extraversion on the skin conductance response to stress and stimulus intensity

JOURNAL

OF

RESEARCH

IN PERSONALITY

11,

129-146 (1977)

The influence of IntroversionlExtraversion the Skin Conductance Response Stress and Stimulus Intensity DON C. FOWLES,

RICHARD

ROBERTS,

AND KIM

on to E. NAGEL

A series of studies compared skin conductance level (SCL) for introverts and extraverts during a series of tones varying in both stimulus intensity (SI) and the amount of stress preceding the tones. When a difficult paired-associate task preceded the tones, both groups were about the same for 83 dB tones but extraverts were much higher at 103 dB, introverts failing to show an increase as a function of SI. There were no differences between the two groups following a simple paired-associate task, both groups showing a similar increase with increased SI. Finally, SCL was higher for introverts than extraverts during tones preceded by a rest period, and this was especially true for the early trials and for lower SI (75 and 83 dB compared with 100 and 103 dB). Taken together, these results suggest that SCL is higher for extraverts at higher levels of arousal but that the reverse is true for lower levels of arousal. This relationship is consistent with the theory that introverts have a “weak nervous system” which develops transmarginal or protective inhibition under stress.

Over the last ten to fifteen years there has been a growing literature concerned with the relationship between electrodermal activity and the personality dimensions of introversionlextraversion (I/E) and neuroticism (N) (Coles, Gale & Kline, 1971; Crider & Lunn, 1971; Galbrecht, Dykman, Reese & Suzuki, 1965; Katkin & McCubbin, 1969; Koriat, Aver-ill & Malmstrom, 1973; Mangan & O’Gorman, 1969; Martin, 1960; MundyCastle & McKiever, 1953; Scott & Wilkinson, 1962). In many of these studies the stimulus conditions have involved a rest period followed by the presentation of a series of stimuli, usually auditory. which are said to elicit either orienting responses or defensive responses. These studies have generally been inspired by the assumption that habituation of the This research was supported by grant MH 25060-01 from the National Institute of Mental Health. The authors wish to thank Linda Powell for her assistance in data collection and Dr. T. C. Weerts for his critical reading of the manuscript. Requests for reprints should be sent to Don C. Fowles, Department of Psychology. University of Iowa, Iowa City, Iowa 52242. 129 Copyright All rights

@I 1977 by Academic Press. Inc. of reproduction in any form reserved.

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orienting response is a fundamental process which ought to be related to personality (e.g., Katkin & McCubbin. 1969; Koriat, Averill & Malmstrom, 1973: Mangan & O’Gorman, 1969). As a result of this interest in habituation, electrodermal responses have received the greatest attention, while electrodermal levels have been somewhat neglected. This is unfortunate because in the case of skin conductance (SC), the most widely used electrodermal measure, levels appear to be less complicated than responses and thus perhaps more likely to yield consistent results. With respect to mechanisms, skin conductance responses (SCRs) are believed to be influenced by at least two different mechanisms (one for rapid recovery SCRs and one for slow recovery SCRs), whereas the primary determinant of skin conductance level (SCL) is thought to be the amount of sweat in the ducts (Edelberg, 1972; Fowles. 1974). On a psychological level, there is some disagreement as to whether SCRs are primarily related to arousal or attentional processes (Raskin, 1973), while SCL is interpreted simply as an index of arousal (Raskin, 1973) or, more conservatively. electrodermal arousal (Stern & Janes, 1973). Moreover, the interpretation of SCL as an index of arousal bears directly on theoretical issues which have arisen in neurophysiological theories of I/E. The best-known theory of I/E and N is that of Eysenck (e.g., 1947, 1967). With respect to SC measurements, he (Eysenck, 1967) has argued that electrodermal activity is related only to I/E, introverts being expected to show greater arousal. A less familiar but equally important theory is the dimension of “the strength of the nervous system” as described by Russian investigators (especially Teplov) and reviewed by Gray (1964b). In this formulation a “weak” nervous system is said to respond with greater excitation to a given stimulus than does a “strong” nervous system. At low stimulus intensities this greater excitation results in greater responsiveness. However, as stimulus intensity and consequently the amount of excitation increases, individuals with a weak nervous system show a diminished response which is said to be due to the development of transmarginal or protective inhibition. Thus, at low levels of stimulation the weak nervous system is expected to show greater reactivity than the strong, but at higher levels of stimulation the reverse is to be expected (Gray, 1964b). Gray (1964a) offered a translation of this theory into a dimension of “arousability” coupled with the familiar inverted-U curve which is hypothesized to describe the relationship between behavioral efficiency and arousal. Originally Gray (1964a) suggested that this dimension of arousability would be related to Eysenck’s neuroticism or to manifest anxiety as measured by the Taylor scale. More recently, however, he has argued that it is more likely to be related to I/E (Gray, 1967), a position also adopted by other authors

