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Changes in the number of active sweat glands (palmar sweat index, PSI) during a distressing film
Biological Psychology 37 (1994) 133-145 0 1994 Elsevier Science B.V. All rights reserved
133 0301.0511/94/$07.00
Changes in the number of active sweat glands (palmar sweat index, PSI) during a distressing film Thomas
Kiihler
* and Irena
Schuschel
Psychologisches Institut III Von-Me&Park (Received
8 December
1992; revised
5, D-2000 Hamburg 13, Germany
12 May 1993; accepted
10 June
19931
Changes of the number of active palmar sweat glands (palmar sweat index, PSI) as assessed by the plastic finger-print method were studied in two groups of female students (n = 21 each). In both samples experiments involved an initial adaptation period, several relaxation phases and an activation period (presentation of a movie). The film was shown 10 min earlier in Group 1, for which in turn follow-up was twice as long. Prints for determination of PSI were taken every 2.5 min from the forefinger and ring finger of the left hand, and recordings of SCL, SF (number of spontaneous fluctuations) and HR were made during the corresponding intervals. Both withinand between-groups comparisons showed an increase of PSI during the activation period and a decrease afterwards. Similar effects were observed for SCL. SF and affective and somatic arousal assessed by a state questionnaire. A decrease of PSI and parameters of electrodermal activity during the first measurements indicated an initial reaction to the assessment procedure itself. Both within-subject and between-subjects correlations between PSI from both fingers showed high parallel test reliabilities, while correlations with electrodermal variables indicated a common physiological basis. Keywords: Palmar sweat index; plastic with electrodermal variables.
finger-print
method;
activation
parameter;
correlation
1. Introduction The PSI (palmar sweat index), the number of active sweat glands in a defined skin area, can be determined easily by means of the plastic impression method, developed by Sutarman and Thomson (19.52). The variable has gained renewed interest in the last years, because its assessment is also possible outside of the laboratory. In a study recently published in this journal we examined changes in PSI values during activation (mental arithmetic) and observed that it rose consistently as a reaction to the activating situation and decreased rapidly thereafter (Kiihler and Troester, 1991). This is in line with our previous findings (cf. Kiihler, Weber & Vogele, 1990; Kdhler, Zander & Dunker, 1991; also cf. Kohler & Vbgele, 1989); however, it is somewhat at variance with the first systematic studies on PSI changes (e.g. * Corresponding
author.
SSDI 0301-0511(93)00922-7
134
T. Kiihler und 1. Schuschel/ Biologicul Psychology 37 (1994) 133-145
MacKinnon, Monk-Jones & Fotherby, 1963; Harrison, 1964), where a decrease was found in response to stress. Other groups of researchers have partly observed a decrease, partly an increase (for an overview cf. KGhler & Troester, 1991). To explain conflicting results, Dabbs, Johnson and Leventhal (1968) proposed that PSI rises only under circumstances demanding interaction with the environment and possibly decreases during other types of situations, especially those where attention is directed inward. We, however, favour another explanation, namely that PSI increases as a response to any activation whatsoever, provided that change values are computed from a baseline period of low arousal, when even the assessment procedure itself is no longer stimulating. This view is based on an observation made in our previous studies, namely that the first two or three PSI values determined under resting conditions were considerably higher than the following ones. Most likely the taking of the prints itself elicits interest initially. Since most researchers have used the very first PSI values for baseline determination it could be the case that activation-related increases have been partly compensated or overcompensated by habituation to the assessment procedure in the course of the experiment. This could easily account for the conflicting results obtained so far (see also Kbhler, Weber & VGgele, 1990). Given the importance of this issue for PSI research it was considered worth reexamining whether PSI values are generally higher during the first assessments and whether the PSI also increases during an activating situation which does not involve interaction with the environment in the sense of actively coping with external stimuli. As in the previously published study (KGhler & Troester, 1991) it seemed useful to employ two groups of subjects with different phase schedules in order to differentiate between the effects of passage of time and of the stimulus situation itself. In addition, we previously obtained the somewhat surprising result that both within- and between-subjects correlations were higher between PSI and the number of spontaneous fluctuations (SF) than between PSI and skin conductance level (SCL). This would seem to require further replication with a different stimulus before its implications are considered. Finally, Weisenberg, Kreindler, Schachat and Werboff (1976) observed different effects in PSI for males and females, and so it seemed sensible to study female subjects this time.
2. Method 2.1. Subjects and experimental design Forty-two female psychology students participated in the study. They were assigned to one of two equal-sized experimental groups (mean age in Group 1 was 27.76 f 6.03 years, while in Group 2 it was 28.71 f 4.70 years, a
T. Ktihler and I. Schuschel/
Biological Psychology 37 (1994) 133-145
135
non-significant difference). Allocation to groups usually was accomplished by alternating subjects on arrival in the laboratory. Occasionally, however, this practice was suspended in order to avoid unequal distribution of ages between groups. There was no systematic difference between groups in the time of day of the testing. The investigation was carried out between July and October 1989, generally in the late mornings or early afternoons. Before the experiment started, subjects sat in a chair for about 30 min with electrodes attached for at least 20 min in order to get used to the laboratory situation. For both groups the investigation involved the same experimental phases, but in a different order. The experimental design is essentially the same as that used Kiihler & Troester (1991), and readers should refer to this for details. In both groups the experiment started with an adaptation period (Phase I> of 6 min, followed by another 10 min of relaxation during which baseline measurements were taken (Phase 111. In Phase III Group 1 was presented with a 10 min sequence from the film “The Shining”; in Group 2 this interval served again for relaxation, In Phase IV (10 min) subjects in Group 1 were told to relax, whereas those in Group 2 watched the film. The film sequence depicts a scene where a woman, spending the winter with her husband in a lonely house, discovers that he had become insane in the last weeks. She is chased up the staircase by the man, until she eventually manages to knock him down and lock him in a room. The final Phase V again lasted 10 min, during which time both groups relaxed. At the end of each phase (except for adaptation) subjects filled out a state questionnaire assessing affective and somatic signs of arousal. The film was preceded by a 1 min information phase during which a short account of the plot was given to make the sequence understandable. This served again as a resting period for the other group. PSI was not assessed during such information phases nor during completion of the questionnaires; other recordings made during these phases were discarded. Throughout the whole experiment subjects were asked not to talk. The experimenter (I.S., female psychology student) spoke with them only to provide brief instructions. 2.2. Psychophysiological
recordings
and questionnaire
During Phases II-IV 4 finger-prints were taken at 2.5 min intervals, the drop being applied to the left forefinger first and afterwards to the left ring finger. The whole procedure including drying time and removal of the film took approximately 60 s. During the adaptation period (Phase I> intervals between PSI assessments were reduced to 2 min in order to obtain three pairs of prints. The method of PSI assessment was the same as in the previous studies. For the recipe of the solution see Kiihler & Troester (1991). Electrodes for the electrocardiogram were attached at the left forearm and the right leg, with the reference electrode on the left leg. To obtain heart
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rate (HR), the QRS waves were filtered, squared and counted in 15 s epochs (amplification of the signals by means of AC/DC amplifiers N-101, Natic UBV 300, parametrisation with interface Par Electronic IF301 controlled by the program NATICBAT). For the assessment of electrodermal activity two Ag/AgCl electrodes 8 mm in diameter were attached to the hypothenar surface of the left hand. “Unibase” with a concentration of 0.05 M NaCl was used as the electrode paste. The number of spontaneous fluctuations (SF) (criterion: > 0.01 PS) and skin conductance level (SCL), integrated over 1.5 s periods, were used as parameters of EDA (amplifier, Natic EDA N-102; parametrisation with interface Par Electronic IF-304, program NATIC.BAT). The questionnaire presented after the different phases comprises 13 items, asking for psychological and somatic indicators of activation (e.g. I am tense, I feel nervous, I feel my heart beating, I have sweaty hands). The intensity of each reaction during the preceding interval had to be marked on a five point scale. The questionnaire had been compiled by ourselves, with some items taken from other inventories. Details concerning the construction and validity of the questionnaire are given elsewhere (KShler & Troester, 1991).
