Delay discounting refers to the fact that an immediate reward is valued more than the same reward if it occurs some time in the future. To examine the neural substrates underlying this process, we studied 13 healthy volunteers who repeatedly had to decide between an immediate and parametrically varied delayed hypothetical reward using a delay discounting task during event-related functional magnetic resonance imaging. Subject's preference judgments resulted in different discounting slopes for shorter (<1 year) and for longer (> or =1 year) delays. Neural activation associated with the shorter delays relative to the longer delays was associated with increased activation in the head of the left caudate nucleus and putamen. When individuals selected the delayed relative to the immediate reward, a strong activation was found in bilateral posterior insular cortex. Several brain areas including the left caudate nucleus showed a correlation between the behaviorally determined discounting and brain activation for the contrast of intervals with delays <1 and > or =1 year. These results suggest that (1) the posterior insula, which is a critical component of the decision-making neural network, is involved in delaying gratification and (2) the degree of neural activation in the striatum, which plays a fundamental role in reward prediction and in time estimation, may code for the time delay.

Stimulant-dependent individuals (SDI) have abnormal brain metabolism and structural changes involving dopaminergic target areas important for the processing of time. These individuals are also more impulsive and impaired in working memory and attention. The current study tested whether SDI show altered temporal processing in relation to impulsivity or impaired prefrontal cortex functioning. We employed a series of timing tasks aimed to examine time processing from the milliseconds to multiple seconds range and assessed cognitive function in 15 male SDI and 15 stimulant-naïve individuals. A mediation analysis determined the degree to which impulsivity or executive dysfunctions contributed to group differences in time processing. SDI showed several abnormal time processing characteristics. SDI needed larger time differences for effective duration discrimination, particularly for intervals of around 1 sec. SDI also accelerated finger tapping during a continuation period after a 1 Hz pacing stimulus was removed. In addition, SDI overestimated the duration of a relatively long time interval, an effect which was attributable to higher impulsivity. Taken together, these data show for the first time that SDI exhibit altered time processing in several domains, one which can be explained by increased impulsivity. Altered time processing in SDI could explain why SDI have difficulty delaying gratification.

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Behavioral studies suggest that visual attention is biased toward stimuli in the region of space near the palm of the hand, but it is unclear whether this effect is universal or selective for goal/task-related stimuli. We examined event-related potentials (ERPs) using a visual detection task in which the hand was placed near or kept far from target and non-target stimuli that were matched for frequency and visual features to avoid confounding factors. Focusing on attention-sensitive ERP components, we found that P3 (350–450 ms) amplitudes were increased for Hand Near conditions for targets only, demonstrating a selective effect consistent with the P3's cross-modal and task-relevance influences. An N1 variant implicated in visuo-tactile integration (central Nd1; 120–190 ms) showed similar target-specific effects. P1 (80–110 ms) effects for target stimuli were also apparent, but may have applied to non-targets as well, which would be consistent with the P1's association with early, pre-categorical increases in sensory gain. Collectively, these findings suggest that by the time stimuli are categorized as relevant/irrelevant for action, the proprioceptive effects of the hand on visual attention are selective for goal/task-related stimuli. At the same time, hand proximity appears to bias attention early, starting with a facilitation of processing for perhaps any visual stimuli near the hand, and continuing with enhancements that are selective to those stimuli categorized as task-relevant.