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Interaction effects of extraversion and neuroticism on detection thresholds
Biological Psychology 9 (1979) 41-47 © North-Holland Publishing Company
41
INTERACTION EFFECTS OF EXTRAVERSION AND NEUROTICISM ON DETECTION THRESHOLDS * Gunnar EDMAN, Daisy SCHALLING and Anita RISSLER Department o f Psychiatry, the Karolinska Institute, and the Department o f Psychology, the University o f Stockholm, Sweden Accepted for publication 19 July 1979 Detection thresholds for electrocutaneous stimulation were measured in young male subjects by two different methods - a method of limits and a forced-choice method - the latter assumed to be less influenced by decision processes. Relations between the threshold measures and scores in the EPI extraversion (E) and neuroticism (N) scales were studied. High N-low E ('unstable introvert') subjects had the lowest thresholds. The result is in line with the predictions made on the basis of Eysenck's personality theory.
1. Introduction A relation between detection thresholds and personality may be assumed, as there is empirical evidence for a centrifugal regulation of sensory input (e.g. Brown and Short, 1974). However individual differences in thresholds have not been extensively explored except for some studies mainly within the framework o f the Pavlovian theory o f 'strength of the nervous system' and the Eysenckian personality theory. In the Soviet studies subjects with a 'weak nervous system' have been found to display a high sensitivity, i.e. low thresholds (cf. Gray, 1964; Strelau, 1975). 'Weakness of the nervous system' has been proposed to correspond to a proneness towards high cortical arousal (Gray, 1964). Thus, there should be a positive relation between cortical arousal and sensitivity. This is supported in experimental studies by Edelberg (1961) and Fuster and Clyeda (1962) reporting that a moderate increase in level of cortical arousal enhanced sensitivity of very weak stimuli. Eysenck (1967) also assumes a positive relation between cortical arousal and sensitivity. Reticular activation of the cortex facilitates potentials evoked by stimulation o f the sensory systems. Cortical arousal can be produced by 'arousal messages' (p. 231) from the reticular formation and from the visceral brain, i.e. the hypothalamus, hippocampus, amygdala, cingulum and septum. Both systems have thresholds which according to Eysenck are related to extraversion (E) and neuroticism (N). Introverts (low E) are assumed to have lower thresholds in the ascending reticular * Address for requests for reprints to Gunnar Edman, Ph.D., Department of Psychiatry, The Karolinska Hospital, S-10401 Stockholm, Sweden.
42
G. Edman et al. / Thresholds and personality
activating systems (ARAS) than extraverts, and unstable (high N) individuals to have lower thresholds in the visceral brain than stable subjects. The arousal state of the cortex may be seen as the net effect of the activity of the two systems. There are only a few empirical studies relating Eysenck's personality dimensions to threshold performance. Smith (1968) found that introvert subjects had lower auditory thresholds. Siddle, Morris, White and Mangan (1969) obtained a significant negative correlation between a measure of visual sensitivity and extraversion. High thresholds for electrocutaneous stimulation in psychopaths have been found by Schoenherr (1964) and Hare (1968). Differences between psychopaths and nonpsychopaths in the same direction although non-significant were obtained in a later study by Hare and Thorvaldson (1970). These studies may be relevant as results indicating low cortical arousal in psychopaths have been reported (Hare, 1978; SchaUing, 1978). Both stimulus modality and method for measuring the thresholds are crucial factors in differential studies of sensory thresholds. Electrocutaneous stimulation offers special advantages when individual differences in the CNS processing of sensory input is the focus of interest. In studies of detection thresholds using other sources of stimulation (visual, auditory, mechanical) the stimuli will act upon specialized receptor organs and the nerve impulse is initiated by a complex process, depending on receptor density and other peripheral factors which may differ between individuals. However, there is empirical evidence that in electrocutaneous stimulation, the stimulus acts also directly on the underlying peripheral nerves, initiating action