An investigation of contrast effects, their relationships and a theoretical resolution

Behavioral interactions associated with differentially reinforced stimuli, which participate in the formation of generalization gradients, are investigated.

Local contrast effects have been found to be one of the major forms of interaction in multiple schedules. A component in a multiple schedule may occur in time between two other components and may be locally influenced by either the preceding or following component.

To investigate the influence of surrounding components, subjects were exposed to a simple three component multiple schedule and local contrast due to the preceding component was found early in training, with some indication that local contrast due to the following component appeared after prolonged training. Such backward-acting contrast was less reliable and less clear than was forward-acting local contrast.

It has been suggested that local contrast may be the precipitating cause of behavioral contrast, an overall response rate interaction which can persist after local contrast has disappeared. To investigate the relation of local to behavioral contrast, a session-by-session measure of each type of contrast was made and their correlation over the course of discrimination training was studied. It was found that the correlation of local contrast with behavioral contrast decreased over discrimination training, although behavioral contrast increased.

Three plausible theories have been advanced to account for local contrast. The two most clearly defined of these, additivity and response competition theories, were compared by examining the effect of the duration of a preceding S+ on contrast effects in a following S-. An adaptation of additivity theory, which applies the effects of signtracking movements, was able to account for two features of the data where behavioral competition theory could not.

The assumptions of the sign-tracking theory were investigated further by taking a measure of sign-tracking movements during a successive discrimination in which the duration of a preceding S- component was shown to have an effect on a following S+. The pattern of movement by the subjects accounted for the duration effect found.