Cortical Evoked Potentials and Extraversion J. FRIEDMAN, PHD, AND R. MEARES, MD Stimulus intensity modulation was studied in extraverts and introverts. Auditory evoked potentials were recorded in 29 subjects and visual evoked potentials in 55. In both sensory modalities, extraverts showed considerably larger amplitudes of the late components of the evoked potentials, suggesting that they were more "open" to stimuli than introverts. In this way, extraverts' responses to simple stimuli could be seen as similar to their interchange with the complex stimuli of the social world.

INTRODUCTION

Personality might be defined as the individual's more or less enduring manner of interrelating with the total sensory environment, of which the social environment is the most important part. This definition allows the problem of personality to be approached through an examination of sensory input regulation. This may be done by electrophysiological means. A stimulus evokes a potential change in the cerebral cortex which is necessarily extracted by averaging techniques. The resultant wave form has several components, of which three are generally studied. The early components are believed to be related to the nervous impulse traversing sensory tracts. The late components seem to be associated with the complex, or "higher," psychological operations which are conceivably related to personality (1-3). The amplitude of the cortical evoked potential varies with the intensity of the stimulus. For some indiFrom the Department of Psychiatry, University of Melbourne, Austin Hospital, Australia. Address requests for reprints to: Dr. Russell Meares, Department of Psychiatry, University of Melbourne, Austin Hospital, Heidelberg, Victoria 3084; Australia. Received for publication June 26, 1978; final revision received March 6,1979.

viduals, the evoked potential grows in amplitude as intensity rises, whereas in others the rise in stimulus intensity is associated with a fall in amplitude of the evoked wave form. The nervous systems of some individuals, therefore, respond as if to amplify or "augment" incoming stimulation, whereas others show responses which suggest damping down or "reduction" of stimulus intensity. The central nervous system can thus be considered to have the capacity of "stimulusintensity modulation," and the evoked potential amplitude-intensity function may serve as an index of the degree of "stimulus-intensity control" (4). This article is concerned with amplitude and "slope" differences, especially in these late components, between extraverts and introverts. METHODS

Subjects The study was carried out in two parts. In an initial pilot study, potentials evoked by light stimuli were recorded in 26 normal male volunteers. In order to generalize and replicate preliminary findings, a second sample of 29 male subjects was obtained and evoked potentials (EPs) in both the visual and auditory modalities were recorded. The overall group of 55 subjects, therefore, provided data on visual evoked potentials, whereas only 29 sub-

Psychosomatic Medicine Vol. 4 1 , No. 4 (June 1979) Copyright c 1979 by the American Psycho Published by Elsevier North Holland, Inc.

279 0033-3174/79/04027908$01.75

J. FRIEDMAN AND R. MEARES jects contributed towards auditory evoked potential findings. All subjects fell in the age range of 18-30 years. Subjects were divided into extraverts or introverts on the basis of their scores on the Eysenck Personality Inventory. Extraverts were defined as those subjects whose extraversion scores fell above the mean of the group. Table 1 shows the mean extraversion scores for the group of subjects as a whole and for the subgroup from whom only auditory EPs were recorded. These values are in close agreement with normative values for students provided by other workers (5, 6). Neuroticism, a possible confounding variable, was similar for both groups as can be seen from mean scores which can also be found in Table 1. In each of the groups, the mean extraversion score was close to the median. Dividing subjects around the mean extraversion score provided 30 high extraverts and 25 low extraverts in the total group, and 15 high extraverts and 14 low extraverts in the subgroup for auditory recording. In order to establish that our methods were reliable, and that an individual's manner of "stimulus intensity control" (7) was relatively stable, all subjects were retested 1 to 2 weeks later under identical conditions.

