Event-related desynchronization in the EEG during emotional and cognitive information processing: Differential effects of extraversion

Biological Psychology 70 (2005) 152–160 www.elsevier.com/locate/biopsycho

Event-related desynchronization in the EEG during emotional and cognitive information processing: Differential effects of extraversion Andreas Fink * Institute of Psychology, University of Graz, Universitaetsplatz 2/III, Graz A-8010, Austria Received 26 November 2004; accepted 7 January 2005 Available online 28 September 2005

Abstract The aim of the present study was to analyze the influence of the personality dimension extraversion/introversion (E) on the level and topographical distribution of event-related desynchronization (ERD) in the EEG whilst participants were engaged in emotional face and cognitive information processing. In this context we build up on former studies dealing with the role of E as a possible moderator variable in cortical activation patterns during performance of mental speed, reasoning and working memory tasks (i.e., cognitive information processing). In a sample of 33 introverts and 33 extraverts (31 were male, 35 female) we found extraverted individuals displaying a lower (left-hemispheric) cortical activation than introverts when their task was to judge the identity of two simultaneously presented facial emotions. This effect was only observed in the upper alpha frequency band (9.6–11.6 Hz). In analyzing E differences during cognitive information processing (i.e., performance of a verbal and a figural-spatial task) E effects – which were moderated by participants’ sex – were restricted to lower EEG (alpha) frequency ranges (5.6–9.6 Hz). The results generally suggest that E is differently involved when different kinds of information are processed. # 2005 Elsevier B.V. All rights reserved. Keywords: EEG; ERD; Extraversion; Alpha band; Emotional face processing; Hemispheric differences

1. Introduction Research into possible physiological basics of the personality dimension extraversion/introversion (E) has been stimulated by Eysenck’s (1967) well-known arousal theory, which links individual differences in the personality dimension E to individual differences in the activity of reticulo-cortical pathways (Ascending Reticular Activation System, ARAS). Starting from Eysenck’s very influential theory, E has been related to different measures or indices of brain activity derived from the human EEG (for extensive reviews, see e.g., Taub, 1998; Matthews and Gilliland, 1999). In the majority of these studies E has been investigated in the EEG alpha frequency range, i.e., roughly in the frequency range between 7 and 13 Hz. This was motivated by assuming the EEG alpha rhythm (which is predominantly observed in a state of relaxed wakefulness) being particularly sensitive to neuronal activity in reticulocortical pathways, which may underlie individual differences in E (Eysenck, 1967). * Tel.: +43 316 380 8505; fax: +43 316 380 9811. E-mail address: [email protected]. 0301-0511/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.biopsycho.2005.01.013

In testing predictions derived from Eysenck’s theory, there is some empirical evidence that individual differences in E are associated with individual differences in EEG alpha power (for more recent studies, see e.g., Gale et al., 2001; Matthews and Amelang, 1993; Tran et al., 2001). These studies typically suggest more alpha power (or larger amplitudes, respectively) in extraverted (E) as compared to introverted (I) individuals. Since more alpha power indicates a more strongly synchronized neural activity (in contrast to desynchronized neural activity, which indicates cortical activation), this finding is in line with the hypothesis of a weaker cortical activation (or arousal) in E versus I. However, there are also numerous research reports that found no convincing evidence in favour of the hypothesis of introverts being more strongly cortically aroused than extraverts (for recent evidence, see e.g., Hagemann et al., 1999; Schmidtke and Heller, 2004). Two important aspects could (among others) be responsible for inconsistencies or contradictory empirical evidence in this field of research. First, as originally suggested by Gale (1983), more or less arousal inducing test conditions might be better or worse suited to allow for activation (arousal) differences between I and E. In light of

