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 choice of immediate versus delayed reward depends on the competition between two distinct neural systems, the mainly limbic and paralimbic “impulsive” system and the cortical fronto-parietal “reflective” system (Bechara 2005; Bickel et al. 2007). In contrast, another neuroimaging study provided evidence that both immediate and delayed rewards are represented within a single reward valuation network that tracks the subjective value of possible rewards during choice (Kable and Glimcher 2007). Importantly, a neurometric discount function reflecting tradeoffs between amount and delay showed a good match with the psychometric discount function captured by behavioral tradeoffs between these variables (Kable and Glimcher 2007).
