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The effect of electrode size on electrodermal measurement Mahon, Mary L. 1986

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T H E E F F E C T O F E L E C T R O D E ' S I Z E ON E L E C T R O D E R M A L M E A S U R E M E N T by M A R Y L. M A H O N B. Sc. , University of Alberta, 1980 A T H E S I S S U B M I T T E D IN P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T S FOR T H E D E G R E E O F M A S T E R O F A R T S in T H E F A C U L T Y OF G R A D U A T E S T U D I E S Department of Psychology We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A F E B U A R Y , 1986 (c) Mary L . Mahon I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e h e a d o f my d e p a r t m e n t o r b y h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f v, csLj /'o^ y  T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 1956 Main Ma l l V a n c o u v e r , C a n a d a V6T 1Y3 D a t e /^U^yL j_<f /f6 i i . A B S T R A C T The effect of the size of electrode contact area on skin conductance (SC) measures has not been clearly resolved. On the basis of the anatomical structure and current electrodermal models of the skin, it is expected that the relationship between S C and the size of electrode contact area will be linear such that increases in contact area will produce corresponding increases in S C . This hypothesis was not supported in the most recent study investigating this relationship (Mitchell & Venables, 1981). However, methodological problems existed with this study which may have contributed to these counterintuitive results. The present study reexamined this relationship while exerting careful experimental control. Forty-eight, right-handed male subjects were randomly assigned to six groups of eight subjects each. Six pairs of electrodes with different size contact areas were placed on six locations on the hands. A' 6 (group) X 6 (size of electrode) latin square design was used, with location of electrode placement as the latinized variable. E a c h group had the electrodes placed on one of six possible location-size combinations. The latinized variable, location of electrode placement, was further broken down in a 2 (hand) X 3 (area on hand) factorial arrangement. Dependent measures were tonic S C level, and the phasic S C amplitude, latency, rise time, and recovery half-time of responses elicited by a series of loud tones, half-time. Differences in tonic and phasic reactivity at the different electrode placement locations were also examined. The results indicated a significant linear relationship between size of electrode and both tonic and phasic activity. Latency measures were not affected by electrode size; however, rise time and recovery i i i . half-time were. No differences in react ivity were found between the right and left hands. Differences were found, however, among the three locations on the hands for both tonic and phasic activity. T h e observed linear relationship between electrode size and S C supports current popular models of electrodermal act ivi ty and has implications for the comparison of results from studies in which different electrode sizes are used. iv. Table of Contents T i t l e Page Abstract i . Acknowledgements i i i . Table of Contents iv. Lis t of Figures vi . Lis t of Appendices v i i . I N T R O D U C T I O N 1. Basic E l e c t r i c a l Circuits 2. The Skin as a Site of E lec tr i ca l Act iv i ty 4. Models of Skin Conductance 6. Measurement of Skin Conductance 8. Terminology 8. Electrode Placement 9. Electrodes and Electrode Paste 9. Temperature ' 11. Size of Electrode Contact Area and Skin Conductance 12. Summary 17. M E T H O D 18. Subjects 18. Apparatus and Recording Techniques 18. Stimuli 19. Procedure 19. Skin Conductance Measures 20. Stat is t ical Procedures 20. V. RESULTS 22. Summary 32. DISCUSSION 38. Skin Conductance and Size of Electrode Contact Area 38. Site Reactivity 40. Time-based Measures and Spontaneous Skin Conductance Responses 41. Conclusion 42. REFERENCES 43. APPENDIX A 45. APPENDIX B 46. APPENDIX C 47. APPENDIX D 48. APPENDIX E 49. VI. List of Figures Number Description Page 1. Basic electrical circuit with one resistor (R) and a voltage source (battery). 3. 2. Simple series resistor circuit with two resistors and a battery. 3. 3. Simple parallel resistor circuit with two resistors and a battery. 3. 4. Electrode sites for skin conductance recording. 5. 5. Parallel resistor model of electroderraal activity (Montagu & Coles, 1966). 7. 6. Schematic skin conductance response. 10. 7. Mean phasic activity (SCR) to tones for each electrode size. 23. 8. Mean tonic activity (SCL) for each electrode size. 24. 9. Habituation of skin conductance responses to eight loud tones at each electrode size. 25. 10. Skin conductance response to the first of the eight tones at each electrode size. 27. 11. Skin conductance response to a cough or deep breath at each electrode size. 28. 12. Differences in site reactivity for skin conductance level. 30. 13. Differences in site reactivity for skin conductance response. 31. 14. Relative latency of skin conductance response at each electrode size. 33. 15. Rise time at each electrode size. 34. 16. Recovery half-time at each electrode size. 35. 17. Number of spontaneous skin conductance responses (SSCRs) at each electrode size. 36. v i i . List of Appendices Page Appendix A Consent Form 45. Appendix B Instructions 46. Appendix C Debriefing 47. Appendix D Experimental Design 48. Appendix E Table 1. Analysis of variance of averaged SCL data. 49. Table 2. Analysis of variance of averaged SCR data. 50. V i i i . Acknowledgements I wish to thank my supervisor, Dr. W. G . Iacono for all his guidance and support during the completion of this project. The helpful instruction and advice of my committee members, Drs. W. Linden and G. J . Johnson, were also greatly appreciated. I also wish to acknowledge the help of Virginia Green for her help with the technical aspects of the data analysis. Final ly, special thanks to my parents, Dr. John Mahon and Florence Mahon for their unwavering support. 1. Electrodermal measurement is a simple and convenient method of psychophysiological recording. Two commonly used measures of electrodermal act ivi ty (EDA) are skin conductance and its reciprocal , skin resistance. Skin conductance (SC) is a measure of the extent to which the skin allows the passage of an e lectrical current. Skin resistance (SR) is a measure of the degree to which skin resists or impedes the flow of e lectrici ty . These measures are based on the react ivity of the eccrine sweat glands and their innervation by the sympathetic branch of the autonomic nervous system. Despite the ease and apparent simplicity of electrodermal measurement, there have been problems comparing results across studies using different procedures. In addition, the underlying physiological mechanism involved in E D A is not fully understood. It has been only relatively recently that models of E D A have been formulated and proposals for standardization of measurement have been made. One remaining unresolved issue concerns the relationship between the size of the electrode contact area and skin conductance. V e r y few studies have been done to investigate this issue and those that have, have involved methodological shortcomings that limit the degree of confidence one can place in the results. There has not been clear support for the hypothesis that increases in the size of the electrode contact area are accompanied by increases in skin conductance. Such results, as will be shown below, are counter to expectation based on the currently favored models of the mechanism underlying S C and on the anatomy of the skin. At the present time it remains unclear what effect electrode size has on S C measurements and comparison of results across studies must be limted to those in which the same electrode sizes are used. T o faci l i tate an understanding of this area of research, it will be helpful to review basic e lectr ical c ircuitry because E D A , as mentioned above, refers to the e lectrical properties of the skin. Following this review will be a discussion of the 2. anatomy of the skin and the proposed mechanisms underlying SC. Terminology involved in SC measures will be presented prior to a discussion of the existing information regarding SC and electrode contact area. Because the present investigation is a methodological study, a detailed critique of the methodology used in prior studies examining the relationship between contact area size and SC will be presented. Basic Electrical Circuits Ohm's law states that the electromotive force (measured in volts, E) is equal to the product of the intensity of the current (measured in amps, I) and the resistance (measured in ohms, R). Figure 1 illustrates the simplest electrical circuit, consisting of a direct current (DC) voltage source (e.g., a battery) and a single resistor. There may be more than one resistor in an electrical circuit and these may be placed in series or in parallel. When resistors follow one another within a circuit, they are said to be connected in series, since the current must flow through the first resistor before reaching the second. The total resistance is equal to the sum of the individual resistances; i.e., Rrp = R^ + + ... + R . See Figure 2. Resistors may also be placed so that they are parallel to one another. Under these circumstances there is more than one pathway through which the current can flow. Thus, there is less resistance to the flow of current than there would be if the same resistors were connected in series. The total resistance is calculated by the following formula: 1 /R^ = 1/R^ + 1 / R 2 + ... 