Die dopaminerge Verankerung der Extraversion: Mechanismen und EEG-Indikatoren

In their psychobiological theory Depue and Collins (1999) linked the agency facet of Extraversion (i.e. assertiveness, dominance, ambition, positive emotionality) to functional properties of the mesocorticolimbic dopamine system, creating individual differences in incentive motivation, positive emotion and cognitive - behavioral flexibility. Although this theory is highly cited, there is still a lack of data probing the main assumptions of this theory. A lack of economic and noninvasive psychophysiological indices and well-established behavioral measures sensitive to both extraversion and manipulations of dopaminergic activity largely contributed to this situation. However, a recent pharmacological study by Wacker, Chavanon and Stemmler (2006) identified posterior versus frontal theta activity (4-8 Hz) in the resting electroencephalogram (EEG) as a possible biological marker for agentic extraversion, which was also sensitive to pharmacological manipulation of central dopamine activity using the selective D2-antagonist Sulpiride (200 mg). Under placebo highly extraverted individuals displayed more posterior vs. anterior theta than introverts, and this was completely reversed under Sulpiride. Besides this promising EEG marker, several lines of research pointed to the possibility that individual differences in working memory functioning and extraversion might be related due to partially overlapping dopaminergic neurobiology. Hence, the present thesis shed light on working memory functioning (study 1) and posterior vs. anterior theta activity (studies 2 and 3). Study 1 aimed at probing the cognitive - behavioral flexibility aspects associated with the concept of agentic extraversion, by pointing to the possible dopaminergic link between agentic Extraversion and working memory functioning. To put this idea to an initial test, the study randomly assigned forty male participants – either extremely high or extremely low in agentic extraversion – to one of two substance groups (placebo vs. 200 mg Sulpiride; double-blind). Participants completed load-graded working memory tasks (n-back tasks) during which anterior vs. posterior EEG activity in the Alpha 1 band (8-10.25 Hz) and reaction times were assessed. The results support the proposal of a dopaminergic basis for agentic extraversion and the suggestion of a dopaminergic link between agentic extraversion and working memory. Study 2 focused on the posterior versus frontal theta activity in the resting EEG as a possible biological marker for the dopaminergic aspects of agentic extraversion. Although this index seems to be consistently related to agentic extraversion the neural sources remained still unanswered. Neuroimaging and EEG research both point to the anterior cingulate cortex (ACC) as a possible source. In order to test this hypothesis resting EEG of 78 healthy, male participants extremely high or low in agentic extraversion was submitted to the low-resolution electromagnetic tomography algorithm. We localized the sources of extraversion-dependent intracerebral theta activity within rostral subdivisions of the ACC. The posterior versus anterior index and theta current density within the rostral ACC were significantly correlated (r = -.52). Study 3 aimed to replicate and extend our work on the posterior vs. anterior theta index by specifying the mechanisms underlying extraversion-dependent reversal effects observed for Sulpiride (see Wacker et al., 2006 above). Three possible mechanisms were identified and probed: (1) inverted U-shaped model, (2) individual differences in time courses of drug effects, and (3) individual differences in pre- versus postsynaptic receptor effects. To this end, we measured the EEG intermittently for about five hours in 80 healthy male volunteers extremely high or low in agentic extraversion who had received either placebo or 50 mg, 200 mg, or 400 mg Sulpiride. Conceptually replicating our earlier findings, we again observed (1) more posterior (versus anterior) theta activity in individuals high versus low in aE under placebo, and (2) aE-related differences in drug-induced changes under Sulpiride. The latter findings support a model postulating stronger post- versus presynaptic receptor effects in individuals high in agentic extraversion compared to those scoring low in agentic extraversion. Overall, the present thesis support the idea that agentic extraversion is based on brain dopamine and especially qualifies the posterior vs. anterior theta as a reliable, dose-sensitive and direct indicator of D2 dopaminergic functioning. This simple measure mirrors -at least partly - the functions associated with rostral ACC theta activity. Examples for application in different areas of research are provided.