2.3. Ecaluation,
data reduction,
statistical
methods
The determination of active sweat glands was carried out as described by Kohler & Troester (1991). It should be noted that in the present study none of the series of finger-prints had to be rejected, whereas in the previous study this had been the case for 19 subjects. In agreement with other researchers (e.g. Martens, 1969, Weisenberg et al., 1976) we have repeatedly found high interrater reliability in determination of PSI values (cf. Kiihler & Viigele, 1989; Kiihler, Weber & Viigele, 1990; Kijhler & Troester, 1991; Kbhler, Zander & Dunker, 1991), with coefficients ranging from .78 to .90. Considering that the evaluator in this study (I.S.) was substantially acquainted with the method and agreement with other raters had been ascertained in several data sets, interrater reliability was not determined for the present data. Only those values of other variables which corresponded to the intervals of the PSI assessment were used. Thus, recordings of HR, SCL and SF (automatically integrated over 15 s) were again averaged across the 1 min intervals following the first application of the drop. To examine differences between groups of subjects and between phases, and to calculate correlations between parameters across subjects, values obtained for each individual in the same phase were again collapsed. Questionnaire answers were scored from 1 to 5, and total scores for both somatic and affective arousal were computed for every subject in each phase.
T. Kiihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
137
Two-way repeated measurements ANOVAs were carried out with experimental group as one factor, phase as the other and psychophysiological parameters or questionnaire indicators of arousal as dependent variables. For closer examination of significant group X phase interactions, experimental groups during Phases III and IV were compared; in Phase III values in Group 1, in Phase IV those in Group 2 were expected to be greater. Since only two between-groups comparisons were considered to be significant and as the direction of the differences was clearly predicted by previous results, one-tailed unpaired t-tests were used. In order to adjust the alpha level, error probabilities obtained were multiplied by 2. Phase effects were tested for each group separately by means of Scheffe’s test. Based on previous findings it was hypothesized that values during activation would be greater than values of adjacent baseline and follow-up phases; furthermore, we assumed that there is a significant decrease, especially in PSI, from adaptation to the following baseline phase. Interindividual correlations for each pair of variables were calculated across subjects of both samples taken together. This was carried out for (i> baseline values (Phase II values in both groups) (ii) values during activation (Phase III recordings in Sample 1, Phase IV recordings in Sample 2) and (iii) reaction values (Phase III values minus Phase II values in Sample 1, Phase IV values minus Phase III values in Sample 2). Intraindividual correlations between variables were determined for each subject separately. For this purpose, Pearson correlations across all points of assessment (in general 19) were computed. Using Fisher’s r to z transformation individual coefficients were subsequently averaged across subjects of each sample. For each pair of variables differences of the z-transformed intraindividual correlations from 0 were tested.
3. Results 3.1. Activation
and adaptation
The ANOVA yielded no significant group differences for any of the variables. This was not unexpected given that, apart from the order of presentation, experimental phases were the same for both groups. However, there were significant effects for phases and group X phase interactions, in all variables other than HR. Results were still significant when the numbers of degrees of freedom were conservatively adjusted by the GreenhouseGeisser epsilon (Geisser & Greenhouse, 1958). 3.1.1. Subjective data For both affective and somatic arousal, phases differed significantly: F(3, 120) = 15.42 and 13.74 (p < .OOOl; no data obtained from the adaptation
138
T. Kiihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
phase). Post hoc comparisons (ScheffC’s test), carried out separately for the two experimental groups, yielded a significant increase from the preceding baseline to the film phase and a decrease thereafter (p < .OOl). There were also significant group X phase interactions: F(3, 120) = 30.27 for affective arousal, 34.08 for somatic arousal. t-tests showed that values during Phase III were higher in Group 1, whose members were watching the film during this experimental period, while Group 2 subjects were still relaxing. During Phase IV when Group 1 relaxed and Group 2 watched the film sequence, values in the latter sample were significantly higher (p < .OOl). 3.1.2. PSI Figure 1 depicts mean values of the two groups in the different phases of the experiment. ANOVA yielded significant phase effects both for PSI
Fig. I. several Group *** p
Means of values for forefinger PSI and ring finger PSI of two groups (n = 21 each) during experimental phases. Differences between adjacent phases as revealed by Schefft’s test. differences in Phases II1 and IV examined by unpaired I-test (* p < .OS; ** p <.Ol; < ,001; n.s. = not significant).