1 /R n - Thus, the total conductance of an electrical circuit with parallel resistors is equal to the sum of the reciprocals of the individual resistances (see Figure 3). Conductance (G) is the reciprocal of resistance and is measured in mhos or siemans (mhos = 1/ohms). Therefore, for Figure 2, 1 / G T = 1 /G^ + 1 / / G2> where and G 2 are the conductances of resistors R^ and Rg respectively. Likewise, for Figure 3, G^, - G^ + G„. For reasons which will become apparent below, electrodermal activity will be 3. Legend ^ resistor l | battery Figure 1. Basic electrical circuit with one resistor (R) and a voltage source (battery). 4 R R Figure 2. Simple series resistor circuit with two resistors and a battery. Figure 3. Simple parallel resistor circuit with two resistors and a battery. 4. discussed in terms of skin conductance rather than resistance. The Skin as a Site of Electrical Activity As mentioned above, electrodermal activity is based on the reactivity of the eccrine sweat glands. Eccrine sweat glands are distributed over the entire body with areas of higher density being located on the palmar and plantar surfaces. These glands are innervated by the sympathetic branch . of the autonomic nervous system and respond to thermal and other perceptual stimuli (e.g., sight, sound, etc.). Eccrine sweat glands, located on the palms and the soles of the feet, are involved in thermoregulation usually only when the environmental temperature exceeds 30 °C. It is believed that the function of palmar or plantar sweating is more for improving the grip than for providing evaporative cooling (Venables & Christie, 1980). Sweat glands originate in the subdermal layer of skin and send their ducts through the dermis and epidermis (outer layer of the skin). The outer layer of the epidermis (corneum) is composed almost entirely of cornified or dead skin and is highly resistant to the flow of electricity. When the sweat ducts are empty, the skin is highly resistant. This resistance 'is determined almost entirely by the epidermis. Sweat is highly conductive since it contains the electrolyte, sodium chloride (NaCl). When the sweat ducts are full, they provide low resistance (or highly conductive) pathways through the epidermis. The physiological mechanisms involved are far more complicated than this basic overview implies; however, this presentation will suffice for the purposes of the present discussion. Electrodes for SC measures are typically placed on the palms of the hands or the soles of the feet because these areas have the highest densities of eccrine sweat glands. The palms are more accessible than the feet and, therefore, are the preferred sites from which to obtain SC recordings. Figure 4 illustrates the surface anatomy of the palm and common electrode placement sites for SC recording, such Lure_4. Electrode s i t e s for skin conductance recording. 6 . as the thenar and hypothenar eminences, and the phalanges of the fingers. In addition, it indicates the dermatomal distribution (i.e., areas of skin innervated by different spinal nerves) of the palm. Models of electrodermal act iv i ty have been formulated on the basis of the anatomy of the skin and using basic e lectrical concepts. Models of Skin Conductance Thomas and Korr (1957) examined the relationship between the number of active sweat glands (i.e., secreting sweat) and electrodermal act ivi ty . The ir results indicated an approximatel linear relationship between the two, with skin conductance increasing as the number of active sweat glands increased. They proposed that sweat glands may act as parallel resistors (or conductors) so that as each additional sweat gland becomes active, S C also increases in an additive, linear fashion. Montagu and Coles (1966) formulated their "parallel resistor" model of electrodermal activity on the basis of this information (see Figure 5). According to this model the sweat glands are seen as binary, potentially conducting pathways arranged in parallel . E a c h sweat gland may contribute additively to the overall conductance depending on whether it is active or inactive. In Figure 5, the sweat gland pathways are represented by r^, r^, r 3 > . . . r j in the diagram. Dry skin contributes in a small way to the overall skin conductance and is represented as R Q . Conductance attributable to the body interior is represented as a series resistor (Rj), but this conductive pathway contributes so little to the overall S C that it may be ignored. Skin capacitance (C) is a relevant factor only when an alternating current (AC) rather than a direct currect (DC) is used in measuring S C . The passage of a D C current through the skin is preferred in the measurement of S C since one can measure simply conductance or resistance without having to consider additional factors such as capacitance. The concept of a parallel resistor model has formed the basis for subsequent F igure 5. P a r a l l e l r e s i s t o r model of e lect rodermal a c t i v i t y Montague and Coles (1966), Legend C = c a p a c i t o r R0= dry s k i n r e s i s t a n c e r , r , r , . . . , r = sweat gland pathways Rj= body i n t e r i o r r e s i s t a n c e 8. models of S C . Edelberg (1972) and Fowles (1974) increased the complexity of the model by focussing on the e lectr ical properties of each individual sweat gland. Included in their formulations are a number of factors found to influence S C levels and responses such as: the initial fullness of the sweat duct; degree of hydration of the corneum (outermost layer of the skin); activity of associated membranes; and the influence of the surrounding areas of the sweat duct. Further detail may be found in Edelberg (1972) and Fowles (1974). However, for the purposes of the ' present investigation, it is important to note that the parallel resistor model of electrodermal act iv i ty is maintained. On the basis of the parallel resistor model, it is much simpler and more logical to measure electrodermal activity in terms of S C rather than resistance. Predictions made on the basis of this model indicate that increases in sweat gland act ivi ty lead to increases in S C . Supportive evidence was found for this prediction in one of the first studies designed to evaluate this relationship (Thomas & Korr, 1957), therefore, it would appear to be simpler to measure S C directly. In addition, unlike the situation for conductors, resistors in parallel are additive in terms of their reciprocals (see Figure 3). Therefore , when measuring conductance, each independent pathway contributes additively to the overall conductance, but a much more complex function exists when resistances are compiled. The individual resistors are interdependent in such a way that the change in overall resistance due to a change in one unit (e.g., a sweat gland), is dependent upon the resistances of the other pathways. Thus, electrodermal activity appears to be more easily measured as skin conductance rather than skin resistance. Measurement of Skin Conductance Terminology. Electrodermal activity includes both tonic (slowly changing baseline levels of activity) and phasic (response) components, which are referred to as the skin conductance level (SCL) and skin conductance response (SCR) 9. respectively. The S C R also involves several components which need to be defined. Figure 6 illustrates a schematic S C response. Latency (lat.) refers to the time between the stimulus onset and response onset; rise time (ris. t.) is the time from the response onset to the response peak; S C R amplitude (amp.) indicates the size of the