The neurotransmitters dopamine and adenosine play a key role in motor control, with drugs affecting these systems strongly stimulating motor processes. The issue we address here is the nature of the cognitive stimulating properties of psychostimulants. Expectation of a stimulus in time and space results in anticipatory preparation of a response, improving both speed and accuracy. Evidence for this comes from the observation that response speed and accuracy is tightly modulated by the prior probability of the stimulus. We were interested in establishing whether psychostimulation modulated the calculation of stimulus prior probability and/or the expression of this calculation. We manipulated dopamine (by administration of amphetamine) and adenosine (using the adenosine A2A antagonist, KW6002). Reaction time and accuracy of rats was measured using a two-choice spatial discrimination task. We manipulated the spatial probability of a stimulus as a function of time, to tease apart the effects on motor preparation of stimulus prior probability, as opposed to absolute preparation time. Reaction times were determined by the time preceding a stimulus (absolute preparation time) as well as prior likelihood of the stimulus (stimulus prior probability). Movement times and anticipatory responses were particularly strongly influenced by both spatial and temporal probability of stimuli, irrespective of absolute preparation time. Both amphetamine and KW6002 stimulated motor activity, as expected. Amphetamine also enhanced the effects of both absolute preparation time and prior probability, whereas KW6002 particularly enhanced the effect of prior probability. The stimulatory effects of psychostimulants are on cognitive processes, implicating the dopamine system, particularly via the dopamine D2 receptor, in the translation of stimulus likelihood into expectation and anticipation for the determination of motor output.
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