T. K6hler and I. Schuschel/Riologicai
Psychology 37 (1994) 133-145
139
5-
4--
3-
A
forefinger (F(4, 160) = 20.33) and for PSI ring finger (F(4, 160) = 9.79) (p < .OOOl for both variables). Scheffe’s post hoc comparisons revealed a significant increase from baseline to film and a decrease thereafter. This held true for both groups and both variables. A significant decrease from adaptation to the following baseline occurred in PSI for the forefinger, but not for PSI ring finger. A significant group x phase interaction was also found: F(4, 160) = 40.65 for PSI forefinger, 20.16 for PSI ring finger (p < .OOOl).In accordance with subjective data (see above) during Phase III values were significantly higher in Group 1, and this was reversed during Phase IV. 3.1.3. Skin c~~~uc~unce level and number of ~pon~aneo~~~uctuatio~ Electrodermal variables changed in a similar way to the PSI: ANOVA yielded significant differences between phases: F(4, 160) = 10.24 for SCL, 13.76 for SF (p < .OOOl), and in both groups and for both variables there
were higher values during the film compared with the preceding baseline and the adjacent foltow-up phase (see Fig. 2). Scheffe’s test also yieIded a significant decrease from adaptation to baseline for both variables. Again group x phase interactions were significant: F(4, 160) = 17.95 for SCL, 22.94 for SF (p < .OOOl). During Phase IV Group 2 subjects (watching the movie sequence) had on average significantly higher SCL and SF values than Group 1, and for SF this was reversed during Phase III. However, for SCL no significant difference was revealed in that phase. 31.4. Heart rate
Although there was a significant overall phase effect for HR (P(4, 160) = 4.39; p < .Ol), post hoc comparisons yielded no significant differences be-
6 --
5’-‘i-L_-I*
A*\\
4-
n.s.
3-
*-
Fig. 2. Means of values for skin conductance level (SCL) and number of spontaneous fluctuations (SF) of two groups (n = 21 each) during several experimental phases. Differences between adjacent phases as revealed by Scheffb’s test. Group differences in Phases III and IV examined by unpaired r-test t * p < .05; * * p < .Ol; * * * p < .OOl; n.s. = not significant).
i? Kiihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
141
Fig. 2 (continued).
tween any pair of phases. group X phase interaction.
HR
was the only variable
with
no significant
3.2. Within-subject and between-subject correlations Intraindividual correlations of variables across points of assessment again reflect the above mentioned findings (see Table 1). They were generally well reproducible in both samples, and indicated similar variations over time for all parameters of skin activity, whereas their covariations with HR were much weaker. In particular, the mean correlations of PSI obtained from two fingers (.79 and .78) indicated high intraindividual parallel test reliability of the measurement. Between-subjects correlations during baseline (Phase II), during activation (Phase III for Group 1, Phase IV for Group 2) and for the changes from preceding baseline to activation are shown in Table 2. Again intercorrela-
T. Ktihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
142 Table 1 Within-subjects
correlations
PSIF
PSIR .19*.49 .78+.50
PSIR
of different
parameters
across
points of assessment
SF 59k.34 .56*.35
*** ***
SCL S6k.38 .56*.45
*** ***
HR .16+.35 * .06 f .35
_
.62+.35 .52i.30
*** ***
.59+.36 .51*.35
*** ***
.18+.35 * .04 * .30
SF
_ _
_ _
.67&.24 .71 k.43
*** ***
.42+.35 *** .21 f .38 *
SCL
_ _
_ _
_ _
First and and rate.
*** ***
.49*.39 .20+.41
*** **
row in each line, means and standard deviations of Group 1 (n = 21); second row, means standard deviations of Group 2 (n = 21); PSIF, PSIR, PSI values obtained from forefinger ring finger, SCL, skin conductance level; SF, number of spontaneous fluctuations; HR, heart * p < .05; * * p < .Ol; * * * p < .OOl.
Table 2 Intercorrelations
of variables
across
data of 42 subjects
PSIR
SF
SCL
HR
AFF
SOM
PSIF
.59 *** .78 *** .71 ***
.45 ** 57 *** .49 ***
.38 * .37 * .60 ***
.14 .34 * .27
.18 .18 .18
.14 .Oh p.11
PSIR
_ _ _
.67 *** .43 ** .57 ***
.31 * .I7 .61 ***
.15 .14 .33 *
.39 ** .19 .31
SF
_ _
_ _
.75 *** .67 *** .70 ***
.34 * .28 .32 *
.17 .04 .21
.13 - .06 - .04
_ _ _
.26 .41 ** .37 *
.21 .03 .28
- .Ol - .03 .09
_ _
_ _ _
.24 .09 .33 *
.38 * .17 .36 *
_ _ _
_ _ _
_ _ _
.62 *** .J3 *** .71***
_ SCL _
HR
AFF
_ _
_ _ _
.30 * .17 .08
First number in each block, intercorrelations of baseline values; second, intercorrelations of values during activation; third, intercorrelations of reaction values; AFF, SOM, questionnaire scores for affective and somatic arousal. * p < .05; * * p < 0.1; ** * p < .OOl. See Table 1 for definition of other abbreviations.
T. Kiihler and I. Schuschel/
Biological Psychology 37 (1994) 133-145
143
tions of parameters of skin activity were generally high and, as a rule, reproducible both during rest and activation and also for the response reactions. Questionnaire scores for somatic and affective arousal also showed high intercorrelations, although, in general, they correlated poorly with skin parameters and HR. The latter variable showed moderate, but frequently significant, correlations with parameters of skin activity.