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Gale’s argumentation so-called ‘‘moderate’’ arousal-inducing conditions, such as when individuals are asked to open or close their eyes alternately, are more likely to confirm Eysenck’s (1967) predictions as compared to low arousal-inducing (‘‘boring’’) conditions, where individuals are instructed to do nothing but simply lie down (where extraverts are assumed of stimulating themselves by cogitation), or – in a high arousalinducing condition – to perform an effortful, cognitively demanding task, where the ‘‘over-aroused’’ introverts are believed to adopt self-relaxation strategies in order to induce a state of calm (cf. Gale, 1983). Another critical point that might account for inconsistencies in this field of research could be the specific alpha frequency range to which E has been related to. Most of the studies dealing with the E  activation relationship analyzed cortical activation within the traditional alpha frequency band, i.e., in the frequency band ranging between 7 and 13 Hz. However, as outlined by Klimesch (1999), broad band analyses of electrophysiological brain activity should be treated cautiously, since they might obscure possible frequency specific effects. Hence, a possible reason for failing to detect E-related effects on cortical activation patterns could also be assumed in the fact that in the majority of studies E has been analyzed in the broad alpha frequency band. We also analyzed the role of individual differences in E in accounting for variability in brain responses to different cognitive demands. In this regard, cortical activation was assessed by means of the event-related desynchronization (ERD; cf. Pfurtscheller and Lopes da Silva, 1999) in the EEG, a method which is based on the well established phenomenon that EEG alpha power (approximately in the frequency range between 7 and 13 Hz) desynchronizes when individuals are mentally active (compared to a resting condition during which no task is performed). The ERD has proved to be a valuable and useful tool in measuring the level and topographical distribution of cortical activation during a wide range of cognitive tasks (e.g., auditory lexical matching task: Karrasch et al., 1998; learning: Neubauer et al., 2004; visual information processing: Pfurtscheller et al., 1994; working memory: Grabner et al. 2004; Krause et al., 2000). In this context, the ERD method has also turned out to be sensitive in indexing different task requirements during reasoning (Neubauer and Fink, 2003) and working memory processing (Stipacek et al., 2003). With respect to its functional significance it is assumed that the ERD of alpha band activity, also called ‘‘alpha blocking’’, reflects an increased excitability level of neurons in the involved cortical areas, which may be related, for instance, to an enhanced information transfer in thalamo-cortical circuits (for a review, see Neuper and Pfurtscheller, 2001). Empirical research has also shown that different patterns of alpha desynchronization can be observed when the broad alpha frequency range is subdivided into lower and upper alpha bands (cf. Klimesch, 1999) with the former presumably reflecting general task demands such as attentional processes (vigilance or arousal) and the latter being more likely to reflect specific task requirements (e.g., intelligence-related processes, semantic memory processes; see e.g., Doppelmayr et al., 2002;

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Grabner et al., 2004; Klimesch et al., 2000; Neubauer and Fink, 2003; Neubauer et al., 2002; cf. also Jausovec and Jausovec, 2004; Neuper and Pfurtscheller, 2001; Verstraeten and Cluydts, 2002). In employing the ERD method, we (Fink and Neubauer, 2004) found some evidence that cortical activation differences between I and E might be mediated by the level of task demands (i.e., experimentally induced activation or arousal level). In this study, Eysenck’s predictions (with more activation in I versus E) could only be confirmed in more effortful or complex (i.e., more arousing) conditions of the test, while in easier (i.e., low arousal inducing) conditions an opposite result pattern was found. In another study (Fink et al., 2005a), extraverts were cortically less activated (i.e., lower ERD) than introverts during performance of two working memory tasks. Interestingly, in these studies we found E related effects on cortical activation patterns more prominently in lower EEG (alpha) frequency ranges (cf. also Fink et al., 2002), i.e., mostly in the frequency range of about 6–8 Hz, to which Klimesch (1999) has referred to as lower 1 alpha band, presumably reflecting unspecific, attentional task demands such as alertness, vigilance or arousal (cf. Klimesch, 1999). Contrary, in the upper alpha band (10– 12 Hz), which is believed to selectively respond to more specific task demands, we observed no E effects at all. Hence, in assuming functional differences between different alpha frequency bands (cf. also Fink et al., 2005b), the analysis of E effects in narrower frequency bands appears to be more suitable here. The specific aim of the present study is to further analyze the relationship between E and electrophysiological brain activity in different EEG alpha frequency bands. Different from our former studies, where we analyzed the role of E as a possible moderator variable in cognitive task performance (i.e., performance of mental speed, reasoning and working memory tasks), the E  activation relationship should be studied in the context of emotional information processing here. The role of individual differences in this research domain is highlighted by Hamann and Canli (2004) in their recent review on ‘‘individual differences in emotion processing’’ by viewing them as ‘‘the rule rather than the exception’’ (Hamann and Canli, 2004, p. 233). Beneath the variable sex (see also Cahill, 2003; Wager et al., 2003), particularly the role of outgoingness or extraversion has been shown to account for variability in brain reactivity to emotional stimuli (Canli et al., 2001). More precisely, we used event-related desynchronization in the EEG in order to test whether individual differences in E were correlated with cortical activity during performance of an emotional face processing task requiring participants to judge the equivalence of two simultaneously presented facial emotions. In addition to this, the ERD during performance of a verbal (judging the semantical identity of words) as well as a figural (judging the identity of arrows) task is measured in order to assess whether cortical activity during emotional face processing might be differentiated from that obtained during performance of ‘‘classical’’ cognitive tasks. An analysis of possible inter-individual variability in EEG baseline activity (EEG alpha power during a resting condition without

