Some temporal properties of local contrast
Pigeons received variable-interval food reinforcement for key pecking during one line-orientation stimulus while key pecking during another line orientation was extinguished (mult VI EXT); the duration of the extinction component was either fixed or variable. When the duration of the extinction stimulus was variable, stable response rate was highest during the initial part of the VI component (positive local contrast) and lowest early in the extinction component (negative local contrast). Early in training the magnitude of positive local contrast was directly related to the duration of the preceding extinction stimulus, but this dependence eventually disappeared. When the duration of the preceding extinction stimulus was constant, positive local contrast disappeared, but was reinstated by a long time-out period introduced part way through each experimental session. These data suggest that local contrast effects represent a temporary disequilibrium between competing activities and the instrumental response.
