Neurobiological bases of inter-temporal choice including DD have been relatively well investigated in animal models. These studies (reviewed in Dalley et al. 2008) strongly implicate the orbitofrontal cortex and the core of the nucleus accumbens as key neural substrates underlying the choice between immediate and delayed reward and suggest that brain regions underlying DD are distinct from those underlying response inhibition, another aspect of impulsivity. More recently, human studies using neuroimaging techniques have shown that the subjective value of a monetary reward is represented by activity in the ventral striatum, the medial prefrontal cortex, and the posterior cingulate cortex (reviewed in Cardinal 2006), although the extent to which activity in this functional network differentiates between immediate and delayed reward is a matter of ongoing debate. In one neuroimaging study, choices of an immediate reward were associated with greater activity in these brain regions, whereas choices of delayed reward were associated with greater activity in the lateral prefrontal and posterior parietal cortical regions (McClure et al. 2004). These findings provide support for a “competing brain systems” hypothesis, according to which the
