Introversion-extraversion: the Bell-Magendie law revisited

Evidence from several sources, including psychophysical, electrodermal and evoked potential measures, indicates that introverts display an enhanced response to sensory stimulation. There is also evidence, primarily from psychophysical studies, which suggests that extraverts may be disposed to emit short, strong bursts of motor activity which facilitate performance on tasks that involve gross motor activity but which impede performance on tasks which require refined motor control. The present paper develops an argument in which the differences between introverts and extraverts in the response to sensory stimulation and in the expression of motor activity may be referred to differences at the level of the sensory and motor nerve.     

Memory scanning,introversion-extraversion,and levels of processing

Individual differences in information processing were studied in the form of the hypothesis that arousal, as indexed by a personality measure of introversion-extraversion, affects the speed with which certain kinds of processing are completed. The Sternberg paradigm was used, and the results suggested that introverts and extraverts scanned for physical features equally rapidly, but that introverts were slower than extraverts at scanning for the semantic features of category membership. There was limited support for the hypothesis that introverts, thought to be more aroused than extraverts, are less able to engage in shared or parallel processing. It was concluded that information processing in introverts and extraverts may differ qualitatively as well as quantitatively.     

Extraversion and sleep: A psychophysiological study of the arousal hypothesis

In order to examine Eysenck's (1967) hypothesis concerning the relationship between extraversion and arousal, autonomic measures were taken from introverts and extraverts immediately before sleep and during 6 hr of sleep. Introverts and extraverts did not differ in heart rate (HR) or skin-potential response rate (SPR) during the pre-sleep period. A variety of sleep-related measures of arousal provided no strong support for the hypothesis that introverts are more aroused than extraverts. The only consistent autonomic difference between introverts and extraverts during sleep was a tendency for introverts to have higher SPRs in the second 3 hr of the night. The results are discussed in terms of the level of environmental stimulation present in the experimental conditions, and suggestions are made for future sleep studies of the relationship between extraversion and arousal.     

The influence of introversion/extraversion on the skin conductance response to stress and stimulus intensity

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.     

Simple visual reaction time,personality and strength of the nervous system: A signal-detection theory approach

Eysenck (1967) has reviewed evidence which suggests that introverts have a higher level of arousal than extraverts. Moreover, Gray (1967) has proposed that introverts have weak nervous systems, in the Russian terminology. Studies of simple reaction time such as that of Mangan and Farmer (1967) have posed problems for these views, but it is suggested that this may have been due to criterion differences between introverts and extraverts. In the present study a simple visual reaction-time task and a signal-detection task were conducted side by side. No significant differences between introverts and extraverts were found in measures of strength derived from the former, or measures of criterion derived from the latter. However, high N Ss were found to have a significantly lower value for Nebylitsyn's index of the slope of the reaction time/stimulus intensity function, thus supporting the view that high N Ss have relatively weak nervous systems.     

Attention and personality in augmenting/reducing of visual evoked potentials

Earlier research has shown that extraverts tend to increase their visual evoked potential amplitudes with increasing light intensity (augmenting), while introverts reach their maximum amplitude at lower intensities (reducing). The evoked response has normally been measured from association areas of the brain (at the vertex). The present study measured VEP amplitudes over visual cortex and at the vertex, using four light intensities in two conditions, where attention was either directed towards the light stimuli, or away from them by a concurrent auditory task. Forty subjects were classified as extraverts or introverts based on the Eysenck Personality Inventory. The results show that attention interacted significantly with extraversion. Introverts exhibited a narrower focus of attention, while higher amplitudes and amplitude-intensity functions when attending to the light flashes and lower when distracted. Extraverts showed smaller differences between conditions, indicating a more evenly distributed attention. Higher arousal in introverts is the probable cause of their narrower focus of attention. There were marked differences in the distribution of activity between vertex and occipital cortex. Introverts showed relatively stronger occipital responses and extraverts stronger vertex responses across all intensities and in both conditions. The predisposition for mainly perceptual responses to aversive stimuli in introverts, and for general alerting and motor preparatory responses in extraverts, are interpreted as supportive of Brebner and Cooper's hypothesis that introverts are ‘geared to inspect’ and extraverts are ‘geared to respond’.     