response; and recovery half-time (rec. t/2) refers to the time taken for the response to return halfway to the baseline (SCL). When many responses are el icited by the presentation of a series of stimuli, the average size of response calculated over all trials to which a response is given is called the amplitude. The average size of a response for all trials on which a response might have been given to a stimulus (not all stimuli elicit a response) is called the magnitude (Venables & Christie, 1980). Occasionally, a response may be given to no apparent stimulus. Such responses are called spontaneous skin conductance responses (SSCRs) and are useful as an indication of overall physiological arousal. The more S S C R s , the more aroused and reactive a subject is. Electrode Placement. Skin conductance is typically measured using bipolar recording, which means that both electrodes are placed on active sites. Act ive sites are those locations with high densities of eccrine sweat glands, for example, the palms. T o avoid obtaining an electrocardiogram (ECG) artifact , electrodes must be placed on the same hand. When electrodes are placed such that their circuit spans the heart, the heart signal may also be recorded. Electrodes should also be placed on the same dermatomal area (see Figure 4). Placement of electrodes within the same dermatome is necessary since, as Venables and Christie (1980) reported, electrodermal activity has been found to vary among different dermatomes. Thus, common sites for bipolar S C recording have been the distal or medial phalanges of the first and second fingers, and the thenar and hypothenar surfaces. These are il l lustrated on Figure 4. Electrodes and Electrode Paste. In measuring S C , it is important to use Figure 6. Schematic skin conductance response. .Response peak Legend l a t . » la tency r i s . t . - r i s e time rec t /2 - recovery h a l f - t i m e amp, A - amplitude SCL SCR lat. Stimulus onset Response onset 11. nonpolarizing electrodes, for example, s i lver/s i lver chloride (Ag/AgCl) electrodes. These electrodes consist of a metal in contact with a solution of its own ions. Polarizing electrodes build up an electrical charge with the passage of current, thus acting as miniature batteries that distort the S C signal. Fowles, Christ ie , Edelberg, Grings, Lykken, and Venables (1981) recommended A g / A g C l electrodes for S C measurements because they have a low polarizing potential. Electrode paste consists of a medium which contains an electrolyte. The preferred electrolyte for S C measurement is sodium chloride (NaCl), since it is the major salt found in sweat. Electrolytes with multivalent ions (i.e., ions carrying e lectr ical charges greater than one) such as zinc (Zn ), calcium (Ca ), and +3 aluminum (Al ) are not recommended because of the potentiating effect they have on S C (Venables & Christ ie , 1980). A potentiating effect means that it would exaggerate the S C measures. A variety of media in which to suspend the electrolyte have been used such as agar and Unibase (a neutral ointment cream). Fowles et al . (1981) recommend Unibase as a preferred medium because of its long shelf-life as an electrode paste. Fowles et al . (1981) noted that the amount of time elapsing after the applicaton of electrode paste is a variable which must be considered in S C measurement. It takes time for the electrode paste to establish an equilibrium with the electrolytes in the skin. They suggest a stabilization time of at least ten minutes, but preferably 15 to 20 minutes. This may be an important factor in studies in which the size of the electrode contact area is varied since the smaller amount of paste required for smaller electrode sizes may reach equilibrium more rapidly than the larger amount required for larger sizes. Temperature. Environmental or ambient temperature has been found to have an effect on S C . Edelberg (1971) reported that the reciprocal of S C (skin resistance) increases by three percent with each degree centigrade decrease in temperature. It 12. is important, therefore, to maintain temperature at a constant level with an experiment involving electrodermal measurement. Size of Electrode Contact Area and Skin Conductance From Thomas and Korr (1957) and the proposed e lectr ical models of electrodermal activity, it would be expected that as the size of the electrode contact area increases, S C would increase concomitantly in a linear fashion. Surprisingly few studies have been carried out investigating this relationship directly and those that have, have obtained equivocal results. Blank and Finesinger (1964) were the first to investigate this relationship. They were concerned with the effect of the size of the electrode contact area on the S C L of essentially inactive skin (i.e., where there is little or no sweat gland activity). They attached pairs of electrodes to the inner surface of each forearm because there are fewer and less active sweat glands at this location, thus minimizing moisture contributed by sweat. Zinc electrodes were used with an electrolyte solution of one percent zinc sulfate (ZnSO^) in five percent agar. They used three sizes of paired electrodes: one, two, and four cm diameters. These were attached one pair at a time, one to each forearm. Electrodes were in place for a maximum of one minute, so that any changes in the skin due to the passage of current or to hydration of the skin were minimized. Repeated measures of S C were taken for each electrode size on the six subjects tested. A four minute recovery period was allowed between measurements. They found that the S C increased as the size of the electrodes increased for all six subjects, but the exact nature of the relationship was not examined. These results are interesting in that they indicate that the conductance of essentially dry skin (i.e., few sweat glands activated) increases with increasing electrode size. Since the present investigation is focussed on sweat gland activity, however, Blank and Finesinger's work is of limited utility. Furthermore, their 13. results were derived from rather crude techniques by present standards (Fowles et al., 1981). For example, they used Zn/ZnSO^ electrodes which have been found to potentiate electrodermal responses because of the contact of the multivalent ions on the skin. Another problem exists with these electrodes, since they tend to acquire an oxide coating over time. This coating acts as an insulator and must be scraped off periodially. A final methodological problem concerns the location in which they placed the electrodes on the body. The electrodes for bipolar recording used in this study were placed on each arm. These placement areas are innervated by different spinal nerves and, as mentioned earlier, variations in EDA have been found within different dermatomal areas (Venables & Christie, 1980). Recordings of EDA taken from these sites would also be subject to ECG artifact. Edelberg and Burch (1962) examined the nature of the relationship between the size of contact area and SC. They reported an "approximately linear relationship". Their procedure was not clearly described and no data are presented. They appear 2 to have varied the size of contact area by using from one to nine 0.3 cm. electrode sites placed "in parallel" (presumably in a row) on one hand. Electrode placements such as these would necessarily include more than one dermatomal area. They used six to ten subjects in the experiment (the exact number was not specified), and it is not clear as to the type of electrode or electrode paste used. Furthermore, no mention was made regarding the duration that the electrodes were in place, or of controlling for other potentially influencing factors such as temperature fluctuation. Thus, although these results suggest a linear relationship between size of electrode contact area and SC, they are far from conclusive. Based upon the results of the two studies discussed above, Lykken and Venables (1971) proposed that because there appeared to be a linear relationship between size of electrode contact area and SC, all measurements should be 2 reported in terms of "specific conductance" (i.e., jumhos/cm ). This proposal has 14. implications for comparison of results from studies that use different sizes of electrodes to record S C . If results were reported using specific conductance units, one should be able to directly compare the results from different studies regardless of the sizes of electrodes used. This proposal has been challenged by Mitchel l and Venables (1980) who also examined the relationship between the size of electrode contact area and E