Intertrial pellets influence the acquisition and expression of timed appetitive responding in rats

Two experiments examined temporally based changes in the conditioned magazine-entries of rats when a target food pellet arrived at a fixed time before the termination of a conditioned stimulus. Both experiments found that increasing the rate of intertrial pellets systematically interfered with the rate of acquisition. When intertrial pellets were featured in acquisition, their omission on test trials also resulted in a very poorly expressed conditioned response. On test trials preceded by intertrial pellets, however, peak times in groups trained with intertrial pellets were shifted in a rightward direction under a long but not short interstimulus interval. This rightward shift was accompanied by a smaller standard deviation. Our key findings are best accommodated by real-time associative models.

► Timing of food pellet delivery by rats was affected by extra random intertrial food pellet deliveries.
► Rates of acquisition of timed responding were reduced.
► Responding on test trials peaked after the learned arrival time, provided the interval was long rather than short.
► Results are well modeled by theories of association formation with both real-time and occasion-setting components.