Procedure To record the EPs, the subject was instructed to sit in a comfortable chair in a darkened room with his eyes closed. An electroencephalogram (EEG) was monitored between the vertex and left mastoid, with the right ear as ground, using an offner type T Beckman EEG. The EEG amplifier characteristics were set with a time constant of 0.3 sec and a high frequency filter at 35 Hz, consistent with the recommendations made by Shagass (8). Each subject was then presented with a total of 250 stimuli in the form of light or sound, at 5 different intensities. The amplified EEG response to each stimulus was averaged separately for each intensity, using a multipurpose digital computer (PDP/8e). In this way, 5 separate EPs were obtained in any one modality, each representing the separate averaged EEG response to 50 stimuli at a particular intensity. The visual stimuli were generated by a Grass PS-22 photostimulator and flashes of 10 ^sec duration could be produced at 5 intensity settings, with relative intensity ratios of 1, 2, 4, 8, and 16. The approximate maximum flash intensity was 1,500,000

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TABLE 1. Extraversion and Neuroticism Scores Mean extraversion score (and SD)

Mean neuroticism score (and SD)

Total group (n=55)

12.8(3.9)

9.0(4.4)

Subgroup (n=29)

12.4(4.5)

8.4(4.1)

candlepower. The flash lamp was positioned 60 cm in front of the subject's face; mufflers were used to eliminate the faint click that was produced artifactually by the emission of each flash. The auditory stimuli were provided by a Hewlett-Packard sine-wave oscillator, and presented through both ears of low resistance headphones placed on the subject's head. Each auditory tone had a frequency of 1000 Hz, a rise time of 2.5 msec, and a duration of 30 msec. The sensation levels above threshold used in this study were 10, 25, 40, 55, and 70 dB. The five intensities of light or sound were presented in pseudorandomized "blocks," where each block consisted of five stimuli of the same intensity. The procedure was such that every block was preceded by every other intensity block, including itself, an equal number of times. The interval between stimuli within a block was 1 sec, whereas the interval between blocks was approximately 1.5 sec. The use of pseudorandomized blocks presumably minimized the development of arbitrary sets that could occur during a long series of stimuli of the same intensity, and also reduced the possible effect on the amplitude of response of a strong stimulus on a weak one, and vice versa. It was also a way of coping with problems of boredom, habituation, fatigue, and attentional drift that Could occur during the course of a session (9).

Analysis Evoked potential components were identified according to their latency from stimulus onset. For visual EPs the successive peaks and troughs were termed IV, V, VI, and VII according to the nomenclature and latency range described by Hall et al. (10). These components were identified within the following latency ranges: 75-140 msec, 90-165 msec, 125-250 msec, 225-300 msec. For auditory EPs, the successive peaks and troughs were labeled Pi, Ni, P2,

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EPs AND EXTRA VERSION N2 according to the nomenclature proposed by Williams et al. (11). These components were defined by their occurrence within the following latency ranges: 40—70 msec, 75 — 150 msec, 180—250 msec, and 225+ msec. The first three components in both modalities correspond to P100, Ni«, and P200, as described by Landau et al. (12). A typical auditory EP is shown in Figure 1. Amplitude was measured as the vertical distance between adjacent components and plotted against log intensity for graphical purposes. Evoked potential amplitude data was analyzed using two-way analyses of variance programs, so that differences between independent groups (e.g., extraverts and introverts) could be ascertained using the five amplitude measures (13). Latencies were not analyzed. Another method of comparing groups involved the calculation of an EP amplitude-intensity "slope" value for each component. This slope value was calculated by means of least-squares technique, yielding a regression coefficient as an index of the relative rate of EP amplitude increase with increasing stimulus intensity (14). Regression coefficients were compared between introverts and extraverts, yielding a t value. Subjects with positive slopes have been designated as "augmenters," whereas those with zero or negative slopes have been called "reducers" (4).

RESULTS

Reliability The test—retest reliability for amplitude was significantly high at each intensity for the three components of the EP, for both light and sound. Correlation coefficients for amplitudes all exceeded the 5 % level of significance and ranged in value from r = 0.38 to 0.72, with the highest values tending to occur at the highest intensity levels. Slope values derived from the linear regression of amplitudeintensity function were also found to be stable across time for the later two auditory components (r = 0.6, p < 0.01; r = 0.5, p < 0.01), and for all three visual components (r = 0.5, p < 0.01; r = 0.5, p < 0.05; r = 0.6, p < 0.01). These results have been reported elsewhere in more detail (15); they are consistent with those from other laboratories which suggest that EPs remain fairly constant (16—18).

stimulus

amplitude

. latency

Fig. 1. Typical EP: Sensory modality—auditory; frequency—1000 Hz tones; intensity—50 dB above threshold; duration—10 msec; electrode positions—unipolar record for vertex to left mastoid; psychological state—overtly relaxed, unmedicated, and (?) unknown subjective state. Pi, Ni are "early" components and P2, N2 are "late" components.