Incentive and feedback effects on target detection: A chronometric analysis of Gray's model of temperament

Chronometric (i.e. reaction time) techniques were used to investigate the response, arousal, and attentional components of Gray's (1971, 1982) model of temperament. Subjects were tested in a game context, where each trial consisted of an incentive warning signal (positive or negative), a visual detection target (left or right visual field), and a feedback signal (positive or negative). Several findings were consistent with Gray's proposal that extraverts tend to respond impulsively given signals of reward whereas introverts respond with inhibition given signals of punishment. Extraverts made more overall errors than introverts in the first experiment, and more errors following positive incentive signals in the second study. In addition, introverts responded more slowly than extraverts following negative feedback in the first study. Evidence was also found in support of Gray's proposal that introverts allocate more attention to negative cues than extraverts. Following negative feedback, introverts showed a reflexive attentional bias in favor of negative locations in the second experiment. However, no support was found for the temperamental differences in phasic arousal suggested by Gray's model. The magnitude and build up of general alerting effects following positive and negative incentives were similar for introverts and extraverts.     

Extraversion and electrodermal activity: arousability and the inverted-U

The Eysenck hypothesis that extraverts are less aroused or more inhibited than introverts has been subjected to a number of tests in the psychophysiological literature. Research to date suggests the need to clearly differentiate between tests of overall group differences in arousal or inhibition and studies of the differential reactions of the two groups to arousal manipulations. Psychophysiological studies of overall arousal differences in our laboratories and elsewhere have yielded somewhat inconsistent results for SCL, nonspecific response frequency, trials-to-criterion habituation and initial response amplitude, though dishabituation results are consistent with the expectation of higher amplitudes in introverts. Far more consistent are findings testing the inverted-U hypothesis, which suggests that introverts reach a point of optimal arousal at lower levels of stimulus input than do extraverts and hence should show earlier asymptotes and decrements in arousal measures. Using stimulus intensity and caffeine to manipulate arousal, we have found supportive results for SCL and several phasic measures, including the initial, test, and dishabituation responses. Overall, findings suggest that the two personality groups differ more clearly in arousability than in overall arousal level and that this difference is most consistent when conditions induce a rapid breakdown of inhibition and increase in arousal. Further research is needed to better understand the conditional relationship of extraversion to arousability, and arousal-manipulation paradigms would appear to be a fruitful initial approach to carrying out the needed research.     

Extraversion and the ability to decode nonverbal communication

This study examined the relationship between extraversion, as measured by the Eysenck Personality Inventory (EPI), and skill at decoding nonverbal forms of communication. Prior research has had mixed success in establishing this relationship. Because extraverts have more experience in social settings than introverts, and because extraverts have a greater desire for sensory stimulation than introverts, it was hypothesized that extraverts would decode nonverbal cues in social interaction more accurately than introverts. The data supported this hypothesis. Extraverts were significantly more accurate in interpreting the meaning of nonverbal communication than introverts; in addition, extraverts were more confident that they were accurate decoders than introverts. The results are discussed in terms of cognitive processing style: the ‘extravert advantage’ in decoding nonverbal communication may be due to extraverts' superior attentive/perceptual skills; their superior interpretive/attributional skills; or both. In addition, the results are discussed in terms of methodological issues in the nonverbal decoding literature and their impact on research on extraversion. The nonverbal decoding task used in the present study differed from that of prior research by presenting scenes of natural, spontaneous, dyadic interactions for which an objective criterion for accuracy existed.     

Effects of noise on sensitivity of introverts and extraverts to signals in a vigilance task

Forty Ss, previously classified as introverts or extraverts on the basis of scores on the Eysenck Personality Inventory, performed a visual vigilance task while being stimulated with noise at an intensity level of either 65 or 85 dB. Introverts given noise of 65 dB intensity showed an improvement in detection rate across trials, whereas introverts given noise of 85 dB intensity showed a decline in detection rate. Extraverts responded to noise of 65 dB intensity with a slight decrease in detection rate, but showed an improvement in detection over trials when noise of 85 dB intensity was given. When noise of the lower intensity was given, introverts showed greater sensitivity to signals than extraverts. When noise of the higher intensity was given, introverts and extraverts were equal in sensitivity. The results are discussed in terms of a hypothesized relationship between stimulation and arousal, with E-I as a moderator variable.     

Extraversion, attention, and habituation of the auditory evoked response

Short-term changes in the auditory evoked response to low-frequency tones (0.5 kHz, 80 dB) were examined for independent groups of introverts and extraverts under attend and ignore conditions. Introverts displayed greater N1-P2 amplitude than extraverts to the first stimulus in a four-stimulus train. The N1-P2 amplitude differences between introverts and extraverts could not be attributed to differences between the groups in either habituation or attention processes. Overall, the effect is indicative of the introverts' enhanced sensitivity to physical stimulation.     

