Stimulus control in fixed interfood intervals

The control exerted by various portions of fixed-time and fixed-interval schedules was assessed with a trace-conditioning procedure. The intervals were segmented into 10 bins. In all but 1 of those bins, the stimuli were presented in different random orders on each trial. In 1 bin, the stimulus was the same on each trial. The position of this trace stimulus was varied across phases. The results indicated that a trace stimulus can come to control behavior and that differential control can extend to even the second tenth of an interfood interval. The results were interpreted as indicating that traditional explanations of the rate loss in earlier portions of an interfood interval are inadequate and that models such as Palya's (1993) bipolar model or Miller and Schachtman's (1985) comparator model may provide a principled framework with which to understand within-trial effects.

A behavior systems view of the organization of multiple responses during a partially or continuously reinforced interfood clock

We examined how a 50% Pavlovian partial reinforcement (PRF) schedule, versus a 100% continuous reinforcement (CRF) schedule, altered the asymptotic amount and distribution of focal and general search behavior in rats during 48-sec trials with and without a four-segment interfood clock (S1-S2-S3-S4-US). Under CRF, but not PRF, average asymptotic focal search (nosing in the feeder) increased across the last two clock segments (S3 and S4), and more for the clock group than the no-clock group. Locomotor general search peaked in the second clock segment (S2) for the CRF-clock and CRF-no-clock groups and in S3 for the PRF-clock groups. Furthermore, the ratio of general search to maximum focal search was higher for the PRF-clock group than for the CRF-clock group. This pattern of results supports the view that predictable reward presentations temporally organize search states and related responses between food presentations. The relative expression of these responses varies with the predictability and proximity of reward and is more sharply defined in the presence of a clock.