Session Duration and the Vi Response Function: Within‐session Prospective and Retrospective Effects

Abstract

Two experiments examined the effects of session duration on responding during simple variable-interval schedules. In Experiment 1, rats were exposed to a series of simple variable-interval schedules differing in both session duration (10 min or 30 min) and scheduled reinforcement rate (7.5 s, 15 s, 30 s, and 480 s). The functions relating response rate to reinforcement rate were predominantly monotonic for the short (10-min) sessions but were predominantly bitonic for the long (30-min) sessions, when data from the entire session were considered. Examination of responding within sessions suggested that differences in the whole-session data were produced by a combination of prospective processes (i.e., processes based on events scheduled to occur later in the session) and retrospective processes (i.e., processes based on events that had already occurred in the session). In Experiment 2, rats were exposed to a modified discrimination procedure in which pellet flavor (standard or banana) predicted session duration (10 min or 30 min). All rats came to respond faster during the short (10-min) sessions than during the first 10 min of the long sessions. As in Experiment 1, the results seemed to reflect the simultaneous operation of both prospective and retrospective processes. The results shed light on the recent controversy over the form of the variable-interval response function by identifying one variable (session duration) and two types of processes (prospective and retrospective) that influence responding on these schedules.

Within‐session Changes in the Vi Response Function: Separating Food Density From Elapsed Session Time

“…Each rat was exposed to a series of four different VI schedules (VI 7.5 s, VI 15 s, VI 30 s, and VI 480 s), the same series of schedules used by Dougan et al (1993). Each schedule was in effect for 24 consecutive days, with the order of schedules counterbalanced across animals to avoid systematic order effects.…”

“…Instead, new experimental analyses that systematically isolate factors responsible for variations in the function are needed. Dougan, Kuh, and Vink (1993) have recently reported such an analysis. They exposed rats to VI schedules with both relatively long (30-min) and relatively short (10-min) sessions.…”

“…However, the degree of bitonicity changed within the 30-min sessions, progressing from monotonic in the first 10-min block, to slightly bitonic in the second 10-min block, to highly bitonic in the third 10-min block. Dougan et al (1993) concluded that session duration is an important variable in determining the form of the VI response function. They were unable to determine, however, which of several processes might be responsible for the observed within-session changes in the function.…”

“…Two characteristics of satiation effects might help to explain the results of Dougan et al (1993). First, because such effects can be caused by the consumption of food early in a session, they would be expected to develop Roll (1993) found late-session decreases in response rate in conditions in which reinforcers were weak or even absent.…”

“…Some of these processes are directly elicited by the presentation of reinforcers (e.g., reinstatement of memory, accumulation of reinforcer-mediated arousal, priming); some of these effects are directly caused by the passage of time (e.g., anticipation of the end of the session); some of these effects are correlated with the passage of time in the session, but are not directly caused by time passage (e.g., waxing or waning interference from a competing response, recovery from handling routines, recovery from or development of information overload, establishment of a response set); and some of these effects are caused by engaging in the operant response (e.g., fatigue). Because all of these effects can potentially change within sessions, it is possible that any or all of them, alone or in combination, might be responsible for the within-session effects observed by Dougan et al (1993). Research examining the contribution of these effects is clearly necessary.…”