4. Discussion The present results are in line with the findings of our previous studies, both in the laboratory and in the field (Kiihler & Vogele, 1989; Kohler, Weber & Vogele, 1990; Kiihler, Zander & Dunker, 1991) In particular, they are in accord with the findings of the study previously reported in this journal (Kiihler & Troester, 1991). PSI values increased during the film and decreased rapidly after its end. Questionnaire data indicated that the short section of the movie was indeed perceived as activating, and this is confirmed by the change both in the number of spontaneous fluctuations and in skin conductance level. Accordingly, the activated group had higher values than the one at rest, except for Phase III in terms of SCL, where significance was not achieved. The behaviour of the PSI proved to be the same, both for the forefinger and the ring finger, as shown both by within-group comparisons and by comparisons between groups under differential activation conditions. Heart rate, although slightly higher in the activation phases, was the only variable that did not change significantly in response to the film. Possibly the movie sequence was thrilling at some moments and mainly attention demanding at others. A decrease in HR during attention demanding stimuli has been described previously (see Graham & Clifton, 1966). Little in the way of HR change during sections of violent films was also observed by Carruthers and Taggart (1973), who reported vagal over-reactivity under these conditions. PSI changes and changes in electrodermal parameters would appear to be less situation specific and most likely reflect activation in general. Values rise during different kinds of stimuli such as mental arithmetic (Kiihler, Weber & Vogele, 1990, Kiihler & Troester, 1991), film sections (Kiihler, Weber & Vogele, 1990, this study) and dental treatment (Kiihler, Zander & Dunker, 1991). Thus PSI increases not only in tasks demanding active coping with external stimuli, but also in situations that are more or less passively experienced. It would be interesting to examine conditions where attention is definitely directed inwardly towards personal thoughts and feelings in this context, but these may not be easy to manipulate in the laboratory. Mental arithmetic has been considered to fulfil this condition by Dabbs, Johnson & Leventhal (1968), but has been regarded as an active coping task by others.
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T. Ktihler and I. Schuschel/Bioiogical
Psychology 37 (1994) 133-145
With regard to intraindividual correlations, all except for some covariations with HR had a mean significantly above 0 in both groups. They were slightly lower in magnitude than the coefficients obtained in our earlier study (Kiihler & Troester, 1991) with mental arithmetic as a stressor. This might best be explained by the fact that the changes in variables as a response to activation were somewhat smaller in this experiment and there was less intraindividual variance. The highest coefficients were observed between PSI from both fingers. Given that the interval between the assessment from the forefinger and the ring finger took at least lo-15 s on average, even higher intraindividual parallel test reliability would be expected if prints are taken simultaneously. One may thus conclude that changes of PSI over time determined at one finger are reproducible on another finger. Interindividual correlations were also high between PSI and electrodermal parameters; they are mostly significant beyond the .OOl level. These are thus unlikely to be chance findings even within a large correlational matrix, and confirm the presumption of a common physiological basis. Coefficients were again highest between PSI from two fingers indicating that parallel test reliability is also satisfactory as far as differences between subjects are concerned. Correlations of parameters of skin activity including PSI with HR or subjective responses were much lower and often not significant. This finding, however, is hardly surprising given that low correlations between electrodermal parameters and HR or measures of subjective arousal have also been found in other studies (cf. Fahrenberg & Foerster, 1982). In line with our previous findings, the very first PSI values from the forefinger under resting conditions were higher than the subsequent ones. This is probably caused by the novelty of the assessment procedure, an assumption which receives support from the fact that SCL and SF as indicators of arousal also fell during the rest period, although electrodes had been attached for at least 20 min. It is somewhat surprising that no significant adaptation effect could be shown for the ring finger PSI. However, it should be kept in mind that Scheffe’s post-hoc comparisons are extremely conservative. In addition only two-tailed error probabilities were considered, although we expected significant differences only between a few phases and had clear expectations as regards the direction of change. Paired t-tests with Bonferoni adjusted one-tailed error probabilities would have yielded a significant decline of ring finger PSI values in both groups. We would thus still argue that PSI adaptation exists for any finger (75% and 69% decline for PSI forefinger, 51% and 78% for PSI ring finger). In the discussion of the previous article we revealed that the print series of a large number of subjects had to be rejected because of holes in the area chosen for evaluation and that this might pose serious limitations to the general applicability of the method. However, this was not the case in the present study, although the recipe of the solution had not changed and the
i? Ktihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
14.5
technique of assessment was the same. Possibly the finger pads of females are better suited for the procedure, being in general smoother and better cared for. It is also possible that the experimenter in this study was more skilful in taking prints. To sum up, the present study replicates the finding that PSI increases during activation and decreases thereafter, changing in the same way as measures of electrodermal activity. Although the type of provocation employed in this study was quite different from the one used in the previous study and although females served as subjects this time, the outcome was very similar. Again we observed that the first resting PSI values were higher than subsequent ones. This supports the assumption that the conflicting results reported in the literature are due to measurement factors rather than to difference in the stressors employed.
References Carruthers, M., & Taggart, P. (1973). Vagotonicity of violence: Biochemical and cardiac responses to violent films and television programmes. British Medical Journal, 3, 384-389. Dabbs, J.M., Johnson, S.E., & Leventhal, H. (1968). Palmar sweating: A quick and simple measure. Journal of Experimental Psychology, 78, 347-350. Fahrenberg, J., & Foerster, F. (1982). Covariation and consistency of activation parameters. Biological Psychology, 15, 151-169. Geisser, S., & Greenhouse, S.W. (1958). An extension of Box’s results on the use of the F-distribution in multivariate analysis. Annuls of Mathematical Statistics, 29, 885-891. Graham, F.K., & Clifton, R.K. (1966). Heart rate change as a component of the orienting response. Psychological Bulletin, 65, 305-320. Harrison, J. (1964). The behaviour of the palmar sweat glands in stress. Journal ofPsychosomatic Research, 8, 187-191. KBhler, Th., & Troester, U. (1991). Changes of the PSI (palmar sweat index) during mental arithmetic. Biological Psychology, 32, 143-154. Kiihler, Th., & Viigele, C. (1989). Laboratory studies on a potential stress indicator in field research: The Palmar Sweat Index. In: D. Siddle, & N. Bond (Eds.) Proceedings of the 24th International Congress of Psychology, Sydney, Australia, Volume 6. Psychobiology. Issues and Applications (pp. 315-323). Amsterdam: Elsevier. KGhler, Th., Weber, D., & Viigele, C. (1990). The behaviour of the PSI (palmar sweat index) during two stressful laboratory situations. Journal of Psychophysiology, 4, 281-287. Kiihler, Th., Zander, O., & Dunker, D. (1991). Interpreting palmar sweat glands. Journal of Psychosomatic Research, 35, 75-81. MacKinnon, P.C.B., Monk-Jones, M.E., & Fotherby, K. (1963). A study of various indices of adrenocortical activity during 23 days at high altitude. Journal of Endocrinology, 26, 555-566. Martens, R. (1969). Palmar sweating and the presence of an audience. Journal of Experimental Social Psychology, 5, 371-374. Sutarman, & Thomson, M.L. (1952). A new technique for enumerating active sweat glands in man. Journal of Physiology, 51P-52P. Weisenberg, M., Kreindler, M.L., Schachat, R., & Werboff, J. (1976). Interpreting palmar sweat prints: A not-so-simple measure. Journal of Psychosomatic Research, 20, l-6.
