The role of trial tracking on rats' successive delayed matching-to-sample modality performance.

The present study tested a trial tracking hypothesis that rats' matching accuracy to sample modality (onset of chamber lights or a tone) in a successive delayed matching task is, in part, determined by their ability to determine whether they are within or between trials during an interstimulus interval. In this discrimination rats had to respond to an initial sample stimulus to obtain a second test stimulus, S2. Responding to S2 was reinforced only when it matched S1. The interstimulus intervals between S1 and S2, the retention interval, and between S2 and the next S1, the intertrial interval, were varied. The prediction that more accurate trial tracking would be maintained by rats that were only reinforced for correctly responding S2 (S1 Nonreinforcement group) than by rats that were reinforced for responding to each S1 as well as to its matching S2 (S1 Reinforcement group) was supported. Increasing retention intervals caused steeper declines in matching accuracy in the S1 Reinforcement group. Although matching accuracy to the tone S1 was generally but slightly reduced by decreasing intertrial intervals from 24 s to 12 s or 6 s, this effect was confined to 1-s and 5-s retention intervals only in the S1 Reinforcement group. A similar prediction based on differences in the required number of presses to each stimulus event was not supported. Also noteworthy were results showing superior matching accuracy to the light S1 than to the tone S1. While increasing retention intervals to 20 s eliminated this difference in the S1 Nonreinforcement group, increasing retention intervals to 5 s or more brought about this effect in the S1 Reinforcement group.