D A . From a preliminary study in which electrode collar sizes ranging from 0.159 to 2 0.786 cm were attached to the volar surfaces of the fingers, it was found that o the predicted monotonic relationship held only for electrode sizes up to 0.503 cm . With larger contact areas, there was no further increase in E D A . They noted, however, that these results were tentative because it was diff icult to attach electrodes, particularly the larger ones, to the convex surfaces of the distal phalanges. Furthermore, they had difficulty ensuring that the electrodes were centered over the larger collar sizes. This could have prevented full exposure of the electrode to the surface area. Consequently, they conducted a series of three experiments to study this relationship with more experimental control. They used eight subjects in each • of the first two experiments. The temperature within the lab varied from 19 to 22 ° C . Four pairs of A g / A g C l 2 electrodes were employed with four collar sizes ranging from 0.159 to 0.786 cm. in 2 the first experiment, and from 0.0 to 0.096 cm in the second. They used 0.5% potassium chloride (KC1) in an agar base as the electrode paste and maintained a constant voltage of 0.5 volts between electrodes while measuring S C . Electrodes were placed on the thenar and hypothenar surfaces of both hands for bipolar recording. They indicated that they used a latin square design to balance electrode location across collar sizes. Subjects were randomly assigned to one of the four site-size combinations. Skin conductance level was measured after an adaptation period of five minutes and at the onset of a series of four tonal stimuli. Skin 15. conductance response measurements (amplitude, latency, rise time, and recovery half-time) were obtained for the responses to each of the four tones. Trend analysis of the results of the first experiment indicated that for S C L , 91% of the variance was accounted for by a linear trend, and for S C R amplitude data, 61% of the variance contributed to a linear trend. The other time-based measurements (latency, rise time, and recovery half-time) did not indicate any significant trends. They also noted that, as in their preliminary study, S C R 2 amplitude did not increase for collar sizes over 0.503 cm . Thus, they concluded that the relationship between the size of electrode contact area and S C appeared to be nonmonotonic, contrary to expectations. 2 Experiment 2 was conducted using collar sizes from 0.0 to 0.096 cm , since they found that the regression lines of S C L and S C R data "did not pass through the axis of origin". They hypothesized that a "different relationship" may exist for 2 collar sizes less than 0.159 cm . They found significant linear trends as a function of size for both S C L and SCR-amplitude data; however, the results of this experiment are highly questionable because higher S C L and SCR-amplitude values were found for smaller mask sizes than' for those used in Experiment 1. The authors pointed out that error in measurement attributable to seepage of electrode paste under the electrode collars and hydration of the corneum may have contributed to these results. They also suggested that the small sample size may not have been adequate to control for differences in reactivity among the four electrode sites. Experiment 3 was designed to determine whether there were any differences in the react iv i ty of the four sites used. They used five subjects and collar sizes of 2 0.159, 0.385, and 0.786 cm . They allowed only three minutes for adaptation and total testing time was less than 20 minutes. Recordings of the S C L s indicated that the hypothenar surface was more reactive than the thenar surface. They also found 16. that despite large individual differences, the general trend was again for S C L s to 2 be actually lower with collar sizes of 0.786 cm than for smaller collar sizes of 2 0.385 cm . On the basis of these three experiments, Mi tche l l and Venables suggested that due to the counterintuitive nature of some of these results (i.e., that larger electrode contact areas should yield lower measures of EDA) , it is necessary for them to be confirmed by a another experiment. Until this has been done, they suggested that all conductances be reported along with the electrode sizes used, 2 and not as specific conductances (jimhos/cm ), as suggested by Lykken and Venables (1971). Furthermore, they pointed out the need to record bipolar measurements from within one dermatomal area and to compare recordings from different electrode sites with caution because of differences in site reactivity. Fowles et al. (1981), in a paper proposing standards for E D A recording, recommended that until the issue regarding the relationship between size of electrode contact area and E D A is resolved, investigators should use areas of 0.786 2 cm , because with smaller areas the problem of seepage is more likely. The purpose of the present study was to resolve the issue regarding the relationship between E D A and size of electrode contact area. In addition to the precautions taken by Mitche l l and Venables with regard to electrode placement, a constant ambient temperature was maintained to eliminate the influence of this variable on E D A measures. Furthermore, a stabilization period of 20 minutes was provided in order for all the electrode contact areas to have enough time to attain equilibrium. Finally, to acheive greater confidence in the results, a larger sample size was tested, and more electrode sizes and response stimuli were used. With these added methodological precautions, it was hypothesized that the results would confirm the theoretically predicted linear relationship between S C and electrode size. If this hypothesis is supported, it will be possible to reconsider the proposal 17. made by Lykken and Venables (1971) to express S C data in specific conductance units. Summary Electrodermal measurement is a widely used psychophysiological technique. Considerable variabil i ty in recording procedures is possible, therefore, proposals for standardization have been made to faci l i tate comparison of results across experiments. One issue that remains to be resolved concerns the relationship between the size of the electrode contact area and S C measures. Skin conductance refers to the degree to which skin allows the passage of an e lectrical current. Sweat glands in skin faci l i tate the passage of e lectrici ty when they secrete sweat because sweat contains the electrolyte, sodium chloride (NaCl). When increasing numbers of sweat glands are active (i.e., secreting sweat), the conductance of the skin would be expected to increase. Theoret ica l models of electrodermal activity have been proposed on the basis of anatomical and physiological knowledge of the skin and basic e lectr ical principles. Using these models, one would predict that a monotonic relationship exists between size of electrode contact area and skin conductance. However, this prediction has not been substantiated thus far. Very few studies have investigated this relationship and because of methodological problems, the results are not conclusive. These studies have typical ly involved small sample sizes and exhibit procedural problems concerning electrode placement area, type of electrodes and electrode paste used, and insufficient adaptation time after application of electrodes. The present investigation involved greater methodological rigor than that used in past studies in order to obtain a resolution to the question regarding the relationship between size of electrode contact area and skin conductance measures. 