Individual differences in components of slow cortical potentials: implications for models of information processing

Individual differences in the effects of pharmacological and behavioural manipulations on slow cortical potentials (SCP), may reflect personality differences in type of informational control under attentional stress. Two experiments were conducted to examine SCP component differences in extraverts and introverts under different attentional tasks and with and without nicotine smoking ‘stressor’ conditions. In an initial experiment a decrease in late negativity for introverts and an increase in late negativity for extraverts suggested that smoking enhanced introverts' stimulus set and extraverts' motor set. In a second experiment, personality differences in SCP were examined again, but within a signal-detection paradigm, which allows separate assessment of the contribution of sensory sensitivity and response bias factors to performance. Smoking increased sensitivity in both personality groups, but response bias (caution) increased in introverts only. Extraverts showed an increase in central negativity during smoking whilst introverts showed a decrease in negativity and a decrease in positive wave components. The results are explained in terms of a motor model of attentional control whereby smoking regulates inhibitory controlled actions in introverts, but activates general motor processes for extraverts.     

Differences in the transmission of sensory input into motor output between introverts and extraverts: Behavioral and psychophysiological analyses

The present study was designed to investigate extraversion-related individual differences in the speed of transmission of sensory input into motor output. In a sample of 16 introverted and 16 extraverted female volunteers, event-related potentials, lateralized readiness potentials (LRPs), and electromyogram (EMG) were recorded as participants performed a visual choice reaction time task. As additional behavioral indicators of performance, measures of reaction time (RT) and response dynamics were obtained. Although extraversion-related differences were found neither for behavioral measures nor for the N1 and P3 components of the evoked potential, introverts showed a reliably shorter latency in stimulus-locked LRP than extraverts. This latter finding supports the notion of faster stimulus analysis in introverts compared to extraverts. Furthermore, there was no indication of extraversion-related individual differences in speed of response organization and response execution as indicated by response-locked LRP and EMG latencies, respectively. However, a significantly higher EMG amplitude observed with introverts pointed to a less accurately adjusted motor output system of introverts compared to extraverts.     

A comment on Paisey and Mangan's neo-Pavlovian temperament theory and the biological bases of personality

In the course of their recent article (Paisey and Mangan, 1982) those authors suggest that the Brebner-Cooper model of extraversion cannot predict a finding by Weisen (1965) that introverts respond more often than extraverts do to avoid strong sensory stimulation. The prediction from the Brebner-Cooper model is elaborated to show how Weisen's findings are dealt with by the model.     

Overhabituation and dishabituation: Effects of extraversion and amount of training

The Eysenck (1967) hypothesis that introverts have higher levels of cortical activation than extraverts has received support in a number of psychophysiological studies, though there is not complete consistency. The present study extended the habituation paradigm used in some previous work to include an examination of differences between extraverts and introverts during and following long-term overhabituation. Electrodermal activity was recorded while 72 extraverts and 72 introverts were subjected to criterion auditory habituation to a 1000-Hz, 90-db tone, followed by 60, 100, or 140 trials of overhabituation. The overhabituation run was followed by a test stimulus of 7000 Hz and a final repetition of the standard stimulus. Results showed that introverts increased in response frequency with increasing overhabituation exposure, while extraverts showed an increase only from 100 to 140 trials. In addition, the hypothesis that differences in test response amplitude favoring introverts would be eliminated by extended overhabituation training was supported. Specifically, extraverts showed increased responses at 100 and 140 trials, while introverts remained approximately constant across the three conditions. Finally, results supported the Sokolov hypothesis that overhabituation training produces test responses of larger amplitude.     

Electrodermal activity and extraversion: Caffeine,preparatory signal and stimulus intensity effects

The Eysenck hypothesis that introverts are more and extraverts less cortically aroused has been supported by the results of most electrodermal studies, though there is some inconsistency in the literature. Further tests of the hypothesis can be carried out using arousal-manipulation paradigms. Some studies have used caffeine or stimulus intensity to increase arousal, as was also the case in the present study. The latter, however, also introduced a preparatory signal, which may be expected under certain conditions, to reduce responses to the stimulus which follows it. Extraverts and introverts were randomly assigned to receive low, medium or high doses of caffeine or a placebo. Each S then heard two sets of tones with the tones in one set preceded by a preparatory signal. Results showed that the preparatory signal reduced phasic amplitudes only at the highest levels of stimulus intensity. Introverts had higher overall SCLs and response magnitudes. In addition, the preparatory signal had little effect on extraverts, but reversed dosage-related response patterns in introverts. Results were supportive of the basic Eysenck arousal hypothesis and also showed that the arousal difference leads to attentional variability between the two groups.     