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performing any task) should finally reveal whether extraversion related effects are apparent during rest as well. 2. Methods 2.1. Participants Out of an original pool of 219 participants, who were recruited through local newspaper advertisements, 76 participants (34 males, 42 females) were selected by means of their classical and emotional intelligence test scores. The social as well as the educational level of the participants covered a considerably broad range. Due to EEG artefacts the data of 10 persons had to be excluded from further analysis. The remaining sample (n = 66) consisted of 31 males and 35 females whose age ranged from 18 to 50 years (M = 34.80, S.D. = 8.14). On the basis of the extraversion (E) scores in a self-report questionnaire (NEO-FFI, see below), the sample was divided into a group of introverted (n = 33; 14 males, 19 females; NEO-FFI-E raw scores ranging between 0.33 and 2.42; M = 1.88, S.D. = 0.49) and a group of extraverted (n = 33; 17 males and 16 females; NEOFFI-E raw scores ranging between 2.50 and 3.67, M = 2.92, S.D. = 0.32) participants. The correlation between sex and E was 0.06 ( p = .65). All participants were right-handed (as determined by self-report), without any obvious signs of medical or psychological disorders, and had normal or corrected to normal vision. The participants were paid for their participation in the EEG session, and all gave informed consent.

In the verbal Posner task (PPV), pairs of words were shown which were either of identical or of different meaning (e.g., chimney and funnel, see Fig. 1); half of the word pairs were composed of synonymous words, half of them contained words of different semantical identity. Participants were instructed to judge whether the presented words are of identical name or not (by pressing either the YES or the NO buttons). Finally, in the figural Posner task (PPF) we presented pairs of arrows which were rotated congruently (e.g., , or ) as semantically identical stimuli or rotated incongruently (e.g., , or ) as semantically different stimuli, respectively. The participants were instructed to judge whether the presented arrows are semantically the same or different (i.e., referring to the congruent or incongruent location of the presented arrows). For both the PPV and the PPF task 10 practice and 50 test items were presented. All experimental tasks were presented in the same manner. As depicted in Fig. 1, each trial started with the presentation of a fixation cross for 2 s, followed by an auditory warning stimulus. Immediately after the warning stimulus, at second 3, the test stimulus was presented, and the participant had to respond as fast and accurately as possible to the stimulus by pressing either the YESbuttons or the NO-buttons, upon which the stimulus was deleted from the screen. For the presentation of the experimental tasks a PC (g.STIMunit, Guger Technologies, Austria) with an external response-console consisting of two horizontally arranged buttons for the YES-responses on the top of the responseconsole and two horizontally arranged buttons for the NO-responses on the bottom of the console was used. To avoid a confounding with hemispheric differences participants were instructed to respond simultaneously with their index fingers in case that a YES-response was required, and with their thumbs in case that the opposite answer NO was required.

2.2. Psychometric tests 2.4. Procedure Prior to the EEG sessions participants were screened with respect to a variety of psychometric test variables (e.g., classical and emotional intelligence). Participants’ personality traits were assessed according to the ‘‘big five’’ model of personality (i.e., extraversion, agreeableness, conscientiousness, neuroticism, openness to experience). For this purpose, the NEO-FFI by Costa and McCrae (translated into German by Borkenau and Ostendorf, 1993) was administered. During the EEG session we assessed temporary mood of the participants by means of a self-report questionnaire (e.g., activation, anger, calmness, weakness) and anxiety by means of a German version of Spielberger’s state-trait anxiety inventory (STAI; Laux et al., 1981). No significant E group and sex differences were found with respect to these control variables (as assessed by means of ANOVAs).

2.3. Experimental tasks In analogy to the classical Posner Paradigm (where pairs of stimuli have to be compared according to a specific instruction; cf. Posner and Mitchell, 1967), four experimental tasks were constructed: (1) an emotional face processing (Posner Paradigm Emotions; PPE) task requiring participants to judge the equivalence of two simultaneously presented facial emotions, (2) a control task (Posner Paradigm Gender; PPG; i.e., judging the equivalence of the faces’ gender without regarding their emotional expressions; the construction of both tasks is described in Ackerer, 2003), and, finally, in order to contrast emotional face with cognitive information processing, we (3) also used a verbal (Posner Paradigm Verbal; PPV) and (4) a figural (Posner Paradigm Figural; PPF) variant of Posner’s classical letter matching paradigm. In the PPE task, male and female facial photos were used as stimuli (see Fig. 1), each of them expressing one of the emotions joy, dolor, surprise, fear, disgust or anger. In each trial, two stimuli were presented simultaneously on the computer screen, and participants’ task was to judge as fast and as accurately as possible whether the emotions expressed in both faces are the same or different (by pressing either the YES or the NO buttons). In the PPG task, the same stimulus material as in the PPE was used. However, in this task a slightly different instruction was given to the participants: they were asked to indicate (by pressing the corresponding response buttons) as fast and accurately as possible whether both faces are of the same or different gender. In both the PPE and the PPG task 80 test items were presented. In order to realize both tasks within a reasonable timeframe, we presented the test trials of the PPE and the PPG tasks in two blocks, each of them comprising 7 practice and 40 test items.

