Positive evidence for Eysenck’s arousal hypothesis: A combined EEG and MRI study with multiple measurement occasions

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Abstract

Eysenck’s arousal hypothesis suggests that introverts have greater cortical activity than extraverts. This prediction was tested in several studies that used the electroencephalogram (EEG) alpha activity to index cortical arousal, but empirical findings are inconsistent. Possibly, external factors of the measurement situation or static factors such as skull thickness may act as nuisance variables on alpha activity and thus mitigate the relation between arousal and extraversion. The aim of the present study was to test the arousal hypothesis with a focus on these methodological issues. Resting EEG was acquired on several occasions of measurement, skull thickness was quantified with anatomical magnetic resonance imaging (MRI), and extraversion was assessed by questionnaire. There was a positive association between alpha activity and extraversion, which was neither affected by external factors nor by skull thickness. This finding is in line with the arousal hypothesis and suggests that external or static factors do not contribute to the inconsistency of empirical findings.

Introduction

Eysenck, 1967, Eysenck, 1994 arousal hypothesis proposes that extraverted individuals have a low arousability of the cortex, i.e. they show a rather small magnitude of phasic change of cortical activity due to stimulation. Moreover, these individuals may also be characterized by a low cortical arousal, i.e. a low level of tonic cortical activity. According to this hypothesis, arousal level is related to the hedonic tone (pleasant vs. unpleasant) such that a low level of arousal is associated with an unpleasant experience. In order to reduce this inconvenient state, extraverts seek situations that may increase their arousal, thus they engage in typically extraverted behaviours.

This theory was put to test in several studies that used the electroencephalogram (EEG) in order to index cortical arousal. In most of these studies, cortical arousal was measured with the EEG alpha activity (i.e. oscillations between 8 and 13 Hz). This operationalization is based on the hypothesis that there is an inverse relation between cortical arousal and EEG alpha activity (Creutzfeldt, 1995). The conjunction of the arousal hypothesis and the latter measurement hypothesis predicts that extraverts show greater alpha activity than introverts. This paradigm yielded empirical evidence that is inconsistent, thus a clear-cut evaluation of the hypothesis is difficult (see the reviews of Eysenck, 1994, Gale, 1983, Matthews and Gilliland, 1999). Several methodological problems hamper a straight-forward test of the theory.

First, the relation of extraversion-arousal may interact with the intensity of the stimulation in any condition due to a mechanism of transmarginal inhibition (Eysenck, 1994). This protective mechanism down-regulates cortical activity in conditions of great stimulus intensities and thus may blur the relationship between extraversion and cortical arousal. Moreover, Gale (1983) suggested that in conditions of low stimulus intensity, extraverts may engage in self-stimulating behaviours in order to increase their arousal up to a pleasant level. In conditions of high stimulus intensities, however, introverts may engage in relaxing behaviours in order to down-regulate their arousal. Taken together, the relation between extraversion and cortical arousal may be consistent with Eysenck’s arousal hypothesis only in moderately-arousing testing conditions.

Gale (1983) provided a methodological analysis of extraversion studies that supported this hypothesis. He classified more than 30 studies according to the stimulus intensity of the experimental conditions and concluded that the arousal hypothesis was more consistently supported by studies that employed moderately-arousing tasks (such as opening and closing the eyes on instruction). However, low-arousing conditions (e.g., lying supine in a soundproofed room) and high-arousing conditions (e.g., performing a difficult or stressful task) yielded rather inconsistent results.

A second problem pertains to the contamination of arousal measures by external factors. In general, psychological measurement never takes place in a situational vacuum, thus a host of situational factors may even affect the measurement of a trait variable to a certain degree (Steyer, Ferring, & Schmitt, 1992). In particular, Eysenck (1994) suggested that external factors such as time of day, emotionally significant events antecedent to testing, and a plethora of further variables may all affect the arousal of the subjects. While most of these variables are largely uncontrollable, they increase the non-trait variance of the arousal data, which in turn may blur the relation between tonic arousal and extraversion. One recent study provided some support for this notion. Hagemann and Naumann (2009) recorded resting EEG on four occasions of measurement each one month apart, and used models of the latent state-trait theory (LST; Steyer et al., 1992) to decompose measures of EEG alpha activity into a consistent component (a latent trait variable) plus occasion-specific components (latent state-residuals), the latter including all effects of external factors that varied between occasions. These occasion-specific components showed non-zero variance and could explain about 5% of the variance of EEG alpha activity. Although this unwanted source of variance of arousal measures was not paramount in this study, it may be reduced by an aggregation of the data across several occasions of measurement (Epstein, 1979). In turn, this procedure will increase the variance proportion that is due to the stable and consistent component of the measurement, i.e., the latent trait (Steyer & Schmitt, 1990).

