Healthy volunteers were tested on 7-s and 17-s peak-interval timing procedures following d-amphetamine (20mg-oral), haloperidol (2mg-oral), and placebo treatments in order to assess the dopaminergic regulation of temporal processing. Individual differences were observed in the drug effects such that two different patterns of timing behavior emerged. In the first pattern, d-amphetamine produced proportional leftward shifts of the timing functions while haloperidol produced proportional rightward shifts. This symmetrical pattern of results suggests that clock speed is regulated by the effective level of dopamine, i.e., d-amphetamine increases clock speed and haloperidol decreases clock speed. The second pattern was the opposite of the first pattern and was revealed by d-amphetamine producing proportional rightward shifts of the timing functions while haloperidol produced no reliable effect. This asymmetrical pattern of results is consistent with an explanation in which attention toward the stimulant-induced euphoria produced by d-amphetamine diminishes the attentional resources available for temporal processing, thereby diluting any drug-induced changes in clock speed. The result of increased competition and time-sharing between these two dimensions (e.g., attention towards feelings of euphoria versus attention towards the passage of time) leads to the underestimation/overproduction of temporal intervals. Interestingly, participants that displayed the 'clock-speed' pattern liked d-amphetamine significantly less than participants that displayed the 'attention' pattern and were more variable in a simple reaction time task than other participants. These results suggest that individuals with a higher degree of sensitivity to time are also more sensitive to their feelings of stimulant-induced euphoria and drug liking-suggesting that internal clock and reward pathways share common dopaminergic pathways.
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