The current results reveal a highly significant association between the level of incentive motivation (as measured by the MPQ Achievement subscale) and the pattern of asymmetry in D2 receptor availability in the putamen, such that greater positive incentive motivation is associated with higher receptor availability in the left relative to the right hemisphere. To the best of our knowledge, this is the first report relating variability in a personality trait to the direction and degree of asymmetry of D2 receptor availability in healthy individuals.
There is ample evidence suggesting that in animals inter-individuals differences in dopaminergic asymmetry covary with, or predict, individual differences in spatial behavior, stress reactivity and drug sensitivity (Carlson & Glick, 1989
). In humans, a recent review reported a population bias of higher D2 receptor binding in the right as compared to left striatum in healthy individuals (Larisch et al., 1998
). This conclusion is based on group averages. However, careful examination of studies that report individual data reveals that in each such study, some subjects had higher values in the right striatum, whereas others showed left-side preference. Thus, notwithstanding the population bias towards higher values of D2 receptor binding in the right striatum, there are considerable individual differences, not only in the direction, but also in the degree of asymmetry. Our findings suggest that the direction and degree of asymmetry in striatal dopaminergic markers may index a behavioral trait of the individual, namely incentive motivation.
As discussed in the Introduction, there is a considerable body of literature suggesting that left (relative to right) frontal and anterior temporal activation (as reflected in EEG power) is associated with positive, approach-related emotion (see Davidson, 2004
, for review). The frontal lobes are extensively connected with the striatum through a series of parallel frontal-subcortical circuits (Alexander et al., 1986
). Dopaminergic neurons from the substantia nigra project to the striatum and affect all frontal–subcortical functions, allowing interaction between the emotional input and motor activity, cognition and motivation (Tekin & Cummings, 2002
). Volkow et al. (2001)
reported a significant correlation between D2 receptor availability in the putamen and FDG metabolism in the orbitofrontal cortex and in the anterior cingulate gyrus. This association could reflect dopamine-mediated striatal regulation of orbitofrontal activity by means of striato-thalamo-cortical pathways (Haber et al., 1995
). Thus, the asymmetry in D2 receptor availability in the putamen may contribute to the asymmetric pattern of activation in the frontal lobes, mentioned above. Based on the reported association between approach behavior and relatively greater left frontal activation level, we expected higher incentive motivation to be associated with a corresponding (L>R) dopaminergic asymmetry in the striatum. Our current findings appear to support this prediction. However, it is the difficult to determine whether the D2 receptor availability represents asymmetry of receptor density or of levels of endogenous dopamine, since [11C]raclopride’s binding is sensitive to competition with endogenous dopamine. Further studies are needed in order to clarify this point. Since [11C]raclopride has a greater sensitivity for detecting binding in the putamen, predominantly to D2 receptors, than in the globus pallidum, where binding is predominantly to D3 receptors (Graff Guerrero et al., 2007
), the current results pertain mostly to asymmetries in dorsal putamen and caudate. Nevertheless, the importance of our findings lies in highlighting the association between asymmetry in dopamine-driven brain circuits and individual differences in motivational and reward-related processes, as reflected in a personality trait representing positive incentive motivation. Another limitation is the small sample which consisted mostly of males, thus limiting the generalizabiliy of our findings.
The current results may have important implications not only for mapping the neurobiological underpinnings of personality in healthy individuals, but also for understanding the pathophysiological correlates of abnormal behavior. Pathological manifestations of motivated behavior occur in a number of neuropsychiatric syndromes (including addiction, depression, schizophrenia and parkinson’s disease). Indeed, reduced motivation and decreased novelty seeking have been described in Parkinson’s disease and this has been shown to be related to the asymmetry of dopamine deficit (Menza et al., 1995
; Tomer & Aharon-Peretz, 2004
). Future studies may clarify the role of asymmetric dysregulation of dopaminergic neurotransmission in mediating behavioral pathology in these disorders.