Abstract
Humans and animals prefer immediate over delayed rewards (delay discounting). This preference for smaller-but-sooner over larger-but-later rewards shows substantial interindividual variability and has been considered as an index of impulsivity. Previous studies have reported that decreased central serotonin levels or lesions of specific parts of cortico- striatal circuitry (e. g. ventral striatum) causes steep rate of delay discounting and consequent impulsive action selection. Recently, Tanaka et al. (2007) found that a graded map of delay discount rate in the striatum and insula using the functional magnetic resonance imaging (fMRI) ; activities in the ventral striatum and anterior insula were correlated with impulsive reward prediction, while those in the dorsal caudate and posterior insula were correlated with deliberate reward prediction. In addition, they showed that the correlated activity in the ventral striatum was dominant at low central serotonin level. These results facilitated our understanding of neural substrates of impulsivity.
Although cumulative evidence has reported dysfunction in serotonergic system or cortico-striatal circuitry in various psychiatric disorders, the relationships between clinical symptoms and these biological backgrounds remain unclear. We suggest the possibility to construct the comprehensive model through the serotonergic dysfunction and related alteration of cotrico-striatal circuitry and impulsive action selection.
