The proportion of fixed interval trials to probe trials affects acquisition of the peak procedure fixed interval timing task

Abstract

A common procedure for studying the ability of animals to time is the peak procedure. With the peak procedure, animals are first trained on a fixed interval schedule (i.e., 30s). After the animals have been well trained on the fixed interval schedule, probe trials are introduced. On probe trials, the stimulus is presented longer (i.e., 90s) and the animal does not receive reinforcement for responding. When animals are first presented with probe trials responding remains flat following the point that reinforcement normally occurs on fixed interval trials. The descending slope that eventually emerges is acquired with experience with probe trials. The present experiments manipulated the percentage of probe trials compared to FI trials across groups of rats. It was hypothesized that the descending limb of peak responding would be acquired more quickly when there were many probe trials per session as this might facilitate extinction of responding beyond the interval that reinforcement normally occurs. It was found, however, that acquisition of peak responding occurred best when there were few probe trials per session.

Figure 1

Mean responses per 5 second bin across the last 5 sessions of training for each group as a function of time.

Figure 2

Mean responding across the 10 blocks of test (30 probe trials per block) for each group as a function of interval.

Figure 3

Mean points of peak responding as a function of group and block (top left graph). Mean maximum response rate as a function of group and block (top right graph). Mean ascending spread as a function of group and block (lower left graph). Mean descending spread as a function of group and block (lower right graph).

Figure 4

Mean responding for the first 10 sessions of probe trial training for each group as a function of interval.

Figure 5

Mean points of peak responding as a function of group and session (top left graph). Mean maximum response rate as a function of group and session (top right graph). Mean ascending spread as a function of group and session (lower left graph). Mean descending spread as a function of group and session (lower right graph).

Figure 6

Mean responding for each proportion of probe trial condition as a function of interval (top left graph). Mean ascending and descending spreads as a function of proportion of probe trials (top right graph). Mean peak times as a function of proportion of probe trials (bottom left graph). Mean maximum responding as a function of proportion of probe trials (bottom right graph).