Fixed-interval (F1) schedules have been used extensively to study timing abilities. In F1 schedules, animals typically show higher response rates immediately after nonreinforced (N) cycles rather than reinforced (R) cycles (the reinforcement-omission effect), and they exhibit the highest rate approximately at the time when the reinforcer is scheduled to occur (peak performance). The present experiments were designed to determine the extent to which factors other than timing contribute significantly to these two learning phenomena. Pigeons were trained in an F116-sec schedule in which half the cycles were R and half were N. When successive cycles were separated by a 2-sec interval, responding early in the F1 interval was higher after an N cycle than after an R cycle. This reinforcementomission effect was eliminated when the interval between cycles was increased to 12 sec, because of an increase in performance after R cycles. In addition, timing of the 16-sec interval was assessed by interpolating 32-sec test cycles (all N cycles) at two rate5-€ither 1 test cycle every other session, or 25 test cycles per session. Peak performance, presumably indexing the animal's ability to time the 16-sec interval, emerged only with 25 test cycles per day, but not with 1 test cycle every other day, despite extensive training with the target, 16-sec-Iong interval. These results suggest that transient demotivation and time-based discrimination contribute significantly to the reinforcement-omission effect and peak performance, respectively.
84Arranging a regular distribution of reinforcers in time, as in a fixed-interval (FI) schedule of reinforcement, is a procedure that has been used extensively to study the timing abilities of organisms (Lejeune & Wearden, 1991). It has long been recognized that unexpected nonreinforced (N) cycles are followed by higher response rates than reinforced (R) cycles are (Staddon & Innis, 1966). This phenomenon, referred to as the reinforcementomission effect, has been linked to a variety of mechanisms, including the role of reinforcement in resetting the internal clock used to estimate the duration of the interval (Mellon, Leak, Fairhurst, & Gibbon, 1995). However, as Mellon et al. noted, when the reinforcement and other associated cues are omitted, animals face what is essentially a longer FI cycle. This situation approaches the conditions of the peak procedure, in which an unusually long test cycle is interpolated with shorter training cycles to assess the animal's ability to estimate the duration of the short interval. Animals exhibit peak performance-that is, a maximum response rate in the long test cycle at approximately the time within the interval when reinforcement is usually scheduled to occur (S. Roberts, 1981). In the present experiments, we examined the contribution of factors other than timing to these learning phenomena.The research reported in this paper was partially supported by TCU/RF Grant 5-23832 to M.R.P. Requests for reprints may be sent to M. R. Papini, Department of Psychology...