potentials, thus bypassing the receptors responding to other modes of stimulation (Rollman, 1974). In the Soviet studies and in other earlier threshold experiments traditional psychophysical methods were applied such as the method of limits, in which the subject's sensory sensitivity and his response criterion, e.g. the cautiousness or the confidence level required in reporting a sensation, jointly determine performance level (Swets, 1964). Thus, individual differences in these thresholds might be due to differences in sensitivity and/or in response criterion. Forced choice procedures for detection threshold measurements (Blackwell, 1953; Green and Swets, 1966) are supposed to give a measure of the sensory threshold, which is less influenced by response criteria or decision processes. Compared to signal detection theory (SDT) analysis (Swets, 1964) it has the advantage of requiring fewer stimulus presentations and avoiding the assumptions that pure sensitivity and response bias can be measured independently (cf. Rollman, 1977). In the forced choice procedure the task is to select that interval among several which contains a stimulus. It has been found that the detection rate of apparently subliminal stimuli is significantly higher than that expected by chance. In the present study detection threshold measurements were made on young male subjects using two different methods - a method of limits and a method of forced choice. On the basis of Eysenck's theory it was hypothesized that the lowest thresholds should be found in unstable introvert subjects, i.e. subjects low in extraversion and high (within a normal range) in neuroticism.
G. Edman et al. / Thresholds and personality
43
2. Method 2.1. Subjects 49 male conscripts participated on a voluntary basis and were paid. Mean age was 21.2 years, range 19.3 to 26.8 years. 2.2. Personality variables The Eysenck Personality Inventory (Eysenck and Eysenck, 1964), in a Swedish translation (Bederoff-Peterson, J/igtoft and Astr6m, 1968) was administered some weeks before the experiment. This inventory contains the scales Extraversion (E), Neuroticism (N) and Lie. 2.3. ThreshoM measurements Detection thresholds for electrocutaneous stimulation were measured with an apparatus described by Schalling (1971). Through a thyristor device, mains supply of 220 V, 50 Hz, the current was transformed to a series of pulses with interpulse intervals of 20 msec, and pulse duration of 2 msec. The current was fed via a variable transformer into two 30 cm 2 stainless steel electrodes in two cups filled with 0.9% NaC1 solution, which was kept at approximately 30°C. Two fingers of the non-dominant hand were immersed into the solution up to the second joint and held in a fixed position by a suitable immobilization in order to secure a constant contact surface. A hydration period of 5 min preceded the threshold measurements. The detection thresholds were measured by two different methods: a method of limits (DTL), and a method of forced choice (DTFC). In the DTL-method the electrical current was continuously increased until the subject reported that a faint vibration was felt. Only ascending series with randomized starting points were given in order to avoid possible after-effects, which may occur in descending series. In the DTFC-method a four-interval temporal forced-choice procedure was applied. The procedure was similar to that used by Hare (1968). Through taperecorded instructions the subjects were told that repeated sequences of the numbers 1, 2, 3 and 4 would be heard, and after one of the numbers a faint vibration would be given. After each sequence, the task was to tell the number that preceded the stimulus. The subject was urged to guess, when he was uncertain. Five stimuli above the detection threshold (obtained by the DTL-method) were first given as previews in order to familiarize the subject with the qualities of the stimulus - such as duration and frequency of the current (cf. Green and Swets, 1966). A screening block followed, consisting of five different stimulus intensities presented five times successively. Stimulus intensities were chosen around the median threshold ob-
44
G. Edman et al. / Thresholds and personality