18. Method Subjects Fifty-nine, right-handed males served as subjects in response to an advertisement posted on the university campus. Eleven of these individuals were not included in the final analysis because: 1) some aspect of the procedure was discovered to be in error (e.g., electrodes were improperly placed); 2) the equipment failed; or 3) the subject responded to less than three of the stimuli presented. The remaining forty eight subjects were randomly assigned to six groups of eight subjects each in order to accomodate a 6 X 6 latin square design. A l l subjects gave informed consent and were paid for their participation in the one hour session. Apparatus and Recording Techniques Six pairs of Beckman 1 cm A g / A g C l electrodes were used in conjunction with the following equally spaced collar sizes: 0.131, 0.262, 0.393, 0.524, 0.655, and 2 0.786 cm . Electrode collars were made before each experimental session out of Micropore waterproof tape. This tape was' tested prior to the study for its ability to resist water and to act as an e lectr ical insulator. The electrolyte consisted of physiological saline (0.9% NaCl) in Unibase according to the proportions provided by Fowles et al. (1981). Electrodes were attached to the thenar and hypothenar surfaces, and to the medial phalanges of the first two fingers of each hand. T o ensure that no leakage of electrode paste occurred between the electrode collars and the skin, two electrode collars were used to attach each electrode. One set of electrode collars was f irst attached to the recording sites and an additional set was attached to the electrodes. The electrodes were then centered and adhered to the collars on the recording sites. Skin conductance was recorded by six channels of a Beckman R612 19. polygraph using six Beckman skin conductance couplers, type 9844. Maximum sensitivity was 0.5 umhos/cm of chart deflection. Stimuli Eight 1000 Hz,105 db tones served as the response elicit ing stimuli. A l l had a duration of one second with rise and fal l times of 40 ms. These stimuli were delivered binaurally through A K G K240 stereo headphones and were presented against a 50 db pink noise background. The tones were presented at pseudorandom intervals ranging from 25 to 90 seconds. Temperature in the laboratory remained constant at 21 ° C for all subjects. Procedure Upon reporting to the experiment, subjects were required to wash their hands with soap and water so that all subjects started with the same degree of hydration. The procedure was described to the subject who was then asked to sign the consent form (see Appendix A). The subject was seated in a comfortable chair and the six pairs of electrodes were attached to his hands. Subjects were pseudorandomly assigned to one of the six possible site-size combinations for electrode attachment. Eight subjects were in each of the six groups. Af ter placement of electrodes, subjects were instructed that they would have to wait 20 minutes before beginning the experiment in order for the electrode paste to stabilize with the electrodermal system. During this adaptation period, they listened to prerecorded music of their choice. After 20 minutes, the subject was informed that the experiment would begin. Headphones were placed on the subject, and a curtain separating the experimenter and polygraph from the subject was drawn. Prerecorded instructions were delivered through the headphones prior to presentation of the eight tones (see Appendix B). The subject was asked to cough three times. He was then asked to hold his breath for fifteen seconds. This was to ensure that the equipment was working properly and to activate the electrodermal 20. system so that there was some sweat in the ducts before the tonal stimuli were presented. This also allowed for the assessment of responses to self-generated stimuli. In order to reduce the habituation effect (i.e., gradual reduction in response to successive stimuli), the subject was asked to pay careful attention to the tones by anticipating them and counting them. At the end of the experiment, which lasted ten minutes, the electrodes were carefully checked upon removal for leakage and also to ensure that they encompassed the entire area of skin exposed by the collar. Every subject was given a thorough debriefing at the end of the session (see Appendix C). Skin Conductance Measures Skin conductance level was measured prior to the onset of each of the eight tones. Skin conductance response (amplitude) was measured as the difference between the baseline level and the peak of the response. A response was defined by a minimum amplitude of 0.05 micromhos. Skin conductance response (amplitude) was measured in response to each of the eight tones plus to the largest of the self-generated responses. The largest response was selected simply because it afforded the greatest precision possible in measurement across the six recording sites. The number of SSCRs were counted during the SC recording period and were defined as responses with a minumum amplitude of 0.05 micromhos which occurred during the period from ten seconds after each tone to the onset of the next tone. Time-based measures (lat., ris.t., and rec.t/2) were taken for the SCR to the first tone (see Figure 6). Latency was measured relative to a common point across the six recording sites because it was not possible to accurately record the onset of the stimuli. Statistical Procedures A (6) group X (6) collar size latin square analysis of variance was done for both tonic (SCL) and phasic (SCR) activity. The main effect of collar size was 21. analysed using trend analysis. It was expected that the relationship between S C and size of electrode contact area would be linear. T h e latinized variable, location, was broken down into "hand" and "location on the hand" which exist in a factorial arrangement. It was expected that no significant differences would exist between the S C measures taken from each hand. Analysis of placement sites, however, was expected to corroborate Mitche l l and Venables finding that the hypothenar surface is more reactive than the thenar. Venables and Christ ie (1980) reported that the finger sites have been found to be slightly less responsive than the other two electrode placement areas, thus analysis was expected to support this finding. Analyses of variance were done on the time-based measures: latency, rise-time, and recovery half-time for the first of the tone-elicited responses. No significant effects effects of size or location were expected, as reported by Mitche l l and Venables (1980). Spontaneous skin conductance responses were analyzed for differences between collar sizes. It was expected that if differences existed, fewer S S C R s would be observed for smaller mask sizes than for larger mask sizes. T o reduce the probability of Type I error in the repeated measures analyses of variance (ANOVAs) described above, the epsilon-correction procedure described by Greenhouse and Geisser (1959) was used to adjust the degrees of freedom. The F tests, which use repeated measures and are presented below, include the unadjusted degrees of freedom together with the value of epsilon associated with the error term and the corrected £ value. 22. Results The results of this investigation will be presented with regard to the major hypotheses. T h e A N O V A source tables are presented in Appendix D. Data were averaged over the eight S C R and S C L measures. These measures were then collapsed over electrode size and possible group differences were analysed with a one way A N O V A . This analysis revealed no significant differences among the groups for both S C L , F(5,42) = 0.30, £ > . 0 5 and S C R , F(5,42) = 0.39, £ > . 0 5 measures. This result was expected since subjects were pseudorandomly assigned to groups. These groups were necessary in order to construct the latin square design. In light of this result, the data were collapsed over groups and repeated measures A N O V A s were done to examine the main effect of size. In addition, trend analyses were carried out. These analyses revealed significant main effects of size for both S C R , F(5,210) = 36.50, £ < . 0 0 1 , and S C L , F(5,210) - 47.67, £ < .001. In addition, a significant linear trend was indicated for both S C R , F(l,47) = 72.97, £ < . 0 0 1 and S C L , F(l,47) = 97.50, £ < . 0 0 1 . No other trends were indicated in the data (see Figures 7 and 8). T o investigate the effects of habituation (i.e., gradual reduction in response to repeated stimuli), a repeated measures A N O V A was carried out on the eight S C R measures. The mean S C R to each tone for each electrode size is plotted in Figure 9. As expected, the analysis revealed a significant size effect, F(5,282) = 13.46, E<.001 and a significant habituation effect, F(7,1974) = 112.85, £ < . 0 0 1 , 1 =.62. In addition, there was a significant interaction between size of contact area and habituation rate, F(35,1974) = 4.69, £ < . 0 0 1 , 1 =.62, indicating that habituation proceeded at a faster rate when larger collar sizes were used. In order to examine response amplitude effects independent of the influence of habituation, an A N O V A was done on the S C R to the first tonal stimulus. Again, a significant effect for size was observed, F(5,235) =31.73, £ < . 0 0 1 , i-=.72. with the 23. Figure 7. Mean phasic act iv i ty (SCR) to tones for each electrode size. 24. I-• 131 .262 «393 . 5 24 . 6 5 5 .786 ELECTRODE SIZE (CM 2 ) Figure 8. Mean tonic act ivi ty (SCL) for each electrode s ize. 25. TONES F igure 9. Habi tuat ion of s k i n conductance responses to e ight loud tones at each e lec t rode s i z e . 