133 0301.0511/94/$07.00
Changes in the number of active sweat glands (palmar sweat index, PSI) during a distressing film Thomas
Kiihler
* and Irena
Schuschel
Psychologisches Institut III Von-Me&Park (Received
8 December
1992; revised
5, D-2000 Hamburg 13, Germany
12 May 1993; accepted
10 June
19931
Changes of the number of active palmar sweat glands (palmar sweat index, PSI) as assessed by the plastic finger-print method were studied in two groups of female students (n = 21 each). In both samples experiments involved an initial adaptation period, several relaxation phases and an activation period (presentation of a movie). The film was shown 10 min earlier in Group 1, for which in turn follow-up was twice as long. Prints for determination of PSI were taken every 2.5 min from the forefinger and ring finger of the left hand, and recordings of SCL, SF (number of spontaneous fluctuations) and HR were made during the corresponding intervals. Both withinand between-groups comparisons showed an increase of PSI during the activation period and a decrease afterwards. Similar effects were observed for SCL. SF and affective and somatic arousal assessed by a state questionnaire. A decrease of PSI and parameters of electrodermal activity during the first measurements indicated an initial reaction to the assessment procedure itself. Both within-subject and between-subjects correlations between PSI from both fingers showed high parallel test reliabilities, while correlations with electrodermal variables indicated a common physiological basis. Keywords: Palmar sweat index; plastic with electrodermal variables.
finger-print
method;
activation
parameter;
correlation
1. Introduction The PSI (palmar sweat index), the number of active sweat glands in a defined skin area, can be determined easily by means of the plastic impression method, developed by Sutarman and Thomson (19.52). The variable has gained renewed interest in the last years, because its assessment is also possible outside of the laboratory. In a study recently published in this journal we examined changes in PSI values during activation (mental arithmetic) and observed that it rose consistently as a reaction to the activating situation and decreased rapidly thereafter (Kiihler and Troester, 1991). This is in line with our previous findings (cf. Kiihler, Weber & Vogele, 1990; Kdhler, Zander & Dunker, 1991; also cf. Kohler & Vbgele, 1989); however, it is somewhat at variance with the first systematic studies on PSI changes (e.g. * Corresponding
author.
SSDI 0301-0511(93)00922-7
134
T. Kiihler und 1. Schuschel/ Biologicul Psychology 37 (1994) 133-145
MacKinnon, Monk-Jones & Fotherby, 1963; Harrison, 1964), where a decrease was found in response to stress. Other groups of researchers have partly observed a decrease, partly an increase (for an overview cf. KGhler & Troester, 1991). To explain conflicting results, Dabbs, Johnson and Leventhal (1968) proposed that PSI rises only under circumstances demanding interaction with the environment and possibly decreases during other types of situations, especially those where attention is directed inward. We, however, favour another explanation, namely that PSI increases as a response to any activation whatsoever, provided that change values are computed from a baseline period of low arousal, when even the assessment procedure itself is no longer stimulating. This view is based on an observation made in our previous studies, namely that the first two or three PSI values determined under resting conditions were considerably higher than the following ones. Most likely the taking of the prints itself elicits interest initially. Since most researchers have used the very first PSI values for baseline determination it could be the case that activation-related increases have been partly compensated or overcompensated by habituation to the assessment procedure in the course of the experiment. This could easily account for the conflicting results obtained so far (see also Kbhler, Weber & VGgele, 1990). Given the importance of this issue for PSI research it was considered worth reexamining whether PSI values are generally higher during the first assessments and whether the PSI also increases during an activating situation which does not involve interaction with the environment in the sense of actively coping with external stimuli. As in the previously published study (KGhler & Troester, 1991) it seemed useful to employ two groups of subjects with different phase schedules in order to differentiate between the effects of passage of time and of the stimulus situation itself. In addition, we previously obtained the somewhat surprising result that both within- and between-subjects correlations were higher between PSI and the number of spontaneous fluctuations (SF) than between PSI and skin conductance level (SCL). This would seem to require further replication with a different stimulus before its implications are considered. Finally, Weisenberg, Kreindler, Schachat and Werboff (1976) observed different effects in PSI for males and females, and so it seemed sensible to study female subjects this time.
2. Method 2.1. Subjects and experimental design Forty-two female psychology students participated in the study. They were assigned to one of two equal-sized experimental groups (mean age in Group 1 was 27.76 f 6.03 years, while in Group 2 it was 28.71 f 4.70 years, a
T. Ktihler and I. Schuschel/
Biological Psychology 37 (1994) 133-145
135
non-significant difference). Allocation to groups usually was accomplished by alternating subjects on arrival in the laboratory. Occasionally, however, this practice was suspended in order to avoid unequal distribution of ages between groups. There was no systematic difference between groups in the time of day of the testing. The investigation was carried out between July and October 1989, generally in the late mornings or early afternoons. Before the experiment started, subjects sat in a chair for about 30 min with electrodes attached for at least 20 min in order to get used to the laboratory situation. For both groups the investigation involved the same experimental phases, but in a different order. The experimental design is essentially the same as that used Kiihler & Troester (1991), and readers should refer to this for details. In both groups the experiment started with an adaptation period (Phase I> of 6 min, followed by another 10 min of relaxation during which baseline measurements were taken (Phase 111. In Phase III Group 1 was presented with a 10 min sequence from the film “The Shining”; in Group 2 this interval served again for relaxation, In Phase IV (10 min) subjects in Group 1 were told to relax, whereas those in Group 2 watched the film. The film sequence depicts a scene where a woman, spending the winter with her husband in a lonely house, discovers that he had become insane in the last weeks. She is chased up the staircase by the man, until she eventually manages to knock him down and lock him in a room. The final Phase V again lasted 10 min, during which time both groups relaxed. At the end of each phase (except for adaptation) subjects filled out a state questionnaire assessing affective and somatic signs of arousal. The film was preceded by a 1 min information phase during which a short account of the plot was given to make the sequence understandable. This served again as a resting period for the other group. PSI was not assessed during such information phases nor during completion of the questionnaires; other recordings made during these phases were discarded. Throughout the whole experiment subjects were asked not to talk. The experimenter (I.S., female psychology student) spoke with them only to provide brief instructions. 2.2. Psychophysiological
recordings
and questionnaire
During Phases II-IV 4 finger-prints were taken at 2.5 min intervals, the drop being applied to the left forefinger first and afterwards to the left ring finger. The whole procedure including drying time and removal of the film took approximately 60 s. During the adaptation period (Phase I> intervals between PSI assessments were reduced to 2 min in order to obtain three pairs of prints. The method of PSI assessment was the same as in the previous studies. For the recipe of the solution see Kiihler & Troester (1991). Electrodes for the electrocardiogram were attached at the left forearm and the right leg, with the reference electrode on the left leg. To obtain heart
136
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rate (HR), the QRS waves were filtered, squared and counted in 15 s epochs (amplification of the signals by means of AC/DC amplifiers N-101, Natic UBV 300, parametrisation with interface Par Electronic IF301 controlled by the program NATICBAT). For the assessment of electrodermal activity two Ag/AgCl electrodes 8 mm in diameter were attached to the hypothenar surface of the left hand. “Unibase” with a concentration of 0.05 M NaCl was used as the electrode paste. The number of spontaneous fluctuations (SF) (criterion: > 0.01 PS) and skin conductance level (SCL), integrated over 1.5 s periods, were used as parameters of EDA (amplifier, Natic EDA N-102; parametrisation with interface Par Electronic IF-304, program NATIC.BAT). The questionnaire presented after the different phases comprises 13 items, asking for psychological and somatic indicators of activation (e.g. I am tense, I feel nervous, I feel my heart beating, I have sweaty hands). The intensity of each reaction during the preceding interval had to be marked on a five point scale. The questionnaire had been compiled by ourselves, with some items taken from other inventories. Details concerning the construction and validity of the questionnaire are given elsewhere (KShler & Troester, 1991).