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including Eysenck (Eysenck, 1966, 1967; Eysenck & Levey, 1972; Mangan, 1972; Zhorov & Yermolayeva-Tomina, 1972; Marton, 1972). Although, in general, the Gray and Eysenck positions are not incompatible-in fact, Eysenck (1967; Eysenck & Levey, 1972) comments on their similarity-there is at least a difference in emphasis where greater levels of stimulation are concerned. As stated, the “weak nervous system” hypothesis states that when stressed the introvert but not the extravert will show a reduction in responsiveness, suggesting that maximum differentiation is likely to be found under these conditions. In contrast, Eysenck (1967) states that normal stimulation will produce I/E differences with more stressful conditions producing differences as a function of N. Moreover, Eysenck has emphasized the greater anxiety and responsiveness of introverts, rather than reduced reactivity under stress. At an empirical level, studies relating electrodermal activity to N and I/E have yielded results which are so inconsistent across laboratories that reviews of the literature (Koriat et al., 1973; Stem & Janes, 1973) have been unable to find any consistent themes other than that interactions between neuroticism and extraversion are to be expected (Stem & Janes, 1973). The methodological differences among studies have been large enough, however, to produce hope that some of the inconsistencies are due to unrecognized method variance (Mangan & O’Gorman, 1969; Sadler, Mefferd & Houck, 1971). Such differences include the methods of selecting Ss, the use of skin potential versus resistance, the method of recording electrodermal activity, the modality of stimulation, the interstimulus intervals employed, and variations in the frequency, duration and intensity of the stimuli. Since these considerations render the negative evidence inconclusive, interest in this area has continued. At least in part, this is because of the theoretical importance of reconciling Russian and Western approaches to the relationship between temperament and physiology (Stem & Janes, 1973). Our own interests also included a concern for providing multimethod validity data for self-report measures in response to criticisms of trait approaches to personality (Mischel, 1971) and in support of interactionist views of personality (Bowers, 1973; Ekehammar, 1974). The above considerations have guided a series of 7 studies involving over 5’00 Ss in our laboratory which have been characterized by the following concerns: (1) the selection of Ss on the basis of both I/E and N; (2) the importance of stimulus intensity (SI) and/or stress; and (3) a search for results which are reliable within the same laboratory from one experiment to the next so that any differences between laboratories can be assumed to be meaningful and not due to chance. All studies involved the measurement of skin conductance activity during the presentation of a

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series of 1,000 Hz tones of one second duration, a procedure which has yielded consistent results with patient populations (Lader, 1967; Lader & Matthews, 1968; Lader & Wing, 1966, 1969; Noble & Lader, 1971). The results to be reported here are limited to SCL data from four studies primarily concerned with l/E, which appear to meet the criterion of replicability and to have important theoretical implications. Experiments 1 and 2 are concerned with the effects of both mild stress and stimulus intensity on XL in Ss selected for high and low I/E with N held constant in the middle range. Experiments 3 and 4 bear on theoretical issues raised in the first two experiments concerning SCL as a function of I/E at low levels of stimulation. EXPERIMENT

1

Method This experiment involved a 2 x 2 x 2 x 20factorial design in which there were two levels of I/E, two stimulus intensities (SI) (83 and 103 dB), and two levels of stress as manipulated by task difficulty, all between Ss factors. In addition, there was a within Ss factor associated with the 20 stimulus tones. The task was a paired-associate verbal learning task which was administered prior to the stimulus tones with its effect measured in terms of its influence on SCL during the tones. Subjects and parsonaliry measures. The dimensions of I/E and N were assessed using Block’s (1964) Ego Control scale (EC-SM) and Ego-Resiliency (ER-S) scale plus the lie scale from the MMPI. These scales represent the first two orthogonal factors of the MMPI and were carefully developed to minimize the influence of acquiesence and social desirability response sets, These factors are considered to be equivalent to I/E and N (Block, 1965) and were felt to be preferable to other scales because they were developed with American subjects and because of the minimal effect of response sets. In an early study from our laboratory using 61 female Ss from Introductory Psychology, Block’s scales were compared with the E and N scales from the Eysenck Personality Inventory (Marchman, 1973). As expected. ER-S was found to be negatively correlated with N (u = -0.64,~ < 0.001) and EC-SF (the Ego-Control scale for females) with E (r = -0.52,~ < O.OOl), while near zero correlations were found for ER-S vs E (r = -0.14) and EC-SF vs N (r = -0.02). Also, as expected, the two factors were orthogonal. Block’s scales correlating -0.02 and the EPI scales 0.07. From these results it is clear that ER-S is a neuroticism factor (low scores indicating high neuroticism) and that EC-SF is an extraversion factor (low scores indicating extraversion). Block’s ER-S and EC-TM scales plus the lie scale from the MMPI were administered with the heading “Brief Questionnaire” to approximately 250 male undergraduates in Introductory Psychology during a large group testing session at the beginning of the semester. 5’s who scored in either the top or bottom 35% on the Ego Control scale, as near to the middle portion of the Ego-Resiliency scale as possible, and 7 or less on the lie scale, were asked to volunteer for the experiment in order to fulfill course requirements. Forty introverts and 40 extraverts obtained in this manner were assigned randomly to the four cells for each group (easy vs difficult task x high vs low SI), resulting in a final count of 10 Ss per cell. In addition to the 80 Ss who completed the experiment and are included in the analysis, 9 Ss were lost due to E error or apparatus failure, and 3 because of inability to master the easy task. The EC scores for the 40 extraverts fell in the range 11-26 (M 21.8, SD 4.47) and for the 40 introverts in the range 35-52 (M 39.7, SD 4.58). Because of the exclusion of Ss who