The EEG session started with mounting of the electrodes and checking the electrode impedances. Subsequently, the participant sat down on a comfortable chair in the EEG recording room, and two 2-min EEG sequences under resting conditions were recorded, the first with eyes closed (recording condition eyes closed, REC), the second with eyes open (recording condition eyes open, REO). Then the participant started to work on the experimental tasks described above. To control for effects of task sequence, the presentation order of the tasks was partially counterbalanced. In each person, the tasks were presented in three consecutive blocks.1 In the first and the last block, the two different tasks with facial stimuli were presented (PPE and PPG), whereby the presentation order was alternated (e.g., PPE–PPG in the first and PPG–PPE in the last block). In the second (middle) block, the verbal (PPV) and figural (PPF) version of the Posner paradigm were administered, again with a counterbalanced presentation order. After the second block, two further 2-min resting EEG sequences (with eyes closed and eyes open, respectively) were recorded. The total time for the EEG session was about 2 h.

2.5. EEG recording The EEG was measured by means of gold electrodes (9 mm diameter) located in an electrode cap in 33 positions (according to the international 10–20 system with interspaced positions); a ground electrode was located on the forehead, the reference electrode was placed on the nose. To register eye movements, an electrooculogram (EOG) was recorded bipolarly between two gold electrodes diagonally placed above and below the inner, respectively, the outer canthus of the right eye. This electrode placement allows for detecting both vertical and horizontal eye movements using only one EOG channel. The EEG signals were filtered between 0.1 and 100 Hz; an additional 50 Hz notch filter was applied to avoid power line contamination. Electrode impedances were kept below 5 kV for the EEG and below 10 kV for the EOG. Trigger signals for the stimulus presentation and the responses were also recorded. All signals were sampled at a frequency of 256 Hz.

1 The presentation order did not differentially affect differences between introverted and extraverted individuals with regard to cortical activation patterns, as a control analysis using the presentation order as a covariate indicates.

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Fig. 1. Schematic time course and EEG measurement intervals for the experimental tasks.

2.6. Selection of frequency bands For reasons given in Klimesch (1999) the frequency borders of the analyzed alpha bands were determined individually for each participant by using the dominant EEG frequency in the alpha band (the so-called individual alpha frequency, IAF) as an anchor point. In contrast to our previous studies we used center of gravity frequency rather than the peak (i.e., maximum of amplitude) in determining the IAF. This was motivated by our findings of a higher psychometric quality of alpha gravity frequency when contrasted to peak frequency in the alpha band (cf. Neuper et al., 2005). Center of gravity frequency in the alpha band was determined as follows: first, for each participant and for each electrode position power spectra were calculated from the resting EEG (for both resting EEG sequences with eyes open). In a next step, the center of gravity in the frequency range between 7 and 13 Hz was calculated for each electrode position. In determining the IAF we aggregated the gravity frequencies over both eyes open-resting conditions and over all leads. Three different frequency windows with a bandwidth of 2 Hz each were defined: lower 1 alpha band (L1 = [IAF–4 Hz] to [IAF–2 Hz]), lower 2 alpha band (L2 = [IAF–2 Hz] to IAF) and upper alpha band (U = IAF to [IAF + 2 Hz]). The mean frequency values for these individually defined bands were 5.57–7.56 Hz for the lower 1 alpha band, 7.57–9.56 Hz for the lower 2 alpha band and 9.57–11.56 Hz for the upper alpha band.

2.7. Quantification of EEG For all experimental tasks, the pre-stimulus interval between 0.5 and 1.5 s served as reference interval (R), and the entire individual reaction time interval (i.e., time period between stimulus onset and response) was used as activation interval (A) in calculating the ERD (cf. Fig. 1). Because of the difficulty of interpreting incorrectly solved test trials (which might be traced back to a lack of competency or to a lack of motivation or both), only correctly solved trials were included in the ERD analyses. In all remaining trials, the reference intervals (during the fixation cross) and the activation intervals (directly before response) were checked individually for artifacts (eye movements, eye blinks, muscle artifacts, etc.) by visual inspection. Time periods containing artifacts were completely eliminated from the ERD analyses. The power of background activity was computed (according to individually defined ranges) for both time intervals (reference and activation) and each trial after squaring band pass filtered signals. The percentage decrease (or increase) in alpha power (mV2)