2.3. Ecaluation,
data reduction,
statistical
methods
The determination of active sweat glands was carried out as described by Kohler & Troester (1991). It should be noted that in the present study none of the series of finger-prints had to be rejected, whereas in the previous study this had been the case for 19 subjects. In agreement with other researchers (e.g. Martens, 1969, Weisenberg et al., 1976) we have repeatedly found high interrater reliability in determination of PSI values (cf. Kiihler & Viigele, 1989; Kiihler, Weber & Viigele, 1990; Kijhler & Troester, 1991; Kbhler, Zander & Dunker, 1991), with coefficients ranging from .78 to .90. Considering that the evaluator in this study (I.S.) was substantially acquainted with the method and agreement with other raters had been ascertained in several data sets, interrater reliability was not determined for the present data. Only those values of other variables which corresponded to the intervals of the PSI assessment were used. Thus, recordings of HR, SCL and SF (automatically integrated over 15 s) were again averaged across the 1 min intervals following the first application of the drop. To examine differences between groups of subjects and between phases, and to calculate correlations between parameters across subjects, values obtained for each individual in the same phase were again collapsed. Questionnaire answers were scored from 1 to 5, and total scores for both somatic and affective arousal were computed for every subject in each phase.
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137
Two-way repeated measurements ANOVAs were carried out with experimental group as one factor, phase as the other and psychophysiological parameters or questionnaire indicators of arousal as dependent variables. For closer examination of significant group X phase interactions, experimental groups during Phases III and IV were compared; in Phase III values in Group 1, in Phase IV those in Group 2 were expected to be greater. Since only two between-groups comparisons were considered to be significant and as the direction of the differences was clearly predicted by previous results, one-tailed unpaired t-tests were used. In order to adjust the alpha level, error probabilities obtained were multiplied by 2. Phase effects were tested for each group separately by means of Scheffe’s test. Based on previous findings it was hypothesized that values during activation would be greater than values of adjacent baseline and follow-up phases; furthermore, we assumed that there is a significant decrease, especially in PSI, from adaptation to the following baseline phase. Interindividual correlations for each pair of variables were calculated across subjects of both samples taken together. This was carried out for (i> baseline values (Phase II values in both groups) (ii) values during activation (Phase III recordings in Sample 1, Phase IV recordings in Sample 2) and (iii) reaction values (Phase III values minus Phase II values in Sample 1, Phase IV values minus Phase III values in Sample 2). Intraindividual correlations between variables were determined for each subject separately. For this purpose, Pearson correlations across all points of assessment (in general 19) were computed. Using Fisher’s r to z transformation individual coefficients were subsequently averaged across subjects of each sample. For each pair of variables differences of the z-transformed intraindividual correlations from 0 were tested.
3. Results 3.1. Activation
and adaptation
The ANOVA yielded no significant group differences for any of the variables. This was not unexpected given that, apart from the order of presentation, experimental phases were the same for both groups. However, there were significant effects for phases and group X phase interactions, in all variables other than HR. Results were still significant when the numbers of degrees of freedom were conservatively adjusted by the GreenhouseGeisser epsilon (Geisser & Greenhouse, 1958). 3.1.1. Subjective data For both affective and somatic arousal, phases differed significantly: F(3, 120) = 15.42 and 13.74 (p < .OOOl; no data obtained from the adaptation
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phase). Post hoc comparisons (ScheffC’s test), carried out separately for the two experimental groups, yielded a significant increase from the preceding baseline to the film phase and a decrease thereafter (p < .OOl). There were also significant group X phase interactions: F(3, 120) = 30.27 for affective arousal, 34.08 for somatic arousal. t-tests showed that values during Phase III were higher in Group 1, whose members were watching the film during this experimental period, while Group 2 subjects were still relaxing. During Phase IV when Group 1 relaxed and Group 2 watched the film sequence, values in the latter sample were significantly higher (p < .OOl). 3.1.2. PSI Figure 1 depicts mean values of the two groups in the different phases of the experiment. ANOVA yielded significant phase effects both for PSI
Fig. I. several Group *** p
Means of values for forefinger PSI and ring finger PSI of two groups (n = 21 each) during experimental phases. Differences between adjacent phases as revealed by Schefft’s test. differences in Phases II1 and IV examined by unpaired I-test (* p < .OS; ** p <.Ol; < ,001; n.s. = not significant).