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participated in another study in the same lab, plus the loss of 12 Ss, the attempt to keep ER-S scores in the middle range was relatively unsuccessful: the range was 12-B (mean 18.0. SD 3.64) for the extraverts and 9-27 (M 18.7, SD 4.26) for the introverts. Tusk. The stress manipulation was accomplished by the presentation of an easy or difficult paired-associate learning task lasting approximately ten minutes. The easy task was created by pairing eight adjectives from Haagen’s (1949) low vividness list (Table 5) with eight adjectives from his high vividness list. Ss were asked to say the correct response aloud and were terminated when they had recited all eight pairings correctly two out of three times. Four different arrangements of the S-R pairs were made to prevent temporal learning as opposed to pairing the words. For the difficult task, the same eight words used as stimulus words in the easy task were used as stimuli. but the response items were merely those same eight words in a different order, thereby creating a double-function list which is unusually difficult to learn (Umemoto & Hilgard, 1961). Ss performing the difficult task were yoked with Ss in the easy task so that each group would spend a comparable time on the task. E scored correct responses on a sheet of paper while sitting in a chair next to S. No Ss were able to correctly respond to all pairs of the difficult task even once, while only three Ss were not able to complete the easy task within a reasonable amount of time and these three Ss were eliminated from the experiment. Thus, for the easy task all Ss included in the analysis completed the task and were assumed to have been moderately aroused but not stressed. For the difficult task, on the other hand, all Ss had been unsuccessful in learning the task in the time allotted and, insofar as they interpreted this as a failure, may have been somewhat stressed by this perceived failure. E did not give them any evaluative feedback at the time, but at the end of the experiment he stressed that the task was extremely difficult, that no Ss were expected to learn it, and that S had done quite well. Stimuli. The stimuli were twenty 1000 Hz tones of 1 second duration and of either 83 or 103 dB intensity (re: .0002 microbars) presented at intervals varying from 45 to 80 seconds with a mean interstimulus interval of 60 seconds. One minute after the final tone, each S received the first of two presentations of a novel stimulus, the second presentation coming one minute later. This stimulus consisted of a loud door bell buzzer (approximately 100 dB) situated two feet from his head. In order to provide a constant background for the stimuli and to mask extraneous noises, a 57 dB white noise was presented between stimuli. beginning approximately one minute prior to the first tone. The masking noise and stimuli were generated and controlled by a BRS solid state programming system and presented through Grayson-Stadler padded stereo earphones. SI was determined by monitoring the RMS voltage across the inputs to the earphones and then reading the dB level from a calibration curve constructed for the headphones using an acoustic coupler and a calibrated condenser microphone.’ Except for SI the presentation of the tones followed the method used by Lader and Wing (1964). The SIs were chosen in order to be comparable to an earlier study in our laboratory. In that study (Schneider, 1974) rough pilot data indicated that 83 dB tones were necessary to produce SCRs to all of the first three tones in approximately 90% of theSs2 The higher intensity (103 dB) was selected. then, simply to provide a 20 dB range of intensity. Ekctrodermal measurements. The technique for electrodermal recording generally followed the recommendations of Edelberg (1967) and Lykken and Venables (1971). Ag-AgCI electrodes with a contact area of 2 cm* were constructed in a manner roughly similar to that recommended by Miller (1968) and described more fully by Fowles and Schneider (1974). ’ We are greatly indebted to Dr. Arnold Small for calibrating the headphones. 2 Eighty-three decibels seems high in view of the many studies which use lower intensities. Schneider suggested that in large part this was due to the use of a masking noise, but that it was also possible that the minimum suitable intensity was overestimated because of the small number of pilot Ss run at each intensity.