from the (aggregated) reference interval (R) to the (aggregated) activation interval (A) was defined as: %ERD = ([R A]/R)  100. Positive %ERD values indicate decreases in alpha power (cortical activation or desynchronization) and negative %ERD values indicate increases in alpha power (cortical deactivation or synchronization, ERS). These changes in alpha band power were analyzed within the three alpha frequency bands described above. In addition to this, for each EEG sequence recorded during rest (four 2-min EEG sequences, two REC and two REO sequences, respectively) alpha power was determined for each frequency band of interest. In order to obtain normally distributed data, for statistical analyses, EEG alpha power values were logtransformed. Based on visual inspection of the topographical distribution of ERD, for further analyses we aggregated the data of different electrode locations, distinguishing the hemispheres as well as frontal, frontocentral, centroparietal, parietooccipital and temporal brain areas. The electrode positions were aggregated as follows: frontal left (FP1, AF3, F3, F7), frontal right (FP2, AF4, F4, F8), frontocentral left (FC1, FC5, C3), frontocentral right (FC2, FC6, C4), centroparietal left (CP1, CP5, P3), and centroparietal right (CP2, CP6, P4), parietooccipital left (PO3, PO5, O1), parietooccipital right (PO4, PO6, O2), temporal left (T3, T5) and temporal right (T4, T6). The midline electrodes (FZ, CZ, PZ) were not included in the analyses (as we were also interested in potential hemispheric differences).

3. Results 3.1. Performance data First, we analyzed possible group differences with respect to performance (i.e., reaction time, RT) in the PPE and PPG task by means of an ANCOVA for repeated measures using task as a within subjects variable, and sex and E as between subjects variables. NEO-FFI neuroticism (N) was used as a covariate.2 2 In order to control for individual differences in the personality dimension neuroticism, which might also be assumed to influence the relationship between E and electrophysiological brain activity, statistical analyses were performed using NEO-FFI neuroticism as a covariate.

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As indicated by a significant main effect of task (F[1,61] = 118.48, p < .001), both tasks differed significantly with respect to their RTs (1450 ms versus 860 ms for the PPE versus PPG task). The remaining ANCOVA effects failed to reach statistical significance. In analyzing possible group differences with respect to RT in the PPV and PPF task, an ANCOVA (task  sex  E, controlling for N) displayed a significant main effect of task (F[1,61] = 5.20, p < .05), indicating a larger RT in the figural (1830 ms) as compared to the verbal task (1699 ms). A significant task by sex interaction (F[1,61] = 7.00 p < .05) suggests that males were more likely to outperform females in the figural task (1712 ms versus 1948 ms for males versus females), while in the verbal task a tendency towards the opposite direction was found (1726 ms versus 1673 ms for males versus females). The remaining ANCOVA effects did not reach statistical significance. 3.2. EEG alpha power during rest In order to analyze possible inter-subject variability in baseline EEG activity, an ANCOVA for repeated measures was performed on the absolute EEG alpha power recorded with both opened (REO) and closed eyes (REC). The ANCOVA design (controlling for N) included the within subjects factors recording condition (REO versus REC), hemisphere (left versus right) and area (frontal, frontocentral, centroparietal, parietooccipital and temporal) as well as the between subjects factors sex and E. In case of violation of sphericity assumptions, degrees of freedom were adjusted by applying the conservative Greenhouse–Geisser procedure. An overview of significant ANCOVA effects is given in Table 1. In the lower 2 and the upper alpha band a significant main effect of condition was observed (cf. Table 1), with the REC condition displaying more EEG alpha power than the REO condition. In the lower 1 alpha band, this effect is – as indicated by a significant interaction between sex and condition – only apparent in males. In all frequency bands, a significant main effect of area was found, suggesting that posterior cortical regions were associated with more EEG alpha power than anterior regions of the cortex. As illustrated for the upper alpha band (see Fig. 2), this effect was much more pronounced in the

Fig. 2. Absolute EEG alpha power (in the upper alpha band) during rest.