26. data significantly following a linear trend, F(l,47) = 113.30, £<.001 (see Figure 10). An additional ANOVA was conducted on the self induced SCR (i.e., SCR elicited by the subject coughing or taking a deep breath) to see if the results were different than those obtained for the externally produced SCRs. As expected, the results were consistent with the above, showing a significant size effect, F(5,210) = 37.29, 2<.001, and a linear trend, F(l,47) = 81.66, £<.001 (see Figure 11). The relationship between SC and electrode size, therefore, is linear. Thus, it is possible to reconsider Lykken and Venables' (1971) proposal to report SC measures 2 in terms of specific conductances (^rohos/cm )• To calculate the specific conductance, an additional data point at the axis of origin was included in a linear regression analysis of the averaged SCR and SCL data. Although not formally tested, it may be assumed that if there is no contact between an electrode and the skin, no EDA will be recorded. Thus, the curves for SCR and SCL must intersect zero on the "y" axis. Assuming this is true, and given the equation for a straight line (y = mx + b), it follows that skin conductance = m (electrode size) + b. If the "y" intercept (b) is zero, the slope of the line (m) is equal to the 2 conductance (in jimhos) divided by the size of electrode (in cm ). Thus, the slope of 2 the regression line is equal to the specific conductance Oumhos/cm ) of these measures, provided the line passes through the origin. A linear regression analysis was performed on the averaged SCR and SCL data, including a data point at the origin ("0,0") for both measures. This analysis produced the following equations for the lines of best fit: SCR = 1.41 (electrode size) + 0.08; and SCL - 5.98 (electrode size) + 0.78. The average of the differences between the SC data and the expected values derived from the regression lines were 0.04 jimhos and 0.36 /lmhos for SCR and SCL respectively. The regression line calculated for tonic (SCL) activity intersects the "y" axis at 0.78 /imhos. This substantial deviation from the origin indicates that the slope 27. •131 .262 .393 .524 .655 .786 ELECTRODE SIZE (CM 2 ) Figure 10. Skin conductance response to the f i r s t of the eight tones at each electrode s i z e . 28. 2.01 eTTSl ,262 »393 .524 .655 V7&6 ELECTRODE SIZE (CM 2 ) Figure 11. Skin conductance response to a cough or deep breath at each electrode s ize. 29. of the line may not be considered to be an estimate of the specific conductance value. The regression line calculated for the phasic (SCR) data, however, intersects the "y" axis at 0.08 ;umhos, which is relatively close to the origin. In addition, as mentioned above the SCR data deviates on average only 0.04 umhos from the expected values. The slope of this line can be considered to be a fairly accurate estimate of the specific conductance value for phasic activity to the stimuli used in this experiment. Differences in reactivity of the sites used for electrode placement were analyzed using a two (hand) X three (site location on hand)) factorial ANOVA for both averaged SCL and SCR measures. For SCL, results indicated no significant difference for hand, F(l,210) = 1.68, £>.05 but a significant effect for area on the hand, F(2,210) = 80.52, £< .001 . There was no significant interaction between hand and location, F(2,210) = 1.06, £>.05 . Results were similar for SCR data with no significant difference between hands, F(l,210) = 0.08, £>.05 and a significant difference for location of electrode placement, F(2,210) = 90.62, £< .001 . There was no interaction between hand and location, F(2,210) = 0.01, £> .05 . The Newman-Keuls post-hoc multiple comparison procedure was used to investigate the significant effect found for the three locations of electrode placement. A .05 significance level was adopted. This analysis revealed that for SCL data: 1) the hypothenar surface was significantly more reactive than the thenar surface; and 2) the hypothenar surface was significantly more reactive than the finger placement sites. This result was also found for phasic (SCR) activity. In addition, the thenar surface was also significantly more reactive than the finger placement sites (see Figures 12 and 13). Time-based measures (latency, rise time, and recovery half-time) were analyzed with repeated measures ANOVAs including trend analyses. Results revealed no significant size effect for latency, F(5,210) = 2.78, £>.05 (see Figure 14). Rise time, 30. 2 SITE Figure 12. Differences i n s i t e r e a c t i v i t y for skin conductance l e v e l (1 = thenar eminence, 2 = medial phalanges, 3 = hypothenar eminence). 31. Figure 13. Differences i n s i t e r e a c t i v i t y for skin conductance responses (1 = thenar eminence, 2 = medial phalanges, 3 = hypothenar eminence). 32. however, did yield a significant effect for size of electrode contact area, F(5,210) - 7.05, £< .001 , with both a significant linear,F(l,47) = 11.10, £<.01 and a cubic trend, F(l,47) = 6.83, £< .01. See Figure 15. Recovery half-time also exhibited a significant size effect, F(5,210) = 3.55, £< .01 . The data, however, conformed significantly to a quadratic, F(l,47) = 5.4, £<.05 and a cubic trend, F(l,47) = 5.27, £< .05 . No linear trend was indicated (see Figure 16). The number of spontaneously emitted SCRs were counted for each electrode size and a repeated measures ANOVA was conducted. Results indicated a significant effect of size, F(5,210) = 11.58, o < .001 and trend analysis revealed a significant linear, F(l,47) = 20.99, £<.001 and quadratic trend, F(l,47) = 5.77, £<.05 (see Figure 17). Summary The results of this study may be summarized as follows: 1) A significant linear trend for size was found for both tonic (SCL) and phasic (SCR) activity. 2) No other trends were evident in the tonic and phasic data. 3) A significant habituation effect was observed for SCR data in addition to an interaction between habituation rate and size of electrode contact area. 4) A significant linear trend was observed for both the self induced SCR (response to cough or deep breath) and for the first of the SCRs to the eight tones. 5) No significant difference between reactivity of the right and left hands was found. 6) Significant differences were observed for the site of electrode placement. For SCL, results indicated the following: a) hypothenar surface significantly more reactive than the thenar surface; and b) hypothenar surface significantly more reactive than the finger placement sites. For SCR data, results indicated the same 33. 10.01 o LL) E L E C T R O D E S I Z E ( C M 2 ) F igure 14. R e l a t i v e la tency of s k i n conductance response at each e lec t rode s i z e . 34. E L E C T R O D E S I Z E ( C M 2 ) Figure 15. Rise time at each electrode s i z e . 35. 20.0J , , , .131 .262 .393 .524 .655 .786 E L E C T R O D E S I Z E ( C M 2 ) Figure 16. Recovery half-time at each electrode s i z e . 36. I6l •TTi 7262 .133 . 5 * 4 .6^5 .7§6 E L E C T R O D E S I Z E ( C M 2 ) Figure 17. Number of spontaneous skin conductance responses (SSCRs) at each electrode size. 37. as above, in addition to the thenar surface being significantly more reactive than the finger sites. 7) No significant difference in latency was found for different sizes of contact area. 8) There was a significant difference in rise time for different sizes of contact area. Both a linear and a quadratic trend were observed. 9) A significant difference in half-recovery times for differenct sizes of contact area was found. Quadratic and cubic trends were indicated in the data. 10) Significant linear and quadratic trends were observed for the number of spontaneous S C R s for different sizes of contact areas. 