"The personality dimensions Extraversion and Neuroticism seem associated with differences in central nervous system function. We used positron emission tomographic (PET) measures of regional cerebral blood flow (rCBF) to investigate central neural differences in Extraversion and Neuroticism, as determined by the Swedish version of the NEO PI-R personality inventory. A median-split defined extraverts from introverts and relatively more and less neurotic subjects. The relative rCBF in the caudate nucleus and the putamen was higher in introverts than extraverts. In introverts, but not extraverts, activity in the putamen was left-lateralized. These areas have high concentrations of dopamine terminals, implicating a dopaminergic basis for individual differences in Extraversion. As a function of Extraversion rCBF did not differ in the prefrontal, orbitofrontal, temporopolar, cingulate, primary visual cortex, the thalamus and the hypothalamus. Thus, individual differences in Extraversion correlate to subcortical rather than cortical brain regions. No rCBF differences were related to Neuroticism. Because introverted subjects displayed an increased neuronal activity in brain regions previously associated with learning, motor and vigilance control, since those behaviors in part define Introversion, a subcortical neostriatal and possibly dopaminergic, rather than a solely cortical correlate of the personality dimension Extraversion is supported."