tained by the DTL-method. In this way, stimulus intensity was adapted to each subject, which is necessary in view of the large inter-individual variation of detection thresholds. The performance in this screening block seved as a basis for the appropriate stimulus intensities to be given during the main block. This block consisted o f five different stimulus intensities, each intensity presented ten times successively. Thus, 50 stimuli were given during the main block. The stimuli were programmed on magnetic tape. On one track the numbers 1 to 4 were read. On the other track a signal, randomized among the numbers, activated the electrical stimulator. The stimulus intensities were adjusted manually. The internumber interval was 1.5 sec, the inter-trial interval 5 sec - long enough to allow for a response - and the interval between the series of 10 stimuli 20 sec. The threshold estimate (DTFC) was expressed as the intensity required for a subject to get 62.5% of his choices correct during the main block, which is equivalent to employing a correction for chance success (Swets, 1964). 2.4. Design
10 measurements by the DTL-method were made, followed by the DTFC-method. Finally, another 10 measurements by the DTL-method were made in order to study possible threshold changes over time. The duration of the threshold measurements was approximately 30 min. 2.5. Statistical analysis
The relations between the two personality variables - extraversion and neuroticism - and the threshold measures were studied by analyses of variance. The relations between each personality variable and the thresholds were obtained as main effects, and their combined effects on the thresholds as interaction effects. The correlation between E and N was - 0 . 1 5 (non-significant), which allowed for a division of the sample into four subgroups of equal size with different combinations of scores in E and N. Nine subjects with scores close to the median were excluded from the analyses of variance leaving 10 subjects in each subgroup. The mean E and N scores * of the subgroups were: group E - N - : 10.3 (3), 1.9 (2); group E - N + : 9.1 (3) and 9.3 (6); group E + N - : 17.1 (7) and 2.0 (2); and group E+N+: 16.5 (7) and 7.8 (5). It can be seen that the N+ (unstable) groups are only moderately high in N.
3. Results A three-way analysis of variance for repeated measures, based on the mean of the two DTLs and the DTFC, was performed. In this analysis the effects of the per* Stanine scores, based on a Swedish standardization group, are given within brackets.
G. Edman et al. / Thresholds and personality
45
sonality variables - extraversion and neuroticism - and method (method o f limits or forced-choice method) and their interactions could be studied. No significant main effects o f either extraversion, F (1,36) = 1.90, n.s. or neuroticism, F (1,36) = 0.42, n.s., was found. The interaction between the two personality variables, however, was significant on the 10% level, F ( 1 , 3 6 ) = 3.10. The N + E - ('unstable introvert') group had the lowest and the N+E+ group the highest thresholds (fig. 1). The difference between the N + E - group and the mean of the other groups were tested post-hoc (cf. Hays, 1963, p. 483). The two separate t-tests were significant; mean DTL: t(38) = 2.11, p < 0.05; DTFC: t(38) = 2.40, p < 0.05. There was no significant triple interaction effect between extraversion, neuroticism, and method, F ( 1 , 3 6 ) = 1.38, n.s. Further, the main effect o f method was highly significant, F (1,36) = 46.35, p < 0.001. Thus, the thresholds obtained by forced-choice method were significantly lower than the thresholds obtained by the method o f limits. The correlation between the mean DTL and the DTFC measures was 0.86. There was also a significant interaction between method and neuroticism, F (1,36) = 5.68, p < 0.05. Subjects with moderately high scores in neuroticism had a greater difference between the mean DTL and the DTFC than subjects with low scores. Finally, the effect of time on the DTL was tested in a three-way analysis o f variance. There was a significant increase in DTL from the first to the second measurement, F (1,36) = 5.64, p < 0.05. There was also a significant interaction between neuroticism and time, F (1,36) = 5.34, p < 0.05. Thus, the increase in DTL was greater in subjects with low scores than in subjects with moderately high scores in neuroticism.
Detection threshold in mA 0.80
-I-+
0.72 0.64
[ ] DTL [] DTFC
, II I',lil
0.56 0.48 0.40
N-E+
N-E-
N+E+
N+E -
Fig. 1. Detection thresholds for electrocutaneous stimulation obtained with a method of limits (DTL) and a forced-choice method (DTFC) in four groups with different combinations of scores in extraversion (E) and neuroticism (N).