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The electrolyte used was 100 ml of 2.1% KCI thoroughly mixed with 295 gm of Parke-Davis Unibase to form a cold cream-like electrolyte with a concentration of 0.5% KCI (Miller. 1968). The electrodes were attached to the thenar and hypothenar eminences of the left hand. Bipolar SC was recorded using a constant voltage Wheatstone bridge circuit with an applied voltage of 1.0 V between each pair of electrodes (Edelberg, 1967) and the output from the bridge amplified by a single direct-coupled channel of a Grass model 5 polygraph. Noting the amount of SC balanced by the bridge on the polygraph record permitted a recording sensitivity of 2 pmhos/cm without loss of information as to the actual SCL. The constant voltage method provides a recording which is linear with respect to conductance, rendering data transformations unnecessary (Lykken & Venables, 1971).

Procedure After reporting to the laboratory, S washed his hands with a mild soap (Johnson’s baby soap) as recommended by Venables and Martin (1967) and then was seated in a comfortable reclining chair in a sound-attenuated room with temperature thermostatically controlled at 72-74°F. He remained in the chair throughout the experiment. S was told that electrodes would be placed on his left hand but would cause no shocks. that he would be doing a memory task, and that he would then hear a series of tones. He then signed an informed consent card, after which E attached the electrodes and left the room to initiate SC recording. Upon returning, the memory drum was placed in front of S. and a tape recording was played to explain the task and purpose of the experiment. In these instructions the requirements of the task were explained, S was encouraged to try hard not to be discouraged by errors and was told that most students find the task a little more difficult than they might first expect. S was also told that his physiological responses would be recorded during the task and the tones which followed and that the experiment was concerned with any possible correlation between these responses and his performance on the task. If S had no question at this point. the task was presented. Upon completion, another, very brief, tape recording was played and then headphones placed over his ears. This recording told S that he would begin to hear some tones in about three minutes and that he should relax and try not to move around too much. keeping his palm up so that there would be no pressure on the electrodes. He was also told that the tones would take about 20 minutes. E left the room and, following a three minute rest period intended to allow for some recovery from the activity associated with the task. presentation of the 20 stimulus tones and 2 buzzes was initiated. At the end of the experiment E returned to the room to debrief S. Approximately half the Ss were monitored on closed-circuit TV and these Ss were assured that this was only to check for excessive movements to make sure they were awake. Scoring anJ arzalysis of’d~ta.~ Skin conductance was scored for any SCR with onset within 1-5 seconds after the stimulus and with an amplitude of at least 0.1 micromho by taking the difference between the prestimulus SCL and the peak of the first response. The prestimulus SCL reading was used as the estimate of SCL during the tones and two buzzer presentations, and thus the 22 readings averaged one reading per minute. In the absence of any response SCL was read as the conductance at the end of the one second stimulus presentation. In addition, SCL was scored four times prior to the first tone: (I) the maximum during the verbal learning task and (2) readings after 1. 2, and 3 minutes during the rest period between the task and the first tone. As mentioned above, only the SCL results will be reported here. Separate analyses were conducted for the following groupings of the SCL data with trials (or time) included as a within Ss factor in (2) and (3): (1) maximum SC during the task; (2) three SCL readings during rest: and (3) twenty SCL readings for tones 2-20 plus the first buzzer. The last 3 The analyses were performed with a computer program developed by Dr. Ray Hohle.

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grouping was, in effect, SCL assessed on the average at one minute after each of the twenty tones and was chosen in order to avoid including the SCL prior to the first tone since this reading could not have been affected by the tones. Of course, the prestimulus SCL for the first buzzer similarly could not have been affected by the buzzer.

