Characterizing the effects of d-amphetamine on temporal discrimination.

Although the effects of dopamine agonists on temporal discrimination have been widely studied, conclusions as to their behavioral and neurophysiological effects are difficult due to a number of discrepant findings in the literature. This study examined whether a previously unexplored procedural difference could account for some of these discrepancies. In three experiments, pigeons categorized the duration of temporal samples during two variants of the interval-bisection procedure. In the position variant, responses to a side key that corresponded to the sample duration produced food. In the color-matching variant, responses to the key color that corresponded to the sample duration produced food. Across experiments, d-amphetamine produced a general disruption of temporal discrimination not accompanied by over or underestimation of time in both procedures. This effect occurred whether pigeons were exposed to both procedures within session (Experiment 1) or across conditions (Experiments 2 and 3) and whether the pigeons were drug experienced (Experiments 1 and 2) or naïve (Experiment 3). Analyses of the cumulative normal functions fitted to the proportion-long response data indicated that d-amphetamine produced its effects by selectively decreasing stimulus control, rather than by affecting timing.