REC condition (i.e., significant REO/REC by area interaction, cf. Table 1). A significant main effect of hemisphere in the upper alpha band suggests more power in the right than the left hemisphere. Two effects were related to E: In the upper alpha band, as flagged out more clearly in Fig. 2, a significant interaction between condition, area and E occurred, suggesting more alpha power in I versus E over parietooccipital cortices during the recording condition with eyes closed. And secondly, in the lower 2 alpha band, an interaction between area, sex and E suggests comparatively strong E group differences (with more alpha power in I versus E) over centroparietal, parietooccipital and temporal regions in females, while in males these differences were far less pronounced. 3.3. EEG alpha power in the reference interval As outlined in the method section, the event-related desynchronization in the EEG is defined as the percentage

Table 1 Absolute EEG alpha power in the lower 1, lower 2 and upper alpha band during rest: overview of significant ANCOVA effects ANCOVA effects

Absolute bandpower (rest) Lower 1

REO/REC REO/REC  sex Area REO/REC  area Hemis REO/REC  area  E Area  sex  E * **

p < .05. p < .01.

Lower 2 F1,61 = 7.28

Upper **

F1,61 = 15.16**

*

F1,61 = 5.52 F1.4,85.0 = 8.05** F1.4,85.0 = 15.13**

F1.4,83.3 = 16.90** F1.4,83.7 = 15.78**

F1.5,93.1 = 41.08** F1.6,97.3 = 25.00** F1,61 = 5.05* F1.6,97.3 = 4.26*

F1.4,83.3 = 4.19

*

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of decrease (or increase) in EEG power from a reference (prestimulus period) to an activation interval (task performance). Hence, the ERD also depends on the power in the reference interval (here, 2500–1500 ms before stimulus onset) which complicates comparisons of ERD values between different groups, since any group differences with respect to activation patterns during performing experimental tasks might also be due to differences in reference power. In order to make sure that reference power does not vary as a function of group, possible sex and E group differences were analyzed in each frequency band of interest. Separate ANCOVAs (controlling for N) for repeated measures were performed for the three alpha bands as well as for the emotional face (PPE/PPG) and cognitive information processing tasks (PPV/PPF). The ANCOVA design included the within subjects factors task (PPE versus PPG and PPV versus PPF, respectively), hemisphere (left versus right) and area (frontal, frontocentral, centroparietal, parietooccipital and temporal) as well as sex and E as between subjects variables. In case of violation of sphericity assumptions Greenhouse–Geisser corrected degrees of freedom were used. An overview of significant ANCOVA effects is given in Table 2. Dealing first with the PPE and PPG task, in all frequency bands a significant main effect of area was found, indicating (similar to the resting condition) an increase in EEG alpha power from anterior to posterior regions of the cerebral cortex. In the lower 1 and the upper alpha band, this effect is somewhat more pronounced in the PPG than the PPE task (i.e., task by area interaction). Moreover, in the lower 1 alpha band, the females were more likely to display more alpha power (M = 7.48) than males (M = 5.29; i.e., main effect of sex), and introverted individuals might be characterized by more alpha power than extraverts (7.12 versus 5.64 for I versus E: i.e., significant main effect of E). In the PPV and the PPF task, alpha power again tends to increase from anterior to posterior cortical regions (i.e., main

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effect of area). In the upper alpha band, the main effect of task reached statistical significance (cf. Table 2), suggesting slightly more power in the PPF than the PPV task. Very similar to the PPE and PPG task, significant ANCOVA effects were most prominent in the lower 1 alpha band. Females showed a significantly larger amount of alpha power than males (7.33 versus 5.12 for females versus males), primarily in the PPF task (7.50 versus 4.88 for females versus males), as a significant task by sex interaction indicates. Introverts were again more likely to display a larger amount of alpha power than extraverted individuals (6.85 versus 5.60 for I versus E). Moreover, in the lower 2 alpha band, an interaction between task, sex and E was found, suggesting comparatively strong E group differences in females during performance of the PPV task (with more alpha power in I versus E) while in the male sample no differences were found. 3.4. Event-related desynchronization (ERD) during task performance In order to analyze possible group differences with respect to cortical activation patterns during performance of the experimental tasks, for each alpha frequency band and for both the emotional (PPE/PPG) and cognitive information processing tasks (PPV/PPF) tasks ANCOVAs (controlling for N) for repeated measures were conducted. The ANCOVA design was equivalent to the design used in the analysis of alpha power in the reference interval. Table 2 provides an overview of all significant ANCOVA effects. Dealing first with the PPE/PPG task, in all frequency bands a significant main effect of area was observed, suggesting a decrease of ERD from anterior to posterior regions of the cerebral cortex. However, a similar topographical activation pattern of the cortex was already found in the pre-stimulus reference interval (see above). Two ANCOVA effects were

Table 2 EEG alpha power during the pre-stimulus reference interval and ERD during performance of the PPE/PPG and PPV/PPF task: overview of significant ANCOVA effects ANCOVA effects

PPE/PPG task Reference power Lower 1

Area Task  area Sex E Task  hemis  E Task  hemis  area  sex  E

ERD Lower 2

*

F1.3,77.0 = 5.07 F1.6,97.5 = 3.91* F1,61 = 5.73* F1,61 = 6.97*

F1.4,86.6 = 4.49

Upper *

Lower 1 **

F1.4,86.6 = 7.17 F2.1,127.0 = 4.07*

F1.5,89.8 = 7.46

Lower 2 **

F1.4,84.0 = 8.19

Upper **

F1,61 = 4.87* F2.8,171.4 = 4.31* PPV/PPF task

Area Task Task  sex Sex E Task  sex  E * **

p < .05. p < .01.