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Extraversion The results of this section are displayed in Figures 2 and 3. Extra verts were found to be clearly differentiated from introverts by the late components of the EPs in both modalities. Amplitude data from components VI-VII and P2N2 revealed that extraverts had considerably larger EPs than did introverts (p < 0.01 in both cases). The early components of the auditory EPs showed no difference between groups using the analysis of variance statistic. This was also so for the earliest visual EP component. However, the middle visual component V-VI showed significant amplitude difference between groups [p < 0.05) and, referring to Figure 3, it is apparent that the greatest difference occurs at the highest intensity. The various EP components were also compared for differences in amplitudeintensity slopes between groups. Only COMPONENT

one significant slope difference between groups emerged, that was for the auditory component P2N2. The slope of this component ranged from 0.05 /xV/log intensity for introverts to 0.12 /xV/log intensity for extraverts (t= 2.2, p < 0.05). This significant difference was confirmed using two-way analysis of variance (groups by stimulus intensity) with linear trend analysis for the repeated intensity dimension (F = 7.42, p < 0.025, 1, 27 df).

DISCUSSION

This study shows that extraverts have considerably larger amplitudes of the late components of the visual and auditory cortical EPs compared to introverts. In addition, auditory EP findings indicate that extraverts show augmenting tendencies and introverts reducing tendencies.

P.N.

z 10

t • INTENSITY

OF

5 TONE-

Fig. 2. Extraversion and the evoked potential. Means and standard errors for the three components of the auditory EP for a group of 29 subjects divided into high (n = 15) and low extraverts (n = 14). The high extraversion (E) group showed significantly larger auditory EP amplitudes (F ratio, p < 0.01) and slope (t = 2.2, p < 0.025) for component P2N2.

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IV-V

HIGH E - - • LOW E

1

2

3

4

5 •*

1 2 3 4 S INTENSITY OF FLASH

1

2

3

4

$



Fig. 3. Extroversion and the visual EP. Means and standard errors for the three components of the visual EP for a group of 55 subjects divided into high (n = 30) and low extraverts (n = 25). The high extraversion (E) group showed significantly larger visual EP amplitudes for components V-VI (F ratio, p < 0.05) and VI-VII (F ratio, p < 0.01).

Previous investigations of extraversion involving the raw EEG, however, have been disappointing. They have either failed to find differences between personality groups (19-21), or else the evidence has been conflicting (22-26). By contrast, studies concerned specifically with cortical EPs seem broadly consistent with our data. Zuckerman et al. (27) found that subjects scoring high on a "sensation seeking scale" had larger EPs at high intensities than those who scored low. It has been suggested that sensation seeking is related to extraversion (28). Also, Soskis and Shagass (29) found a trend relating augmentation of the visual EP slope to extraversion. This trend, however, was not marked, perhaps because their study was confined to components of the EP with latencies less than 200 msec, while the present investigation showed that the

major differences were to be found in the later components. It is possible that differences in EPs between extraversion groups emerge only under specific stimulus conditions and that the different components are differentially sensitive to personality factors. For instance, Stelmack et al. (30) present findings which at first sight are contrary to this study. They showed that for low frequency tones introverts had higher Ni—P2 amplitudes than extraverts at 55 and 80 dB intensity. These differences were accentuated in their study by the use of extreme groups of both extraverts and introverts and the establishment of a third category of ambiverts. In our study we also find that for the earlier components there is a tendency for introverts to show larger amplitudes than extraverts at some intensity points. However, our most striking findings were for the later compo-