46
G. Edman et at / Thresholds and personality
4. Discussion
As predicted the lowest detection thresholds for electrocutaneous stimulation were found in the high neuroticism-low extraversion subgroup. Irrespective of method for measuring the thresholds, the unstable-introvert subjects had the highest sensitivity. No significant differences were obtained between extravert and introvert subjects, which is inconsistent with the results by Smith (1968) and Siddle et al. (1969) in other modalities. Neither were there any differences between subjects low and those moderately high in neuroticism. One possible interpretation of the higher sensitivity in the unstable-introvert subjects is that these individuals have low thresholds in both the physiological systems - the ARAS and the visceral brain - assumed by Eysenck to be the biological basis for the personality dimensions. 'Arousal messages' from both systems may facilitate the detection of weak stimulation by raising the cortical arousal. However, it should be noted that according to Eysenck, an interaction between the two systems in producing cortical arousal takes place only in extremely stressful situations (Eysenck, 1967, p. 233), while our interpretation is based on the assumption of such an interaction also in non-stressful conditions, perhaps even more pronounced in monotonous, weak stimulation situations like the one prevailing in the present study. The lack of a significant triple interaction between extraversion, neuroticism, and method indicates that the interaction between the two personality dimensions was independent of method. Thus, when a forced-choice procedure was applied, which is assumed to reduce the influence of response or decision processes, the interaction did not disappear. This result is supported by the fact that the correlation between the two threshold estimates was very high, indicating that they had a common variance of approximately 75%. A greater decline in performance (i.e. increase in DTL from the first to the second measurement) was found in subjects low in neuroticism than those moderately high. The inhibition presumably built up in the cortex during the monotonous task may have been counteracted by 'arousal messages' from the visceral brain. This compensatory loop may be absent or insufficient in low neuroticism subjects. Subjects moderately high in neuroticism tended to show a greater difference between the mean DTL and the DTFC. It might be assumed that this difference reflects the influence of response or decision processes on the DTL, e.g. more cautiousness in high neuroticism subjects. The present results are in line with an accumulating evidence suggesting a biological basis for important personality dimensions. With increased knowledge of the biochemical and electrocortical processes underlying sensory processing and attention the implications of individual differences in detection thresholds will become more clear.
G. Edman et al. /Thresholdsand personality
47
Acknowledgements The study has been financially supported by grants f r o m the Swedish Medical Research Council (21X-4545).
References Bederoff-Peterson, A., J~igtoft, K. and Astr6m, J. (1968). Eysencks Personality Inventory (EPI). Skandinaviska Testf6rlaget: Stockholm. Blackwell, H.R. (1953), Psychophysical Threshold: Experimental Studies of Methods of Measurement. University of Michigan Press: Ann Arbor. Brown, A.G. and Short, A.D. (1974). Effects from the somatic sensory cortex on transmission through the spino-cervical tract. Brain Research, 74,338-341. Edelberg, R. (1961). The relationship between the galvanic skin response, vasoconstriction, and tactile sensitivity. Journal of Experimental Psychology, 62, 187-195. Eysenck, H.J. (1967). The Biological Basis of Personality. Charles Thomas: Springfield, 111. Eysenck, H.J. and Eysenck, S.B.G. (1964). Manual of the Eysenck Personality Inventory. University of London Press: London. Fuster, J.M. and Clyeda, A. (1962). Facilitation of tachistoscopic performance by stimulation of midbrain segmental points in the monkey. Experimental Neurology, 6,384-406. Gray, J.A. (1964). Pavlov's Typology. Pergamon Press: New York. Green, D.M. and Swets, J.A. (1966). Signal Detection Theory and Psychophysics. Wiley: New York. Hare, R.D. (1968). Detection thresholds for electrical shock in psychopaths. Journal