An event-related potential analysis of extraversion and individual differences in cognitive processing speed and response execution

Individual differences in cognitive processing speed and response execution were examined in relation to extraversion. Event-related potentials (ERPs) were recorded concurrently with reaction time and movement time (MT) measures as participants (N = 67) performed simple reaction time and stimulus-response compatibility tasks. Slower processing speed for extraverts, as indicated by longer latency of a late positive ERP wave, P3, was only evident in conditions in which stimulus information was in conflict with response selection demands. As previously reported, the salient effect in all conditions of both tasks was faster MT for extraverts, an effect that is indicative of differences in fundamental motor processes. On the simple reaction time task, amplitudes of the N1 component, an early negative ERP wave, were smaller for extraverts than for introverts in response to auditory tones, an effect that affirms the enhanced sensory reactivity of introverts to punctate physical stimuli.     

The interactive effect of personality, time of day, and caffeine: a test of the arousal model

The personality dimension of introversion/extraversion is one of the few personality dimensions that can be reliably identified from study to study and investigator to investigator. The importance of this demension within personality theory is due both to the stability of the trait and the influential theory of H. J. Eysenck. The basic assumption in Eysenck's theory of introversion/extraversion is that the personality differences between introverts and extraverts reflect some basic difference in the resting level of cortical arousal or activation. Assuming that there is a curvilinear relationship (an inverted U) between levels of stress and performance leads to a test of this arousal theory. That is, moderate increases in stress should hinder the performance of introverts who are presumably already highly aroused. However, the same moderate increase in stress might help the performance of the presumably underaroused extraverts. Revelle, Amaral, and Turriff reported that the administration of moderate doses of caffeine hindered the performance of introverts and helped the performance of extraverts on a cognitive task similar to the verbal test of the Graduate Record Examination. Assuming that caffeine increases arousal, this interaction between introversion/extraversion and drug condition supports Eysenck's theory. This interaction was explored in a series of experiments designed to replicate, extend, and test the generality of the original finding. The interaction between personality and drug condition was replicated and extended to additional cognitive performance tasks. However, these interactions were affected by time of day and stage of practice, and the subscales of introversion/extraversion, impulsivity, and sociability, were differentially affected. In the morning of the first day, low impulsives were hindered and high impulsives helped by caffeine. This pattern reversed in the evening of the first day, and it reversed again in the evening of Day 2. We concluded that the results from the first day of testing require a revision of Eysenck's theory. Instead of a stable difference in arousal between low and high impulsives, it appeared that these groups differed in the phase of their diurnal arousal rhythms. The result is that low impulsives are more aroused in the morning and less aroused in the evening than are the high impulsives. A variety of peripheral or strategic explanations (differences in caffeine consumption, guessing strategies, distraction, etc.) for the observed performance increments and decrements were proposed and tentatively rejected. It seems probable that some fundamental change in the efficiency with which information is processes is responsible for these performance changes.     

Extraversion and rapid tapping: reactive inhibition or general cortical activation as determinants of performance differences

Two samples of subjects (n_tot = 22) scoring high or low on EPI-E tapped a morse-key as fast as possible over a 2-min period. As predicted, extraverts showed a lower tapping performance than introverts with differences between groups increasing significantly over time due to greater delays among the extraverts. Within the E dimension tapping differences could be shown to depend significantly only on the subfactor impulsiveness but not on sociability. There were no differences between groups, however, in frequency of involuntary rest pauses. Therefore the effects of extraversion on tapping performance should be explained more in terms of underlying differences in cortical arousal level than in terms of intermittent blocks as part of inhibitory potentials.     

Time perception: effects of introversion/extraversion and task interest.

32 extraverted and 32 introverted college students rated the interest value and estimated the duration of 3-min. intervals filled with reading. There were 3 dull and 3 interesting readings. It was hypothesized that time perceptions of extraverts and introverts would differ when their interest in the task was dissimilar. But no personality differences in time perception and few in interest were found; both extraverts and introverts judged the intervals to be shorter when the reading was interesting.