F1.7,102.8 = 4.34*

F1.3,77.6 = 5.60*

F1.4,84.0 = 4.77*

F1.4,87.1 = 7.31** F1,61 = 4.20*

F1.5,93.8 = 16.88**

F1,61 = 4.45* F1,61 = 5.84* F1,61 = 4.49* F1,61 = 6.55*

F1,61 = 5.43*

F1.3,81.8 = 4.01*

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significance in the upper alpha band. As depicted in Fig. 3, in the PPE task extraverted individuals were more likely to display a lower ERD than introverts, primarily over the left hemisphere ( p < .05, as assessed by means of the Scheffe´ test), while in the PPG task no substantial ERD differences between I and E were found. During performance of the PPV and PPF task, the main effect of area (found in both lower alpha bands, cf. Table 2) again indicates a decrease of ERD from anterior to posterior regions of the cortex. In addition to this, the interaction between task, sex and E reached statistical significance in the lower 1 alpha band. As depicted in Fig. 4, males and females appear to be more likely to display the inverse E  activation relationship in that domain they usually perform better (cf. also behavioral results): males in the figural task and females in the verbal task. However, it should be stressed that these differences were not significant in specific post hoc comparisons by means of the Scheffe´ test. The remaining ANCOVA effects failed to reach statistical significance. 4. Discussion Fig. 3. Interaction between task, hemisphere and E in the upper alpha band during performance of the PPE and PPG task.

related to the personality dimension E. First, a complex higher order interaction between task, hemisphere, area, sex and E in the lower 1 alpha band. The pattern of this interaction suggests E group differences in males (with more ERS in E versus I) over right hemispheric posterior (i.e., parietooccipital, temporal) cortices, predominantly in the PPG task. And secondly, the interaction between task, hemisphere and E reached statistical

Fig. 4. Interaction between task, sex and E in the in the lower 1 alpha band during performance of the PPV and PPF task.

The aim of the present study was to further analyze differential effects of the personality dimension extraversion/ introversion on the event-related desynchronization in different individually adjusted EEG alpha frequency bands. Building up on our former research, physiological E effects have not only been studied during performance of cognitive tasks here, but during emotional face processing as well. The findings of the present study generally suggest that baseline differences in EEG alpha power (resting condition) appear to be of rather low magnitude (cf. also Gale, 1983); differences between intro- and extraverts were only observed over parietooccipital cortices during the resting condition with eyes closed (see Fig. 2). Contrarily, in analyzing cortical activation during performance of different tasks, the ERD findings suggest that E is differently involved when different kind of information processing is required. When participants task was to judge whether the presented facial emotions were equivalent or not (PPE task), extraverted individuals were more likely to display a lower cortical activation (lower ERD) than introverts, primarily in the left hemisphere (cf. Fig. 3). In contrast, when the presented facial stimuli had to be judged with respect to the identity of their sex (PPG task), no obvious differences were found between both groups. Interestingly, this effect was only observed in the upper alpha band, i.e., roughly in the frequency range between 9.6 and 11.6 Hz. It is important to note that in the upper alpha band no significant between group differences in pre-stimulus reference power (used for ERD calculation) were found, herewith suggesting that cortical activation differences are indeed associated with the performance the task. In analogy to the neural efficiency concept of human intelligence which suggests a more strongly localized (i.e., efficient) cortical activation during task performance (i.e., recruitment of task-relevant cortices) in brighter as compared to less intelligent individuals (for a review see Neubauer and Fink, 2005), the ERD differences found here could also point to a