of Abnormal Psychology, 73,268-272. Hare, R.D. (1978). Electrodermal and cardiovascular correlates of psychopathy. In: Hare, R.D. and Schalling, D. (Eds.) Psychopathic Behaviour: Approaches to Research. Wiley: New York, 107-144. Hare, R.D. and Thorvaldson, S.A. (1970). Psychopathy and response to electrical stimulation. Journal of Abnormal Psychology, 76,370-374. Hays, W.L. (1963). Statistics. Holt, Rinehart and Winston: London. Rollman, G.B. (1974). Electrocutaneous stimulation. In: Geldard, F.A. (Ed.) Conference on Vibrotactile Communication. The Psychonomic Society: Austin, Texas. Rollman, G.B. (1977). Signal detection theory measurement of pain: A review and critique. Pain, 3,187-211. Schalling, D. (1971). Tolerance for experimentally induced pain as related to personality. Scandinavian Journal of Psychology, 12, 271-281. Sehaling, D. (1978). Psychopathy-related personality variables and the psychophysiology of socialization. In: Hare, R.D. and SchaUing, D. (Eds.) Psychopathic Behaviour: Approaches to Research. Wiley: New York, 85-106. Schoenherr, J.C. (1964). Avoidance of noxious stimulation in psychopathic personality. Doctoral Dissertation, University of California, Los Angeles. University Microfilms: Ann Arbor, Mich. No. 64-8334. Siddle, D.A., Morris, R.B., White, K.D. and Mangan, G.L. (1969). Relation of visual sensitivity to extraversion. Journal of Experimental Research in Personality, 3, 264-267. Smith, S.L. (1968). Extraversion and sensory threshold. Psychophysiology, 5,293-299. Strelau, J. (1975). Pavlov's typology and current investigations in this area. Nederlands Tijdschrfft voor de Psychologic, 30, 177-200. Swets, J.A. (1964). Indices of signal detectability obtained with various psychophysical procedures. In: Swets, J.A. (Ed.) Signal Detection and Recognition by Human Observers. Wiley: New York.
41
INTERACTION EFFECTS OF EXTRAVERSION AND NEUROTICISM ON DETECTION THRESHOLDS * Gunnar EDMAN, Daisy SCHALLING and Anita RISSLER Department o f Psychiatry, the Karolinska Institute, and the Department o f Psychology, the University o f Stockholm, Sweden Accepted for publication 19 July 1979 Detection thresholds for electrocutaneous stimulation were measured in young male subjects by two different methods - a method of limits and a forced-choice method - the latter assumed to be less influenced by decision processes. Relations between the threshold measures and scores in the EPI extraversion (E) and neuroticism (N) scales were studied. High N-low E ('unstable introvert') subjects had the lowest thresholds. The result is in line with the predictions made on the basis of Eysenck's personality theory.
1. Introduction A relation between detection thresholds and personality may be assumed, as there is empirical evidence for a centrifugal regulation of sensory input (e.g. Brown and Short, 1974). However individual differences in thresholds have not been extensively explored except for some studies mainly within the framework o f the Pavlovian theory o f 'strength of the nervous system' and the Eysenckian personality theory. In the Soviet studies subjects with a 'weak nervous system' have been found to display a high sensitivity, i.e. low thresholds (cf. Gray, 1964; Strelau, 1975). 'Weakness of the nervous system' has been proposed to correspond to a proneness towards high cortical arousal (Gray, 1964). Thus, there should be a positive relation between cortical arousal and sensitivity. This is supported in experimental studies by Edelberg (1961) and Fuster and Clyeda (1962) reporting that a moderate increase in level of cortical arousal enhanced sensitivity of very weak stimuli. Eysenck (1967) also assumes a positive relation between cortical arousal and sensitivity. Reticular activation of the cortex facilitates potentials evoked by stimulation o f the sensory systems. Cortical arousal can be produced by 'arousal messages' (p. 231) from the reticular formation and from the visceral brain, i.e. the hypothalamus, hippocampus, amygdala, cingulum and septum. Both systems have thresholds which according to Eysenck are related to extraversion (E) and neuroticism (N). Introverts (low E) are assumed to have lower thresholds in the ascending reticular * Address for requests for reprints to Gunnar Edman, Ph.D., Department of Psychiatry, The Karolinska Hospital, S-10401 Stockholm, Sweden.