T. K6hler and I. Schuschel/Riologicai
Psychology 37 (1994) 133-145
139
5-
4--
3-
A
forefinger (F(4, 160) = 20.33) and for PSI ring finger (F(4, 160) = 9.79) (p < .OOOl for both variables). Scheffe’s post hoc comparisons revealed a significant increase from baseline to film and a decrease thereafter. This held true for both groups and both variables. A significant decrease from adaptation to the following baseline occurred in PSI for the forefinger, but not for PSI ring finger. A significant group x phase interaction was also found: F(4, 160) = 40.65 for PSI forefinger, 20.16 for PSI ring finger (p < .OOOl).In accordance with subjective data (see above) during Phase III values were significantly higher in Group 1, and this was reversed during Phase IV. 3.1.3. Skin c~~~uc~unce level and number of ~pon~aneo~~~uctuatio~ Electrodermal variables changed in a similar way to the PSI: ANOVA yielded significant differences between phases: F(4, 160) = 10.24 for SCL, 13.76 for SF (p < .OOOl), and in both groups and for both variables there
were higher values during the film compared with the preceding baseline and the adjacent foltow-up phase (see Fig. 2). Scheffe’s test also yieIded a significant decrease from adaptation to baseline for both variables. Again group x phase interactions were significant: F(4, 160) = 17.95 for SCL, 22.94 for SF (p < .OOOl). During Phase IV Group 2 subjects (watching the movie sequence) had on average significantly higher SCL and SF values than Group 1, and for SF this was reversed during Phase III. However, for SCL no significant difference was revealed in that phase. 31.4. Heart rate
Although there was a significant overall phase effect for HR (P(4, 160) = 4.39; p < .Ol), post hoc comparisons yielded no significant differences be-
6 --
5’-‘i-L_-I*
A*\\
4-
n.s.
3-
*-
Fig. 2. Means of values for skin conductance level (SCL) and number of spontaneous fluctuations (SF) of two groups (n = 21 each) during several experimental phases. Differences between adjacent phases as revealed by Scheffb’s test. Group differences in Phases III and IV examined by unpaired r-test t * p < .05; * * p < .Ol; * * * p < .OOl; n.s. = not significant).
i? Kiihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
141
Fig. 2 (continued).
tween any pair of phases. group X phase interaction.
HR
was the only variable
with
no significant
3.2. Within-subject and between-subject correlations Intraindividual correlations of variables across points of assessment again reflect the above mentioned findings (see Table 1). They were generally well reproducible in both samples, and indicated similar variations over time for all parameters of skin activity, whereas their covariations with HR were much weaker. In particular, the mean correlations of PSI obtained from two fingers (.79 and .78) indicated high intraindividual parallel test reliability of the measurement. Between-subjects correlations during baseline (Phase II), during activation (Phase III for Group 1, Phase IV for Group 2) and for the changes from preceding baseline to activation are shown in Table 2. Again intercorrela-
T. Ktihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
142 Table 1 Within-subjects
correlations
PSIF
PSIR .19*.49 .78+.50
PSIR
of different
parameters
across
points of assessment
SF 59k.34 .56*.35
*** ***
SCL S6k.38 .56*.45
*** ***
HR .16+.35 * .06 f .35
_
.62+.35 .52i.30
*** ***
.59+.36 .51*.35
*** ***
.18+.35 * .04 * .30
SF
_ _
_ _
.67&.24 .71 k.43
*** ***
.42+.35 *** .21 f .38 *
SCL
_ _
_ _
_ _
First and and rate.
*** ***
.49*.39 .20+.41
*** **
row in each line, means and standard deviations of Group 1 (n = 21); second row, means standard deviations of Group 2 (n = 21); PSIF, PSIR, PSI values obtained from forefinger ring finger, SCL, skin conductance level; SF, number of spontaneous fluctuations; HR, heart * p < .05; * * p < .Ol; * * * p < .OOl.
Table 2 Intercorrelations
of variables
across
data of 42 subjects
PSIR
SF
SCL
HR
AFF
SOM
PSIF
.59 *** .78 *** .71 ***
.45 ** 57 *** .49 ***
.38 * .37 * .60 ***
.14 .34 * .27
.18 .18 .18
.14 .Oh p.11
PSIR
_ _ _
.67 *** .43 ** .57 ***
.31 * .I7 .61 ***
.15 .14 .33 *
.39 ** .19 .31
SF
_ _
_ _
.75 *** .67 *** .70 ***
.34 * .28 .32 *
.17 .04 .21
.13 - .06 - .04
_ _ _
.26 .41 ** .37 *
.21 .03 .28
- .Ol - .03 .09
_ _
_ _ _
.24 .09 .33 *
.38 * .17 .36 *
_ _ _
_ _ _
_ _ _
.62 *** .J3 *** .71***
_ SCL _
HR
AFF
_ _
_ _ _
.30 * .17 .08
First number in each block, intercorrelations of baseline values; second, intercorrelations of values during activation; third, intercorrelations of reaction values; AFF, SOM, questionnaire scores for affective and somatic arousal. * p < .05; * * p < 0.1; ** * p < .OOl. See Table 1 for definition of other abbreviations.
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143
tions of parameters of skin activity were generally high and, as a rule, reproducible both during rest and activation and also for the response reactions. Questionnaire scores for somatic and affective arousal also showed high intercorrelations, although, in general, they correlated poorly with skin parameters and HR. The latter variable showed moderate, but frequently significant, correlations with parameters of skin activity.
4. Discussion The present results are in line with the findings of our previous studies, both in the laboratory and in the field (Kiihler & Vogele, 1989; Kohler, Weber & Vogele, 1990; Kiihler, Zander & Dunker, 1991) In particular, they are in accord with the findings of the study previously reported in this journal (Kiihler & Troester, 1991). PSI values increased during the film and decreased rapidly after its end. Questionnaire data indicated that the short section of the movie was indeed perceived as activating, and this is confirmed by the change both in the number of spontaneous fluctuations and in skin conductance level. Accordingly, the activated group had higher values than the one at rest, except for Phase III in terms of SCL, where significance was not achieved. The behaviour of the PSI proved to be the same, both for the forefinger and the ring finger, as shown both by within-group comparisons and by comparisons between groups under differential activation conditions. Heart rate, although slightly higher in the activation phases, was the only variable that did not change significantly in response to the film. Possibly the movie sequence was thrilling at some moments and mainly attention demanding at others. A decrease in HR during attention demanding stimuli has been described previously (see Graham & Clifton, 1966). Little in the way of HR change during sections of violent films was also observed by Carruthers and Taggart (1973), who reported vagal over-reactivity under these conditions. PSI changes and changes in electrodermal parameters would appear to be less situation specific and most likely reflect activation in general. Values rise during different kinds of stimuli such as mental arithmetic (Kiihler, Weber & Vogele, 1990, Kiihler & Troester, 1991), film sections (Kiihler, Weber & Vogele, 1990, this study) and dental treatment (Kiihler, Zander & Dunker, 1991). Thus PSI increases not only in tasks demanding active coping with external stimuli, but also in situations that are more or less passively experienced. It would be interesting to examine conditions where attention is definitely directed inwardly towards personal thoughts and feelings in this context, but these may not be easy to manipulate in the laboratory. Mental arithmetic has been considered to fulfil this condition by Dabbs, Johnson & Leventhal (1968), but has been regarded as an active coping task by others.
