEds.), Tec/miques ill ps)"chop/}ysi%gy (pI" 70-\J1). New York: Wilq·. Lt...;ming, M . N., Ray, c., & l'lowland, W. S. (1970). Low-"ollage, direcI·current burns. Jour>~11 of the Americ:w Meilic.1/ Associmioll, 214(9), 1681 - 1684.

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Mathieu, P. A., & Sullivan , S. J. (1990). Frcqllcncy characteristics of signals and instrumentation, Implication for Ei'dG bioftt