42
G. Edman et al. / Thresholds and personality
activating systems (ARAS) than extraverts, and unstable (high N) individuals to have lower thresholds in the visceral brain than stable subjects. The arousal state of the cortex may be seen as the net effect of the activity of the two systems. There are only a few empirical studies relating Eysenck's personality dimensions to threshold performance. Smith (1968) found that introvert subjects had lower auditory thresholds. Siddle, Morris, White and Mangan (1969) obtained a significant negative correlation between a measure of visual sensitivity and extraversion. High thresholds for electrocutaneous stimulation in psychopaths have been found by Schoenherr (1964) and Hare (1968). Differences between psychopaths and nonpsychopaths in the same direction although non-significant were obtained in a later study by Hare and Thorvaldson (1970). These studies may be relevant as results indicating low cortical arousal in psychopaths have been reported (Hare, 1978; SchaUing, 1978). Both stimulus modality and method for measuring the thresholds are crucial factors in differential studies of sensory thresholds. Electrocutaneous stimulation offers special advantages when individual differences in the CNS processing of sensory input is the focus of interest. In studies of detection thresholds using other sources of stimulation (visual, auditory, mechanical) the stimuli will act upon specialized receptor organs and the nerve impulse is initiated by a complex process, depending on receptor density and other peripheral factors which may differ between individuals. However, there is empirical evidence that in electrocutaneous stimulation, the stimulus acts also directly on the underlying peripheral nerves, initiating action potentials, thus bypassing the receptors responding to other modes of stimulation (Rollman, 1974). In the Soviet studies and in other earlier threshold experiments traditional psychophysical methods were applied such as the method of limits, in which the subject's sensory sensitivity and his response criterion, e.g. the cautiousness or the confidence level required in reporting a sensation, jointly determine performance level (Swets, 1964). Thus, individual differences in these thresholds might be due to differences in sensitivity and/or in response criterion. Forced choice procedures for detection threshold measurements (Blackwell, 1953; Green and Swets, 1966) are supposed to give a measure of the sensory threshold, which is less influenced by response criteria or decision processes. Compared to signal detection theory (SDT) analysis (Swets, 1964) it has the advantage of requiring fewer stimulus presentations and avoiding the assumptions that pure sensitivity and response bias can be measured independently (cf. Rollman, 1977). In the forced choice procedure the task is to select that interval among several which contains a stimulus. It has been found that the detection rate of apparently subliminal stimuli is significantly higher than that expected by chance. In the present study detection threshold measurements were made on young male subjects using two different methods - a method of limits and a method of forced choice. On the basis of Eysenck's theory it was hypothesized that the lowest thresholds should be found in unstable introvert subjects, i.e. subjects low in extraversion and high (within a normal range) in neuroticism.
