International Journal of Comparative Psychology

Organisms are constantly extracting information from the temporal structure of the environment, which allows them to select appropriate actions and predict impending changes.  Several lines of research have suggested that interval timing is modulated by the dopaminergic system.  It has been proposed that higher levels of dopamine cause an internal clock to speed up, whereas less dopamine causes a deceleration of the clock.  In most experiments the subjects are first trained to perform a timing task while drug free.  Consequently, most of what is known about the influence of dopaminergic modulation of timing is on well-established timing performance.  In the current study the impact of altered DA on the acquisition of temporal control was the focal question.  Thirty male Sprague-Dawley rats were distributed randomly into three different groups (haloperidol, d-amphetamine or vehicle).  Each animal received an injection 15 min prior to the start of every session from the beginning of interval training.  The subjects were trained in a Fixed Interval (FI) 16s schedule followed by training on a peak procedure in which 64s non-reinforced peak trials were intermixed with FI trials.  In a final test session all subjects were given vehicle injections and 10 consecutive non-reinforced peak trials to see if training under drug conditions altered the encoding of time.  The current study suggests that administration of drugs that modulate dopamine do not alter the encoding temporal durations but do acutely affect the initiation of responding.