RESULTS XL during tusk. Although there was a trend (p = 0.10) toward a main effect of I/E in which extraverts had the higher SCL, this did not replicate in Experiment 2 and will not be discussed further. SCL during rest. During the rest period there was, of course, a main effect for trials (p < .OOl). There was also a Trials x Task interaction @ < .05) in which SCL was the same for both task conditions during the first two minutes but higher for the difficult task at the end of the third minute. A weak trend (p = .153) toward higher SCL for extraverts was also found but was not replicated in Experiment 2. Thus, the rest period yielded no personality findings of interest. It is worth noting that the effects of task difficulty could only be seen at the end of the brief rest period, suggesting that the demands of the easy task were sufficient to produce considerable immediate arousal. XL during tones. The Trials x SI x Task x I/E ANOVA for SCL during the tones yielded a number of significant results. A decline in SCL over trials (p < .OOl) and an increase in SCL with increased SI (p < .Ol) were found as expected. Both of these were generally found and will not be reported elsewhere except in those instances in which they were not found. Inspection of the results showed that substantial differences appeared as a function of task difficulty and this was supported by several interactions at significant or near-significant levels: Trials x Task x I/E (p < .OOl);Trials x SI x Task x I/E (p < .lO); SI x Task x I/E (p = 0.18). Consequently, Trials x SI x I/E ANOVAs were computed separately for each task condition. There were no significant findings for I/E during the tones following the easy pretask and the only trend obtained was not replicated in Experiment 2. The difficult pretask condition, on the other hand, produced two findings of interest. There was a difference between the extraverts who received the 103 dB stimuli and the other three groups: the 103 dB extraverts started out slightly higher at the beginning of the tones and failed to decline over trials, while the other three groups showed the usual decline over trials (Trials x I/E,p < .OOl; Trials x SI x I/E,p = .002). In addition, as shown in the left portion of Fig. 1, the extraverts showed a large SI effect while the introverts showed essentially no change in SCL with an increase in SI. Although this SI x I/E interaction was only a trend in this study (p = . lo), it was supported by a similar finding in Experiment 2.

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ROBERTS

Exp. ’

AND

NAGEI Exp 2

12.0 1

‘\ 11.03 2 $0.0 2: b-)

-

9.0 -

8.0

t

Cl’/’ d’

0 EXTVRT 0 INTVRT -fast Strssl

,; 2 83dB

103dB

: I 83dB

103dE

STIMULUS INTENSITY FIG. I. Comparison of the prestimulus SCL results for Experiments 1 and 2 collapsed across trials. Dashed lines: easy task (low arousal): solid lines: difficult task (high arousal). Open circles: extraverts; closed circles: introverts.

Summarizing these results, the only I/E differences which were found were in the response to SI following the difficult pretask. Under these conditions extraverts showed a large SI effect but the introverts did not, and this difference increased across trials for the 103 dB tones. This result underscores the importance of both task difficulty and SI in determining whether differences will be seen between introverts and extraverts. Further discussion of these results will be postponed until after presentation of Experiment 2. EXPERIMENT

2

Method With the exception of one small procedural detail, Experiment 2 was a literal replication (Lykken, 1968) conducted one year later with a different E and half the number of Ss @/cell) used in Experiment 1. Its purpose was to determine which results would replicate with the least amount of variability one could expect from one study to the next-i.e., a change of experimenter. Lower significance levels were anticipated because of the reduced sample size. The procedural change was the shortening of the period between the task and the first tone by having the onset of the first tone coincide with the end of the three-minute rest period. For technical reasons, in Experiment 1 there was a delay of approximately 60 seconds between the end of rest and the onset of tone 1 and this was eliminated in Experiment 2. The EC scores for the 20 extraverts ranged from 12 to 27 (M 22.5, SD 3.62) and for the 20 introverts from 36 to 47 (M 40.5, SD 3.38). This time the ER-S scores were successfully kept

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near the median: the range was 16-21 (M 17.9, SD 1.52) for the extraverts and 16-20 (M 18.0, SD 1.39) for the introverts.

RESULTS SCL during task. There were no significant findings or even trends for SCL during the task. SCL during rest. Aside from a trials effect (p < .OOl), there were no significant findings. There was a trend toward a Trials x Task x I/E interaction (p = .l l), but nothing similar to this was found in Experiment 1. The Trials x Task interaction from Experiment 1 did not replicate; instead, there was only a very weak trend toward a generally higher SCL for the difficult task (p = .31). Comparing the two studies, task difficulty had an effect during rest only after three minutes in Experiment 1 and did not even approach significance in Experiment 2, indicating little effect of the task on SCL during the rest period. SCL during tones. The difficult task produced a somewhat higher SCL during the first few tones than did the easy task, but the difference disappeared by the 11th tone (Trials x Task, p < .05). In the personality realm, task difficulty was again found to be important, as indicated by a nearly significant SI x Task x I/E interaction (p = .06) which replicated the trend found in Experiment 1. In the separate analysis of the easy task condition no significant results were obtained involving I/E. For the difficult task condition the introverts and extraverts showed opposite effects of SI: the extraverts showed the usual increase in SCL with greater SI while the introverts showed a smaller change in the opposite direction (SI x I/E, p = .09). The divergence over trials reported in Experiment 1 for introverts and extraverts during the 103 dB tones was not found in this experiment. DISCUSSION