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more efficient use of the cortex in E versus I when engaged in performing the emotional face processing task. At first sight, the existence of E effects in the upper alpha band appears to run counter to our previous work, revealing E effects primarily in the lower alpha bands. However, as has been repeatedly demonstrated by Klimesch and colleagues (for review see Klimesch, 1999) the upper alpha band selectively responds to specific task demands, while activity in the lower alpha bands is believed to reflect more general task demands, such as expectancy or alertness. We also found intelligence-related (i.e., more task-related) effects more prominently in the upper alpha rather than the lower alpha bands (e.g., Neubauer et al., 2002; Grabner et al., 2004). Hence, in the context of emotional face processing we might then assume individual differences in E being more directly involved than in cognitive information processing, and, as a consequence, being more likely reflected in the (task-specific) upper alpha band. Very similar to our previous studies, differential effects of E during performance of the (cognitive) verbal and figural tasks (PPV and PPF) were exclusively restricted to the lower alpha bands, which are assumed of reflecting more unspecific (i.e., attentional) task demands. In the upper alpha band we found no E effects at all. As indicated in Fig. 4, the pattern of this interaction suggests that females were more likely to display the expected inverse E  activation relationship in the verbal, and males in the figural task—exactly in that domain they usually perform better (as it was also reflected in our behavioral data). This finding nicely parallels our findings of the influence of task modality and sex on the brain-IQ relationship (Neubauer et al., 2002), which we observed in the upper alpha band. However, given that specific post hoc comparisons of this interaction failed to reach statistical significance and given the fact that both sexes and E groups already differed in pre-stimulus reference power in the lower 1 alpha band (cf. Table 2), an interpretation of this finding – which awaits replication in further studies – should be treated cautiously. Nevertheless, the prominent role of E in the lower alpha frequency bands during cognitive information processing – as well as the absence of these effects in the task specific upper alpha band, respectively (see also Fink et al., 2005a; Fink and Neubauer, 2004; Fink et al., 2002) – could point to the more general role of lower alpha oscillations (which are possibly different for I versus E) in providing some kind of basic alertness, arousal or vigilance what is necessary in order to perform successfully on a given cognitive task. Topographical analyses of the ERD data show that in performing the PPE and PPG task, E related effects were more strongly related to left- than right-hemispheric regions of the cerebral cortex. This finding seems insofar surprising, as emotional information processing or in particular facial emotion processing is usually assigned to the right hemisphere (for a recent review see Posamentier and Abdi, 2003). This finding also contradicts empirical reports dealing with E group differences in lateralized hemispheric processing of emotion by Smith et al. (1995), who found that cortical activation differences between introverted and extraverted individuals

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were minimal in the left but substantial in the right hemisphere. One possible reason for this finding might be assumed in task procedure itself. Contrary to Smith et al. (1995) study, in which non-verbal human sounds (e.g., woman laughing or baby cooing) were used in order to minimize verbal processing, in the present study, participants were explicitly (verbally) told at the beginning of the task that in the subsequent test faces were shown, each of them expressing emotions such as ‘‘joy’’, ‘‘dolor’’, ‘‘surprise’’, ‘‘fear’’, ‘‘disgust’’ or ‘‘anger’’. In this connection, we might speculate that participants made use of some kind of verbal encoding or rehearsal strategies in judging the equivalence of the facial emotions. Some support for this presumption is provided by a recent study by KucharskaPietura et al. (2003), who compared left and right hemisphere damaged patients and healthy controls during performance of facial and vocal affect perception tasks of specific emotions. The authors conclude that - beneath the primacy of the right hemisphere in processing emotional stimuli - the left hemisphere could have a particular role in the perception of emotion conveyed through meaningful speech. Our data also corroborate this presumption to some extent: First, though the interaction between hemisphere, area and E during performance of the PPE/PPG task just failed to reach statistical significance ( p = .06), (the more communicative and talkative) extraverts (as compared to introverts) tended to display a lower cortical activation over left-hemispheric frontocentral and centroparietal, temporal cortices—regions that are known to be active during processing of speech or language (e.g., Blank et al., 2002). And secondly, if this presumption of verbal rehearsal involvement in performing the PPE task were true, the recruitment of speech based cortices should be more pronounced in the PPE than the PPG task. As separate analyses for the PPE as well as the PPG task reveal, the hemisphere by area and E group interaction was only significant in the PPE (F[2.75, 167.91] = 3.63, p < .05) but not in PPG task. However, whether this verbal rehearsal hypothesis could indeed be a possible explanation of the leftlateralization of E effects here, should be addressed more specifically in future studies. On the whole, the results of the present study clearly indicate that introverted and extraverted individuals differ in the usage of their brains when confronted with emotional face processing and cognitive tasks, herewith highlighting the role of individual differences in these domains. The more distinct association of E effects with either the lower alpha or the upper alpha bands depending on the kind of information processed further hints at possible functional differences between different alpha bands. Acknowledgments This research was supported by a grant from the Austrian Science Foundation (Fonds zur Fo¨rderung der wissenschaftlichen Forschung; P16393). The author wishes to express his large gratitude to Beate Staudt and Mathias Benedek for organizing and conducting the EEG test sessions and for analyzing the EEG data. Moreover, the help of Sabine Zierler in

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