Operant Discrimination, Neutralization, and Reversal with Variations in SΔ Exposure

Abstract

Pigeons were trained on a series of red-green discrimination reversals. Each discrimination was preceded by a neutralization treatment in which the pecking response was reinforced equally often in both stimuli. During discrimination, pecking in Sd was reinforced with food according to a 90-sec. variable interval (VI) schedule. Ten 15-min. exposures of Sd occurred in each discrimination session. Each discrimination was continued to a criterion. Three groups of 2 birds differed in the number and duration of SΔ exposures in each discrimination session. A fourth group had a history of repeated extinction and VI reinforcement in the presence of both discriminative stimuli. Under both neutralization and discrimination stimulus exposures were isolated by timeouts. Both the session duration and the temporal distribution of periods of reinforcement availability were held constant over all conditions. An increase in the rate of formation of successive reversals was obtained only with the shortest SΔ exposure times (equal to the Sd exposure). This increase, over 8 successive discriminations, was of small magnitude. Longer SΔ exposures led to more rapid discrimination throughout a series without increases in the later discriminations. A previous history of extinction and reconditioning in the presence of both stimuli was without effect. These data raise the question of the boundary conditions for the commonly observed phenomenon of increased speed of discrimination in a series of reversals. Procedural variables implicated in the control of this phenomenon are discussed.