OF EXPERIMENTS

1 AND 2

The extraverts in both studies showed the expected higher SCL during strong than during weak tones, whether the tone series followed a stressful or a nonstressful experience. After the nonstressful pretask, the introverts gave this same pattern of increased SCL with increased tone intensity. However, when the tones were preceded by a stressful task experience, the introverts in both studies showed a different relation of arousal to tone intensity; their SCL was not higher during the strong, as compared to the weak, tones and, indeed, in the second experiment their SCL actually varied inversely with tone intensity. The increase in SCL with tone intensity, found for both groups after the nonstressful pretask, was significant in both experiments 0, < .05; p < .05). The failure of the introverts to increase SCL with tone intensity after the stressful pretask

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was also reliable, as indicated by the SI x I/E interaction for this condition which, although marginal in the separate experiments, is significant for the pooled data (p < .02). This result can be stated in the following form: under moderate levels of stimulation both introverts and extraverts show increased SCL in response to increased stimulus intensity, but when stressed only extraverts show this increase while introverts show either no increase (Experiment 1) or a decrease (Experiment 2). This result is consistent with the Russian hypothesis that introverts have a weak nervous system which develops transmarginal inhibition when stimulus intensity is high. The necessity of having Ss perform a difficult task prior to presentation of the tones in order to produce a transmarginal inhibition effect is paralleled by the Russian use of caffeine for a similar purpose. In these studies caffeine is said to have increased the “background excitability of the cortex,” thus making it possible to reach levels of excitation sufficient to produce transmarginal inhibition (Gray, 1964b, p. 159). Without caffeine, transmarginal inhibition has seldom been obtained, just as it was not found without the difficult task in these experiments. It is worth noting that the interaction with trials in Experiment 1 (which did not replicate) is of some theoretical interest. The strength of the nervous system is defined in two ways: (a) the maximum response to a single application of the stimulus and (b) the maximum duration of the response to repeated application of the stimulus (Gray, 1964b, p. 161). The SI x I/E interaction corresponds to the first definition while the Trials x SI x I/E interaction corresponds to the second. Both results would be predicted by the theory of the strength of the nervous system. In view of this, it should be stressed that because of the small sample in Experiment 2 the power was not great and the failure to replicate is by no means conclusive (Tversky & Kahneman, 1971). In fact. the interaction is still significant when the data from the two studies are pooled 0, < .05). Consequently, the effect of trials should be of interest in future studies. EXPERIMENT

3

The results of Experiments 1 and 2 supported the strength of the nervous system formulation of I/E and this, in turn, suggests that at very low intensities of the excitatory process, introverts will show greater reactivity than extraverts. Since there was no evidence of higher SCLfor introverts in the least stimulating conditions of Experiments 1 and 2 (i.e., the easy task followed by the 83 dB tones), it seemed likely that the predicted differences would be found only with low SI and in the absence of any prior task. We looked, therefore, at other studies in our series to find those in which there was minimal stimulation prior to the stimulus

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tones and which included tones of low to moderate intensity. Two studies met these requirements and are presented as Experiments 3 and 4. Method This experiment (which was actually run concurrently with Experiment 1) differed from the first two primarily in that there was no task. Only a ten-minute rest period preceded the tones. The design was a 3 x 2 x 20 ANOVA with 3 levels of extraversion (high, medium and low), 2 stimulus intensities (83 and 103 dB), and twenty tones. There were two Es: a male who ran IS Ssicell (who also ran all Ss in Experiment 2) and a female who ran 5 Ssicell for a total of 120 Ss for the experiment as a whole. Since the Ss were all female, extraversion scores were based on the EC-SF. Ss were selected who fell in the upper, lower, and middle thirds on this scale with all Ss having Ego Resiliency (neuroticism) scores in the middle 30% of the distribution. Thus, all Ss received ER-S scores in the range 16-19, while the ranges for the I/E groups were 7-13 (extraverts), 14-17 (ambivetts), and 18-28 (introverts) on the EC-SF scale.

Results The results for XL during the tones are presented in the left portion of Fig. 2. There was an overall effect of L’E in which introverts were highest, ambiverts next, and extraverts lowest (p = .Ol). The apparent interaction, in which larger differences between introverts and extraverts are seen at the lower intensity, was only a weak trend @ = .17). It does, however, suggest that lower intensities may maximize the probability of obtaining significant differences, and a similar result was obtained in Experiment 4. There was no effect of SI. In order to better compare these results with the other studies, the medium EC groups were deleted and a new analysis performed, This procedure increased the significance of the main effect for I/E (p = .003) and revealed a Trials x I/E interaction (p = 0.02) in which the introverts Exp. 4

75dB STIMULUS

1COdB

INTENSITY

FIG. 2. Comparison of the prestimulus SCL results for Experiments 3 and 4 collapsed across trials. Open circles: extraverts; half-filled circles: ambiverts; filled circles: introverts. In Experiment 4 solid lines indicate Session 1 and dashed lines indicate Session 2 (one week later). The results for Experiment 4 are collapsed across Ego Resiliency.

