When humans are offered the choice between rewards available at different points in time, the relative values of the options
are discounted according to their expected delays until delivery. Using functional magnetic resonance imaging, we examined
the neural correlates of time discounting while subjects made a series of choices between monetary reward options that varied
by delay to delivery. We
... [Show full abstract] demonstrate that two separate systems are involved in such decisions. Parts of the limbic system
associated with the midbrain dopamine system, including paralimbic cortex, are preferentially activated by decisions involving
immediately available rewards. In contrast, regions of the lateral prefrontal cortex and posterior parietal cortex are engaged
uniformly by intertemporal choices irrespective of delay. Furthermore, the relative engagement of the two systems is directly
associated with subjects' choices, with greater relative fronto-parietal activity when subjects choose longer term options.