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With regard to intraindividual correlations, all except for some covariations with HR had a mean significantly above 0 in both groups. They were slightly lower in magnitude than the coefficients obtained in our earlier study (Kiihler & Troester, 1991) with mental arithmetic as a stressor. This might best be explained by the fact that the changes in variables as a response to activation were somewhat smaller in this experiment and there was less intraindividual variance. The highest coefficients were observed between PSI from both fingers. Given that the interval between the assessment from the forefinger and the ring finger took at least lo-15 s on average, even higher intraindividual parallel test reliability would be expected if prints are taken simultaneously. One may thus conclude that changes of PSI over time determined at one finger are reproducible on another finger. Interindividual correlations were also high between PSI and electrodermal parameters; they are mostly significant beyond the .OOl level. These are thus unlikely to be chance findings even within a large correlational matrix, and confirm the presumption of a common physiological basis. Coefficients were again highest between PSI from two fingers indicating that parallel test reliability is also satisfactory as far as differences between subjects are concerned. Correlations of parameters of skin activity including PSI with HR or subjective responses were much lower and often not significant. This finding, however, is hardly surprising given that low correlations between electrodermal parameters and HR or measures of subjective arousal have also been found in other studies (cf. Fahrenberg & Foerster, 1982). In line with our previous findings, the very first PSI values from the forefinger under resting conditions were higher than the subsequent ones. This is probably caused by the novelty of the assessment procedure, an assumption which receives support from the fact that SCL and SF as indicators of arousal also fell during the rest period, although electrodes had been attached for at least 20 min. It is somewhat surprising that no significant adaptation effect could be shown for the ring finger PSI. However, it should be kept in mind that Scheffe’s post-hoc comparisons are extremely conservative. In addition only two-tailed error probabilities were considered, although we expected significant differences only between a few phases and had clear expectations as regards the direction of change. Paired t-tests with Bonferoni adjusted one-tailed error probabilities would have yielded a significant decline of ring finger PSI values in both groups. We would thus still argue that PSI adaptation exists for any finger (75% and 69% decline for PSI forefinger, 51% and 78% for PSI ring finger). In the discussion of the previous article we revealed that the print series of a large number of subjects had to be rejected because of holes in the area chosen for evaluation and that this might pose serious limitations to the general applicability of the method. However, this was not the case in the present study, although the recipe of the solution had not changed and the
i? Ktihler and I. Schuschel /Biological Psychology 37 (1994) 133-145
14.5
technique of assessment was the same. Possibly the finger pads of females are better suited for the procedure, being in general smoother and better cared for. It is also possible that the experimenter in this study was more skilful in taking prints. To sum up, the present study replicates the finding that PSI increases during activation and decreases thereafter, changing in the same way as measures of electrodermal activity. Although the type of provocation employed in this study was quite different from the one used in the previous study and although females served as subjects this time, the outcome was very similar. Again we observed that the first resting PSI values were higher than subsequent ones. This supports the assumption that the conflicting results reported in the literature are due to measurement factors rather than to difference in the stressors employed.
References Carruthers, M., & Taggart, P. (1973). Vagotonicity of violence: Biochemical and cardiac responses to violent films and television programmes. British Medical Journal, 3, 384-389. Dabbs, J.M., Johnson, S.E., & Leventhal, H. (1968). Palmar sweating: A quick and simple measure. Journal of Experimental Psychology, 78, 347-350. Fahrenberg, J., & Foerster, F. (1982). Covariation and consistency of activation parameters. Biological Psychology, 15, 151-169. Geisser, S., & Greenhouse, S.W. (1958). An extension of Box’s results on the use of the F-distribution in multivariate analysis. Annuls of Mathematical Statistics, 29, 885-891. Graham, F.K., & Clifton, R.K. (1966). Heart rate change as a component of the orienting response. Psychological Bulletin, 65, 305-320. Harrison, J. (1964). The behaviour of the palmar sweat glands in stress. Journal ofPsychosomatic Research, 8, 187-191. KBhler, Th., & Troester, U. (1991). Changes of the PSI (palmar sweat index) during mental arithmetic. Biological Psychology, 32, 143-154. Kiihler, Th., & Viigele, C. (1989). Laboratory studies on a potential stress indicator in field research: The Palmar Sweat Index. In: D. Siddle, & N. Bond (Eds.) Proceedings of the 24th International Congress of Psychology, Sydney, Australia, Volume 6. Psychobiology. Issues and Applications (pp. 315-323). Amsterdam: Elsevier. KGhler, Th., Weber, D., & Viigele, C. (1990). The behaviour of the PSI (palmar sweat index) during two stressful laboratory situations. Journal of Psychophysiology, 4, 281-287. Kiihler, Th., Zander, O., & Dunker, D. (1991). Interpreting palmar sweat glands. Journal of Psychosomatic Research, 35, 75-81. MacKinnon, P.C.B., Monk-Jones, M.E., & Fotherby, K. (1963). A study of various indices of adrenocortical activity during 23 days at high altitude. Journal of Endocrinology, 26, 555-566. Martens, R. (1969). Palmar sweating and the presence of an audience. Journal of Experimental Social Psychology, 5, 371-374. Sutarman, & Thomson, M.L. (1952). A new technique for enumerating active sweat glands in man. Journal of Physiology, 51P-52P. Weisenberg, M., Kreindler, M.L., Schachat, R., & Werboff, J. (1976). Interpreting palmar sweat prints: A not-so-simple measure. Journal of Psychosomatic Research, 20, l-6.