38. Discussion The effect of the size of electrode contact area on SC has not been clearly resolved. On the basis of anatomical structure and current electrodermal models of the skin, it was hypothesized that SC will increase with increasing sizes of electrode contact area. In addition, this relationship was expected to be linear. This hypothesis was not supported in the most recent study investigating this relationship (Mitchell & Venables, 1980). However, it was noted that methodological problems existed within this study which may have contributed to these counterintuitive results. Further investigation was warranted, therefore, with careful consideration being given to methodological control. The present investigation was designed for this purpose following the guidelines for electrodermal measurement outlined by Fowles et al. (1981). In addition, a much larger subject sample was examined in the present study than were in the Mitchell and Venables' study, the temperature in the laboratory was kept constant, and the electrode collars were carefully tested to ensure that they were both water resistant and electrical insulators. Skin Conductance and Size of Electrode Contact Area The results of the present study clearly support the hypothesis that SC (both tonic and phasic activity) increases monotonically with increasing sizes of electrode contact area. This relationship was also found to follow a linear trend. No other trends were found in the data. This effect was evident for phasic activity, both in response to loud tones and to self-induced SCRs (i.e. response to cough or deep breath). Thus, the relationship holds for these two general types of stimuli. A significant habituation effect was observed over the SCRs to the eight tones, in addition to an interaction between habituation rate and size of electrode contact area. Habituation to repeated stimuli is virtually unavoidable and the interaction indicates that the rate of decline in SCR amplitude is faster with 3 9 -larger electrode sizes. This could be explained by a "floor effect" such that smaller electrode sizes allow less initial electrical conductance and therefore, have a relatively smaller range through which SC can decrease. This habituation effect and interaction with size, however, may have exaggerated the linear relationship between SC and electrode size. The results of an additional analysis on the SCRs to the first tone indicated that the monotonic, linear relationship still held for responses not subject to habituation. As discussed previously, Mitchell and Venables' found that with electrode sizes 2 larger than 0.503 cm , the SC measures did not increase concommitantly to the expected values based on a linear regression analysis. They concluded that the relationship between SC and electrode size was nonmonotonic. The discrepancy between Mitchell and Venables' results and those of the present study may be explained by the difference in the time allowed for adaptation (i.e., stabilization of electrode paste with the electrodermal system). Mitchell and Venables allowed only five minutes for adaptation before beginning their experiment. This may not have 2 been sufficient to allow areas greater than 0.503 cm to stabilize. Thus, optimal responses would not be obtained from these electrodes. In addition, they tested only eight subjects which may not have been sufficient to reduce the influence of individual differences, which is substantial in psychophysiological measures. It appears, therefore, that with careful methodological control, the relationship between EDA and size of contact area is monotonic and linear as predicted. Calculation of the regression line for phasic activity (including a data point at the of origin) produced a line that passed very close to the origin ("y" intercept = 0.08 jimhos). The average deviation of the actual SCR data from the regression line was only 0.04 jumhos. Thus, the slope of this line is a good estimate of the specific conductance value for phasic activity. This estimation of the specific conductance value should be replicated, however, before it can be considered to be 40 . a constant across laboratories and experiments. If it is replicated, it would be possible to use the regression line to interpolate S C values for different electrode sizes. In addition, it supports Lykken and Venables (1971) proposal to report E D A in specific conductance units which would allow direct comparison of E D A data across experiments. T h e regression line calculated for tonic act ivi ty deviated substantially from the origin ("y" intercept = 0.78 yumhos). In addition, S C L data deviated to a greater extent than S C R data from the respective lines of best fit for these data. The slope of the S C L line, therefore, may not provide an accurate estimate of the specific conductance. As discussed earlier, i f there is no contact between an electrode and the skin, no E D A will be recorded. Thus, there must be a data point at the origin. There are two possible explanations for the failure of the S C L regression line to pass through the origin. The first is that there may have been a systematic error in the recording of S C such that the whole curve was elevated by a constant value (the value of the "y" intercept). This is unlikely, however, because of the careful calibration of the recording equipment and the methodological control implemented in the study. T h e second possibility is that a nonlinear relationship exists between S C L and electrode sizes smaller than 0.131 2 cm . It is possible that when smaller electrode sizes are used, the number of sweat glands that will be in contact with the electrode is more variable than when larger collars are used.. If this is so, the parallel resistor model may only be applicable for electrode sizes large enough to encompass a large number of sweat glands. Further investigation is needed to verify these hypotheses. Site React iv i ty As expected, there were no significant differences in electrodermal reactivity between the right and left hands. This result was found for both tonic and phasic act ivi ty . Differences were expected for E D A among the three placement sites on 41. the hand. The results of the present study confirmed Mitchel l and Venables' finding that for tonic act ivity , the hypothenar surface is more reactive than the thenar. In addition, this study indicated that the hypothenar (but not the thenar surface) is more reactive than the finger placement sites. Mitche l l and Venables did not report any data for phasic activity, however, the present investigation found results similar to those for tonic activity, with the additional finding that both the hypothenar and thenar surfaces are more reactive than the finger placement sites. Thus, the most sensitive measures of E D A (SCL and SCR) will be obtained from the thenar and hypothenar surfaces. These results on site reactivity indicate that skin conductance recorded from either hand is comparable, provided that homologous sites are used. In addition, for tonic activity, data recorded from the thenar eminence and finger placement sites is comparable. A similar relationship does not hold between the hypothenar eminence and either of the other two sites. For phasic activity, the different palmar electrode placement sites yield different extimates of reactivity, thus rendering difficult the comparison of values recorded across sites. Time-based measures and spontaneous skin conductance responses. The results from the present study on latency were similar to those reported by Mi tche l l and Venables, indicating that latency was not affected by electrode size. This dependent measure, therefore, may be compared across E D A recordings taken with different electrode sizes. Rise time and recovery half-time, however, were significantly affected by electrode size, contrary to expectation. The relationship between electrode size and these measures was found to be complex and further investigation is necessary before these results can be explained. Comparison of rise time and recovery half-time data should be restricted to those instances in which the same electrode sizes are used. T h e number of S S C R s was also affected significantly by the size of the 42. electrode contact area. The results showed a strong linear trend which confirms the expectation that larger electrode sizes allow the recording of more S S C R s . Thus, if one is using the number of S S C R s as a dependent measure, larger electrode sizes would allow for greater accuracy and sensitivity in recording. The significant quadratic trend that was observed in the data cannot be explained on the basis of our current understanding of the electrodermal act ivi ty of the skin. Caution is advised when comparing S S C R data taken with different sizes of electrode contact areas. Conclusion The careful methodological control employed in this study has resulted in data that challenge the finding of Mitche l l and Venables (1980). In contrast to their study, in which it was found that a nonmonotonic relationship existed between E D A and electrode size, the present investigation provides evidence that a monotonic, linear relationship exists between these variables. This finding supports the parallel resistor model of electrodermal activity (Montagu & Coles, 1966; see Figure 5) Thus, sweat glands appear to act as parallel resistors (or conductors). This model 2 may not hold for very small electrode contact areas (i.e., less than 0.131 cm ), possibly due to the unpredictability of the number of sweat glands in contact with the electrode. Large electrodes yield the most sensitive measures of E D A and therefore, they may be preferred for S C recording. The linear relationship between E D A and electrode size argues for the reporting of results in terms of specific conductances. 