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declined more rapidly over trials. The SI x I/E trend was unchanged (p = .17). EXPERIMENT

4

The results reported here are based on a reanalysis of the data from a dissertation (Marchman, 1973) which was the first study in the series on I/E and N and the one which first emphasized the importance of SI.* The analysis at that time, however, was based primarily on the scoring procedures used by Lader and Wing (1966) and thus failed to detect the results reported below. Method The design was that for a 2 x 2 x 2 x 2 x 20 ANOVA consisting of 2 levels each of extraversion (high and low), neuroticism (high and low) and stimulus intensity (75 and 100 dB with a white noise background of approximately 65 dB), all between Ss variables with IO Ss/cell. Within Ss factors were two identical sessions held one week apart and the usual 20 tones. Ss were chosen from a total of 261 females who completed the EC-SF and ER-S scales. In order to be included, S must have scored in the upper or lower 35% of the distribution on both scales and have a lie score of 7 or less. This was prior to the acquisition of the BRS stimulus presentation equipment and so the stimulus tones were recorded on tape. Also, stimulus intensity was determined by clasping the headphones over a sound level meter. A comparison of the two methods of stimulus presentation by Schneider (1974) indicated that. compared to estimates based on the calibrated headphones used in our more recent studies, the sound lever meter underestimated the stimulus intensity. That is, Schneider estimated that the intensities used in Marchman’s study would be about 80-85 dB and I05 dB using the more accurate techniques we now have available. Thus, the intensities were roughly comparable to those of the other studies. The background white noise was higher (65 versus 57 dB) which could have had some effect on SCL and also on the subjective intensity of the tones. The technique for recording SC was similar to that of the other studies except that different Ag-AgCI electrodes were used. Since this resulted in a somewhat smaller area of contact, a comparison with absolute SCL from the other studies is inappropriate.

Results For SCL during the tones the major finding which involved I/E was an initially higher SCL for introverts which showed a more rapid decline over trials to a level not too far above that for extraverts (Trials x IIE,p < .OOl). However, the more rapid decline over trials for introverts was found only for Session 1, producing a trend toward an interaction with sessions (Trials x l/E x Sessions, p = .08). The overall effect of I/E in which introverts showed higher SCL was only a weak trend @ = .20). From inspection of the results (see the right hand portion of Fig. 2), it is 4 The authors would like to thank James Marchman for making his data available for reanalysis.

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obvious that this effect of I/E was found only for the 75 dB tones, but the apparent interaction between I/E and SI was not significant (p = .22). DISCUSSION OF EXPERIMENTS 3 AND 4 The results of Experiment 4 are consistent with those of Experiment 3 and confirm the prediction that introverts will be more aroused than extraverts under minimally stressful conditions. The failure to find as large a difference during the later trials of Experiment 4 appears to be simply a matter of greater adaptation over trials for the initially more aroused group. In both studies the greater SCL for introverts was more obvious at the lower intensity, as would be expected from the theory of strength of the nervous system, although in neither study did this SI x I/E interaction reach significance. In spite of the fact that N did not enter into any of these results, it would be a mistake to conclude that it is not important. There were some trends indicating that I/E differences were less likely to be obtained for high N Ss, suggesting that N should not be ignored in future studies. SUMMARY

AND DISCUSSION

Description of Results In the first two experiments it was found that when a stressful task preceded the presentation of a series of tones, extraverts showed an increase in SCL as a function of an increased intensity of the tones but introverts did not. As a result, for the stressed, 103 dB condition (the most arousing in our experiments), SCL was higher for extraverts than for introverts. At the other end of the arousal continuum, Experiments 3 and 4 in which there was no task or other stressor showed that at least for the lower intensity tones (75 dB, 83 dB) SCL was higher for introverts than for extraverts. Experiments in our laboratory which utilized only intense tones (100 or 103 dB) have not yielded consistent results. Thus, the empirical generalization seems warranted that at low to moderate levels of stimulation SCL is greater for introverts but that the reverse is true at high levels, while no differences are to be found at intermediate levels of stimulation. The time course of these differences varied from study to study, but when there was an interaction with trials the largest differences were found during the early trials for the low stress conditions and during the later trials for the high stress conditions. This suggests that the differences produced by the stressful task + 103 dB tones continue for as long as twenty-five minutes after the task has ended, whereas those produced simply by the 75 or 83 dB tones may be more transient, presumably because of fairly rapid adaptation to the tones.