G. Edman et al. / Thresholds and personality
43
2. Method 2.1. Subjects 49 male conscripts participated on a voluntary basis and were paid. Mean age was 21.2 years, range 19.3 to 26.8 years. 2.2. Personality variables The Eysenck Personality Inventory (Eysenck and Eysenck, 1964), in a Swedish translation (Bederoff-Peterson, J/igtoft and Astr6m, 1968) was administered some weeks before the experiment. This inventory contains the scales Extraversion (E), Neuroticism (N) and Lie. 2.3. ThreshoM measurements Detection thresholds for electrocutaneous stimulation were measured with an apparatus described by Schalling (1971). Through a thyristor device, mains supply of 220 V, 50 Hz, the current was transformed to a series of pulses with interpulse intervals of 20 msec, and pulse duration of 2 msec. The current was fed via a variable transformer into two 30 cm 2 stainless steel electrodes in two cups filled with 0.9% NaC1 solution, which was kept at approximately 30°C. Two fingers of the non-dominant hand were immersed into the solution up to the second joint and held in a fixed position by a suitable immobilization in order to secure a constant contact surface. A hydration period of 5 min preceded the threshold measurements. The detection thresholds were measured by two different methods: a method of limits (DTL), and a method of forced choice (DTFC). In the DTL-method the electrical current was continuously increased until the subject reported that a faint vibration was felt. Only ascending series with randomized starting points were given in order to avoid possible after-effects, which may occur in descending series. In the DTFC-method a four-interval temporal forced-choice procedure was applied. The procedure was similar to that used by Hare (1968). Through taperecorded instructions the subjects were told that repeated sequences of the numbers 1, 2, 3 and 4 would be heard, and after one of the numbers a faint vibration would be given. After each sequence, the task was to tell the number that preceded the stimulus. The subject was urged to guess, when he was uncertain. Five stimuli above the detection threshold (obtained by the DTL-method) were first given as previews in order to familiarize the subject with the qualities of the stimulus - such as duration and frequency of the current (cf. Green and Swets, 1966). A screening block followed, consisting of five different stimulus intensities presented five times successively. Stimulus intensities were chosen around the median threshold ob-
44
G. Edman et al. / Thresholds and personality
tained by the DTL-method. In this way, stimulus intensity was adapted to each subject, which is necessary in view of the large inter-individual variation of detection thresholds. The performance in this screening block seved as a basis for the appropriate stimulus intensities to be given during the main block. This block consisted o f five different stimulus intensities, each intensity presented ten times successively. Thus, 50 stimuli were given during the main block. The stimuli were programmed on magnetic tape. On one track the numbers 1 to 4 were read. On the other track a signal, randomized among the numbers, activated the electrical stimulator. The stimulus intensities were adjusted manually. The internumber interval was 1.5 sec, the inter-trial interval 5 sec - long enough to allow for a response - and the interval between the series of 10 stimuli 20 sec. The threshold estimate (DTFC) was expressed as the intensity required for a subject to get 62.5% of his choices correct during the main block, which is equivalent to employing a correction for chance success (Swets, 1964). 2.4. Design
10 measurements by the DTL-method were made, followed by the DTFC-method. Finally, another 10 measurements by the DTL-method were made in order to study possible threshold changes over time. The duration of the threshold measurements was approximately 30 min. 2.5. Statistical analysis
The relations between the two personality variables - extraversion and neuroticism - and the threshold measures were studied by analyses of variance. The relations between each personality variable and the thresholds were obtained as main effects, and their combined effects on the thresholds as interaction effects. The correlation between E and N was - 0 . 1 5 (non-significant), which allowed for a division of the sample into four subgroups of equal size with different combinations of scores in E and N. Nine subjects with scores close to the median were excluded from the analyses of variance leaving 10 subjects in each subgroup. The mean E and N scores * of the subgroups were: group E - N - : 10.3 (3), 1.9 (2); group E - N + : 9.1 (3) and 9.3 (6); group E + N - : 17.1 (7) and 2.0 (2); and group E+N+: 16.5 (7) and 7.8 (5). It can be seen that the N+ (unstable) groups are only moderately high in N.
3. Results A three-way analysis of variance for repeated measures, based on the mean of the two DTLs and the DTFC, was performed. In this analysis the effects of the per* Stanine scores, based on a Swedish standardization group, are given within brackets.