A third problem is specific to the measurement of cortical arousal with the EEG. Although Eysenck (1994) has suggested that the EEG alpha activity is the standard measure of cortical arousal, recent research provided evidence that the inverse relationship between EEG alpha and cortical activity is not universal. In particular, several studies reported increased alpha power in conditions of mental imagery, meditation, and autogenic training, and other studies reported decreased alpha power after yoga, massage therapy, and administration of benzodiazepines (see Klimesch, Doppelmayr, Rohm, Pollhuber, & Stadler, 2000). However, there is ample evidence from studies that combine EEG and imaging methods that EEG alpha power shows an inverse relationship with regional cerebral blood flow (rCBF), regional cerebral metabolic activity, and hemodynamic activity when these measures are recorded in a resting situation (for a review, see Sotero & Trujillo-Barreto, 2008). Thus, as long as the EEG measurement takes place in a resting situation, EEG alpha activity may be used to index cortical activity.

A fourth problem is that individual differences in alpha activity may be due to factors other than cortical arousal, such as differences in brain geometry between subjects, or individual variations in skull thickness (Myslobodsky, Coppola, & Weinberger, 1991). The latter proposal is based on Ohm’s law, which suggests that the amplitudes of the EEG may be affected both by the strength of the cortical current sources and by the electrical resistance of the skull. Because skull resistance is considerably larger than that of other head tissues, the thickness of the skull may be a significant moderator of the EEG amplitudes. Therefore, individual differences in the stable and consistent component of EEG alpha activity may be – at least in part – due to static factors such as skull thickness. In particular, Hagemann, Hewig, Walter, and Naumann (2008) showed a significant negative association between skull thickness and alpha activity of r = −.35 at frontal sites. This finding suggests that individual differences in EEG alpha are partially due to anatomic features, which implies that these static factors may increase the non-arousal variance of EEG alpha activity and thus may blur the relation between tonic arousal and extraversion.

The aim of the present study was to test the arousal hypothesis with EEG alpha activity serving as a measure of cortical arousal. We hypothesize that (a) extraversion is related to cortical arousal and (b) that cortical arousal may be indexed with EEG alpha activity. The conjunction of both presumptions allows the prediction that there is a positive correlation between extraversion and alpha activity. This test of the arousal hypothesis aims to account for the methodological problems listed above. First, we selected a testing condition that was moderately-arousing according to the classification of Gale (1983). In particular, the subjects’ task was to sit on a chair and open and close their eyes upon instruction. This condition is unlikely to evoke transmarginal inhibition or self-regulating behaviours, and thus may facilitate an assessment of tonic cortical arousal (Eysenck, 1994, Gale, 1983). Moreover, in this specific condition, EEG alpha may serve as an inverse index of cortical activity (Sotero & Trujillo-Barreto, 2008). Second, we acquired measures of cortical arousal on three occasions each separated by five weeks and aggregated the data across the occasions. This procedure will reduce the distorting effects of external factors. Third, we acquired anatomical MRI scans to control skull thickness in the analyses of the relation between EEG alpha activity and extraversion.

Section snippets

Subjects

Forty-nine right-handed subjects (25 female and 24 male, mean age = 24 years, SD = 2.3 years) were recruited from the student population of the University of Trier. Informed consent was obtained for all subjects. The study was approved by the local ethics committee.

Procedure

All participants were assessed individually at four occasions of measurement each separated by approximately 5 weeks. On the first three occasions, a resting EEG was acquired and on the fourth occasion, an MRI scan was performed. A detailed

Results

In a first step, we analysed the retest correlations of alpha power density values prior and after controlling for the variance that is due to skull thickness with first-order and partial correlations (where skull thickness served as a control variable). The results of this analysis are presented in Table 1. There were hardly any differences between the first-order and the respective partial correlations, which suggest that individual differences in skull thickness do not substantially

Discussion

In the present study, there was a positive relation between EEG alpha in a resting state and extraversion scores, which suggests that extraverted individuals are cortically less aroused than introverts. This finding is consistent with the predictions that were derived from Eysenck, 1967, Eysenck, 1994 arousal hypothesis and the presumption that EEG alpha activity is an inverse measure of cortical arousal. However, the magnitude of this relation did not exceed a correlation of .30, which is not

Acknowledgements

The authors are grateful to Renate Freudenreich, Maja Neurath, and Helmut Peifer for technical support, and Astrid Kronbergs and Olaf Schweisthal for data processing. This research was supported by the Stiftung Innovation Rheinland-Pfalz project 647 and a grand by the DFG to the first author (HA 3044/5-1).

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