142 The Dimension

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These results are generally consistent with the hypothesis that l/E corresponds to the dimension of strength of the nervous system as described by Russian investigators (Gray, 1964a, 1964b, 1967: Eysenck & Levey, 1972) or the reinterpretation of this formulation into a dimension of arousability (Gray, 1964a). The greater responsiveness at low stimulus intensities and the decline in responsiveness at high stimulus intensities as a result of transmarginal inhibition among introverts, according to this theory, have already been described in the introduction. The direction of the differences in SCL and their limitation to the extremes of the arousal continuum are as expected from the theory. Moreover, the necessity of using a stressful task in order to bring out the transmarginal inhibition effect among introverts has its parallel in the use of caffeine by Russian investigators in order to produce transmarginal inhibition in individuals with a “weak” nervous system (Gray, 1964b). Although the absence of consistent l/E differences at intermediate levels of stimulation is to be expected from the inverted-U curve associated with the theory of the strength of the nervous system, the situation may be even more complicated in the case of SCL. There is a normal adaptation or decline in SCL over trials when stimuli of low intensity are presented to unselected populations (Raskin, 1973; Raskin, Kotses & Bever, 1969). Transmarginal inhibition, on the other hand, is said to produce a decrement in response to repeated, intense stimulation in individuals with a “weak” nervous system (Gray, 1964a, 1964b). Assuming that the second process is stronger and the first possibly weaker among introverts, it is likely that at intermediate levels of stimulation both processes may be operative, albeit in different individuals: extraverts will show a decline over trials attributable to normal adaptation, while at the same time introverts will show a decline attributable more to transmarginal inhibition. Only at the extremes of the arousal continuum are these processes clearly separated allowing unambiguous predictions.

As an index of arousal, SCL bears on an important difference between Russian and Western theorizing. In Russian theorizing transmarginal inhibition reduces the central excitatory process as well as peripheral responsiveness, whereas in arousal theory as formulated by Westerners (e.g., Hebb, 19.55; Malmo, 1959) central arousal continues to increase even though efficiency of performance decreases. Gray (1964a) called for the use of measures of arousal to resolve this question. Thus, insofar as SCL does in fact reflect a central arousal process, the results of the experiments reported here support the Russian view. However, data

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consistent with the Western view have been reported by Belanger and Feldman (Malmo, 1966; Malmo & Belanger, 1967): heart rate in rats showed a monotonic increase with the number of days of water deprivation while rate of bar pressing showed an inverted-U relationship to water deprivation. Thus, the measure of arousal (heart rate) did not show a reduction at higher levels of deprivation. Differences in the species, stimulus conditions, and response system being measured make comparisons between these studies all but impossible, but both sets of results should be incorporated into any final resolution of the issue. It is possible that different central processes are being assessed in the two studies. The results reported here also have implications for the use of SCL as an index of arousal: extraverts gave results which nicely fit an increasing, monotonic function, whereas introverts showed at least a flattening (Experiment 1) and perhaps a decrease (Experiment 2) at higher levels of stress and stimulus input. Thus, the I/E dimension moderates the SCL response to stimulation and must be added to the already long list of factors which affect the assessment of arousal. Failure to recognize this may account for some of the difficulties encountered in the use of SCL for this purpose in the past. Personality Traits These experiments address basic issues in the field of personality. Mischel (1971) questioned the extent to which personality factors are attributes of subjects as opposed to being conceptual dimensions commonly shared by subjects and observers in our culture. The findings of physiological correlates of I/E, combined with other validity data for this factor, demonstrates that at least a significant portion of variance on this factor reflects an attribute of the subject. In a broader context, the finding of a Situations x Personality interaction, in which introverts show a higher SCL at low levels of stimulation and the reverse at high levels of stimulation, supports the interactionist view of personality (Bowers, 1973; Ekehammar, 1974). Finally, a cautionary note should be sounded. Sex differences may prove to be important in this area, in which case our discussion of experiments utilizing male Ss (Experiments 1 and 2) and female Ss (Experiments 3 and 4) as if they were from the same population may not be warranted. Similarly, the effects of stimulus intensity have been discussed as if this were a within Ss manipulation when in fact it was a between Ss variable. While from a theoretical point of view a within Ss intensity effect is possible and desirable, practical problems may be encountered, particularly with sequence effects if both intensities are to be administered in the same session. For example, if the 83 dB tones were presented on trials l-10 with the 103 dB tones on trials 1l-20, the ten

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