G. Edman et al. / Thresholds and personality
45
sonality variables - extraversion and neuroticism - and method (method o f limits or forced-choice method) and their interactions could be studied. No significant main effects o f either extraversion, F (1,36) = 1.90, n.s. or neuroticism, F (1,36) = 0.42, n.s., was found. The interaction between the two personality variables, however, was significant on the 10% level, F ( 1 , 3 6 ) = 3.10. The N + E - ('unstable introvert') group had the lowest and the N+E+ group the highest thresholds (fig. 1). The difference between the N + E - group and the mean of the other groups were tested post-hoc (cf. Hays, 1963, p. 483). The two separate t-tests were significant; mean DTL: t(38) = 2.11, p < 0.05; DTFC: t(38) = 2.40, p < 0.05. There was no significant triple interaction effect between extraversion, neuroticism, and method, F ( 1 , 3 6 ) = 1.38, n.s. Further, the main effect o f method was highly significant, F (1,36) = 46.35, p < 0.001. Thus, the thresholds obtained by forced-choice method were significantly lower than the thresholds obtained by the method o f limits. The correlation between the mean DTL and the DTFC measures was 0.86. There was also a significant interaction between method and neuroticism, F (1,36) = 5.68, p < 0.05. Subjects with moderately high scores in neuroticism had a greater difference between the mean DTL and the DTFC than subjects with low scores. Finally, the effect of time on the DTL was tested in a three-way analysis o f variance. There was a significant increase in DTL from the first to the second measurement, F (1,36) = 5.64, p < 0.05. There was also a significant interaction between neuroticism and time, F (1,36) = 5.34, p < 0.05. Thus, the increase in DTL was greater in subjects with low scores than in subjects with moderately high scores in neuroticism.
Detection threshold in mA 0.80
-I-+
0.72 0.64
[ ] DTL [] DTFC
, II I',lil
0.56 0.48 0.40
N-E+
N-E-
N+E+
N+E -
Fig. 1. Detection thresholds for electrocutaneous stimulation obtained with a method of limits (DTL) and a forced-choice method (DTFC) in four groups with different combinations of scores in extraversion (E) and neuroticism (N).
46
G. Edman et at / Thresholds and personality
4. Discussion
As predicted the lowest detection thresholds for electrocutaneous stimulation were found in the high neuroticism-low extraversion subgroup. Irrespective of method for measuring the thresholds, the unstable-introvert subjects had the highest sensitivity. No significant differences were obtained between extravert and introvert subjects, which is inconsistent with the results by Smith (1968) and Siddle et al. (1969) in other modalities. Neither were there any differences between subjects low and those moderately high in neuroticism. One possible interpretation of the higher sensitivity in the unstable-introvert subjects is that these individuals have low thresholds in both the physiological systems - the ARAS and the visceral brain - assumed by Eysenck to be the biological basis for the personality dimensions. 'Arousal messages' from both systems may facilitate the detection of weak stimulation by raising the cortical arousal. However, it should be noted that according to Eysenck, an interaction between the two systems in producing cortical arousal takes place only in extremely stressful situations (Eysenck, 1967, p. 233), while our interpretation is based on the assumption of such an interaction also in non-stressful conditions, perhaps even more pronounced in monotonous, weak stimulation situations like the one prevailing in the present study. The lack of a significant triple interaction between extraversion, neuroticism, and method indicates that the interaction between the two personality dimensions was independent of method. Thus, when a forced-choice procedure was applied, which is assumed to reduce the influence of response or decision processes, the interaction did not disappear. This result is supported by the fact that the correlation between the two threshold estimates was very high, indicating that they had a common variance of approximately 75%. A greater decline in performance (i.e. increase in DTL from the first to the second measurement) was found in subjects low in neuroticism than those moderately high. The inhibition presumably built up in the cortex during the monotonous task may have been counteracted by 'arousal messages' from the visceral brain. This compensatory loop may be absent or insufficient in low neuroticism subjects. Subjects moderately high in neuroticism tended to show a greater difference between the mean DTL and the DTFC. It might be assumed that this difference reflects the influence of response or decision processes on the DTL, e.g. more cautiousness in high neuroticism subjects. The present results are in line with an accumulating evidence suggesting a biological basis for important personality dimensions. With increased knowledge of the biochemical and electrocortical processes underlying sensory processing and attention the implications of individual differences in detection thresholds will become more clear.
G. Edman et al. /Thresholdsand personality
47
Acknowledgements The study has been financially supported by grants f r o m the Swedish Medical Research Council (21X-4545).
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