Effects of dl-amphetamine under concurrent VI DRL reinforcement

Three adult, food-deprived rats were given IP injections of dl-amphetamine sulfate under DRL and concurrent VI DRL reinforcement schedules. The drug results were as follows.(1) The IRT distributions of DRL responses shifted to the left, but some temporal discrimination remained. (2) The IRT distributions of VI responses shifted slightly to the left. (3) The distinguishing characteristics of VI and DRL IRT distributions were preserved. (4) The frequency distribution of number of VI responses between two consecutive DRL responses was relatively unaffected. (5) Over-all response rates on the two components of the concurrent schedules increased more or less proportionately. These findings imply that the primary behavioral effect of dl-amphetamine was a motor excitatory one. The drug's disruption of timing behavior was not due to a derangement of internal timing mechanisms, nor to interference with the topography or pattern of behavior. Rather, it might be a secondary result of the accelerated emission of overt behavior patterns mediating the temporal spacing of DRL bar presses.