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nents, for which Stelmack et al. did not present data. Similarly, in an earlier study Stelmack and Campbell (31) found that extraverts tended to be more sensitive to auditory stimuli than introverts in high frequency conditions, whereas in low frequency conditions introverts were more sensitive. Somatosensory EPs cannot often be directly compared with visual and auditory potential data, since many of these studies concern short latency, early components. Nevertheless, Shagass and Schwartz (32) were able to show that young extraverts had larger early components of the somatosensory EP than introverts, but the reverse was found for older subjects. He could not replicate this finding in a later study (33). Animal studies are also consistent with our data. For example, exploratory activity in cats has been related to augmentation (34). CONCLUSIONS

The findings of this study, that stimuli evoked larger potentials in the brain of extraverts, suggest that extraverts are more "open" to stimuli than introverts. This is consistent with the behavior of extraverts, whose gregariousness might be seen as a receptivity, or even "a hunger," for the stimuli of the social world. However, this suggestion seems hard to reconcile with the theories of Eysenck (35). Eysenck has proposed, largely on the evidence of conditioning experiments, that extraversion is associated with high levels of cortical inhibition. If this were so, one might expect small, rather than large, cortical potentials to be evoked in extraverts. The failure to confirm Eysenck's predictions experimentally has been reported by others (20).

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Alternatively, the findings can be explained in a manner not contrary to Eysenck's theories. Zuckerman et al. (27) proposed that individuals differ in the demarcation of the set neural point which limits the amount of high levels of stimulation they accept before their corticoreticular inhibitory threshold is reached. People high on extraversion may have a regulatory threshold set to permit high levels of sensory input, which would be reflected by larger EP amplitudes at high intensities. This suggestion is consistent with the report of Stelmack and Campbell (31) that extraverts are more sensitive than introverts only at high frequency levels of stimulation. It would also take into account Eysenck's own discussion (36) about extraversion and the relationship between hedonic tone and strength of sensory stimulation. Finally, the origin of the differences of brain function apparent in different personalities remains unexplained. It is possible that they may be induced by learning. It is equally possible that they are genetically given, so that rudimentary "styles" of organizing sensory input, which are present at birth, influence subsequent patterns of cognitive processing and the evolution of personality (37). In either case, it can be postulated that the behavioral tendencies of extraverts cannot be seen as distinct from the function of their nervous system. This study was supported by the National Health and Medical Research Council of Australia and the Australian Tobacco Research Foundation. The authors thank Dr. Monte S. Buchsbaum, NIMH, Bethesda, for assisting in the analysis of the data and providing an ANOVA program with trend analysis.

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J. FRIEDMAN AND R. MEARES 27. Zuckerman M, Murtaugh T, Siegel J: Sensation seeking and cortical augmenting-reducing. Psychophysiol 11:535-542,1974 28. Farley F, Farley SV: Extraversion and stimulus seeking raotivation. J Consult Psychol 31:215-216,1967 29. Soskis DA, Shagass C: Evoked potential tests of augmenting-reducing. Psychophysiol 11:175-190, 1974 30. Stelmack RM, Achorn E, Michaud A: Extraversion and individual differences in auditory evoked response. Psychophysiol 14:368-374,1977 31. Stelmack RM, Campbell KB: Extraversion and auditory sensitivity to high and low frequency. Percept Mot Skills 38:875-879,1974 32. Shagass C, Schwartz M: Age, personality and somatosensory cerebral evoked responses. Science 148:1359-1361,1965 33. Haseth K, Shagass C, Staumanis JJ: Perceptual and personality correlates of EEG and evoked response measures. Biol Psychol 1:49-60,1969 34. Silverman J: Stimulus intensity modulation and psychological disease. Psychopharmacologia 24:42-80,1972 35. Eysenck HJ: The Biological Basis of Behaviour. Springfield, 111. Charles C Thomas, 1967 36. Eysenck HJ: Experiments with Drugs. London, Pergamon Press, 1963 37. Silverman J: Research with psychedelics: some biopsychological concepts and possible clinical applications. Arch Gen Psychiatry 25:498-510,1971

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Cortical evoked potentials and extraversion.

Cortical Evoked Potentials and Extraversion J. FRIEDMAN, PHD, AND R. MEARES, MD Stimulus intensity modulation was studied in extraverts and introverts...
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