Impulsivity and neural systems of rewards
Martin, Laura E.
Potts, Geoffrey F.
Doctor of Philosophy
Behavioral studies of decision-making in impulsive individuals demonstrate biases for immediate rewards. Decision-making includes evaluating motivational values of both options and actions. In the current studies, event related potentials (ERPs) and functional magnetic imaging (fMRI) were used to assess the temporal and spatial properties of the ventral tegmental (VTA) dopamine (DA) reward system during item evaluation and action/outcome-monitoring in impulsivity. A passive reward evaluation and an active action/outcome-monitoring task were used to test a model predicting that dysregulation of the VTA DA reward system leads to overestimations of reward value in the prefrontal cortex and underestimations of punishments in the anterior cingulate cortex (ACC) during reward evaluation and action/outcome-monitoring respectively. Passive reward evaluation ERP results showed a prefrontal P2a ERP index of reward expectation that did not differ with impulsivity. FMRI reward evaluation results indicated that the basal ganglia and left superior frontal cortex were sensitive to rewarding outcomes, whereas the medial ACC was sensitive to non-rewarding outcomes. Left superior frontal cortex fMRI activation showed greater differentiation between better than expected and worse than expected outcomes among high impulsive participants. Action/outcome-monitoring ERP results showed an ERN ERP index the ACC in the action/outcome-monitoring with the greatest response on error trials that led to punishments. The ERN was also larger among high impulsives, compared to the low impulsives, when errors led to missed rewards. The fMRI results showed that the basal ganglia responded like the ERN to errors resulting in punishments. The ACC and right middle frontal regions showed greater activation to correct responses, and a right ACC region showed larger activations when correct responses led to rewards. However, these activations did not differ with impulsivity. In conclusion, the current results suggest differential responsiveness of the mesotelencephalic reward system in impulsive individuals which may lead to reward hypersensitivity but not punishment hyposensitivity during item evaluation and action/outcome-monitoring among high impulsive participants, as seen in the left superior frontal fMRI activation during item evaluation and in the ACC ERP activity during outcome-monitoring. Differences between the ERP and fMRI results may reflect the different physiological processes that give rise to the signals.