43. References Blank, I. H., & Finesinger, J. E. (1946). Electrical resistance of the skin. Archives of Neurology and Psychiatry. 56, 544-557. Coulson, C. A., & Boyd, T. J. M. (1979). Electricity (2nd ed.). London: Langman Group Limited. Edelberg, R. (1972). Electrical activity of the skin. In N. S. Greenfield & R. A. Sternbach (Eds.), Handbook of psychophysiology (pp. 367-417). U.S.A.: Rinehart & Winston, Inc.. Edelberg, R., & Burch, N. R. (1962). Skin resistance and galvanic skin response. Archives of General Psychiatry, 7, 163-169. Fowles, D. C. (1974). Mechanisms of electrodermal activity. In R. F. Thompson & M. M. Patterson (Eds.), Methods in physiological psychology. Vol. 1: Bioelectric recording techniques. Part C: Receptor and effector processes (pp. 231-271). New York: Academic Press. Fowles, D. C., Christie, M. J . , Edelberg, R., Grings, W. W., Lykken, D. T., & Venables, P. H. (1981). Committee report: Publication recommendations for electrodermal measurements. Psychophysiology. 18, 232-239. Greenhouse, S. W., & Geiser, S. (1959). On methods in the analysis of profile data. Psychometrika, 24, 95-112. Grings, W. W. (1974). Recording of electrodermal phenomena. In R. F. Thompson & M. M. Patterson (Eds.), Methods in physiological psychology. Vol. 1: Bioelectric  recording techniques. Part C: Receptor and effector processes (pp. 273-296). New York: Academic Press. Lykken, D. T . , & Venables, P. H . (1971). Direct measurement of skin conductance: A proposal for standardization. Psychophysiology. 8, 656-672. Mi tche l l , D. A . , & Venables, P. H . (1980). The relationship of E D A to electrode size. Psychophysiology. 17, 408-412. Montagu, J . D. , & Coles, E . M . (1966). Mechanism and measurement of the galvanic response. Psychological Bulletin, 65, 261-279. Montagu, J . D. , & Coles, E . M . (1968). Mechanism and measurement of the galvanic response: A n addenum. Psychological Bulletin, 69, 74-76. Smith, B. V . (1966). Simplifying electricity: A programmed test. New York: Bantam Books Inc.. Thomas, P. E . , & Korr , I. M . (1957). Relationship between sweat gland activity and electrical resistance of the skin. Journal of Applied Physiology, 10, 505-510. Venables, P. H . , & Christ ie , M . J . (1980). Electrodermal activity. In I. Mart in & P. H . Venables (Eds.), Techniques in psycholophysiology (pp. 3-67). New York: John Wiley & Sons, L t d . . 45. Appendix A The relationship between skin conductance and the size of the electrode contact area. M a r y M a h o n / D r . W. G. Iacono Department of Psychology University of Brit ish Columbia Vancouver, B . C . V 6 K 2E5 Consent Form I have been asked to participate in a study in which my body's responses will be recorded while I perform a simple task. This task involves sitting comfortably while listening to brief tones. The response of my body that will be recorded is the activity of my sweat glands. To make these recordings, sensors will be attached to the palms of my hands, but no discomfort or danger to myself is involved. I understand that the study will take one hour and that I have the option of obtaining course credit or being paid $5.00 for my participation. Any questions regarding the procedure will be answered to my satisfaction by the experimenter. I understand that all the information obtained in this profect will be kept confidential and used only for the purposes of this study. By signing this form, I agree to participate although I realize that I am free to withdraw from this study at any time. Signature Date Appendix B 46. Instructions Okay, we're ready to begin. The first thing were want to tell you is, there's nothing to worry about. Those wires connected to the palms of your hands just measure tiny signals that your body generates. Nothing unpleasant will happen. I'll tell you about everything in advance so there'll be no surprises. Before we start, I want to check the recording equipment to make sure it's working correctly. I would like to see how your body reacts when you cough. Please cough once now (10 second pause). Cough again (10 seconds). And one more time (10 seconds). Good. Now, take a deep breath and hold it, take a deep breath now (5 second pause). Hold it, don't let any air out ( 5 seconds). Keep holding (5 seconds). Okay, you can breath normally now. For about the next ten minutes, I'd like you to sit still so take a few moments to get yourself comfortable. Just let your hands rest comfortably on your lap, with the palms facing up. We're going to listen to some tones. They will be fairly loud. I want you to pay careful attention to these tones since they may differ and will come at unpredictable times. Try to anticipate when the next one will come, and count how many you hear. Before the tones start, I will switch on a white noise generator. This produces a sound like a fan running and is nothing to worry about. Remember, anticipate the tones, count them, and try to remain still throughout the next ten minutes. Okay, I'm going to switch on the white noise. Here we go. 47. Appendix C Skin Conductance as a Function of the Size of the Electrode Contact Area Electrode measurement is a widely used psychophysiological technique. There is considerable variability in recording procedures and therefore, proposals for standardization have been made. Standardized procedures are necessary in order to facilitate comparison of results across experiments. One issue in the recording of electrodermal activity that remains to be resolved, concerns the relationship between the size of the electrode contact area and skin conductance measures. Skin conductance refers to the degree to which skin allows the passage of an electrical current. Sweat glands in skin facilitate the passage of electricity when they secrete sweat, since sweat contains a highly conductive electrolyte (NaCl). When increasing numbers of sweat glands secrete sweat, the conductance of the skin would be expected to increase. Thus, one would expect that a monotonic relationship exists between the size of the electrode contact area and skin conductance. Very few studies have been done to investigate this prediction and of those that have, the prediction has not been substantiated. These studies, however, contain serious methodological flaws which leaves the issue unresolved. The purpose of this study is to investigate this relationship while maintaining careful methodological control. Six sizes of electrode contact areas were placed on the palms of your hands. The tones you heard elicited skin conductance responses which were recorded on the polygraph. It is hoped that the greater methodological control employed in this study will substantiate the prediction of a monotonic relationship between skin conductance and size of electrode contact area. Thankyou for helping me in my research. Mary Mahon Department of Psychology University of British Columbia Appendix D Experimental Design S I Z E S l S 2 S 3 1 H 1 A 1 H 1 A 2 H l A 3 2 H 1 A 2 H 1 A 3 H 2 A 3 3 H 1 A 3 H 2 A 3 H 2 A 2 4 H 2 A 3 H 2 A 2 H 2 A 1 5 H 2 A 2 H 2 A 1 H 1 A 1 6 H 2 A 1 H 1 A 1 H 1 A 2 •= left hand = right hand = thenar surface = medial phalanges of first two fingers = hypothenar surface 4 9 . Appendix E Table 1. Analysis of variance of averaged SCL data. Source SS df MS F P Total 1571.50 287 5.48 Between subjects 47 Group 23.11 5 4.62 .30 n.s. Subjects/group 649.56 42 15.47 10.88 <.001 Within subjects 240 * S i z e 338.69 5 67.74 47.67 <.001 S i z e X s u b j e c t s 235 Size X groups 25 Hand 2.39 1 2.39 1.68 n.s. Area 228.67 2 114.34 80.52 <.001 Hand X Area 3.00 2 1.50 1.06 n.s. LSR 27.70 20 1.38 0.97 n.s. Size X Ss/groups 298.41 210 1.42 'Trend Analysis of Size Variable Size (linear) 330.41 1 330.41 97.50 <.001 Subjects X Size (lin) 159.27 47 3.39 50. Appendix E cont. Table 2. Analysis of variance of averaged SCR data. Source SS df MS F P Total 125.27 287 Between subjects 47 Group 2.09 5 .42 .39 n.s. Subjects/group 45.04 42 1.07 8.10 <.001 Within subjects 240 "Size 24.14 5 4.83 36.50 <.001 Size X subjects • 235 Size X groups 25 Hand 0.01 1 0.01 0.08 n.s. Area 23.57 2 11.78 90.62 <.001 Hand X area 0.00 2 0.00 0.01 n.s. LSR 2.63 20 0.13 1.00 n.s. Size X Ss/groups 27.78 210 0.13 "Trend Analysis of Size Variable Size (linear) 23.68 1 23.68 72.97 <.001 Subjects X size (lin) 15.25 47 0.32 

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