The role of stimulus comparison in perceptual learning: an investigation with the domestic chick

In two experiments an imprinting procedure was used to familiarize chicks with two stimuli, A and B, that subsequently served as the discriminanda in a simultaneous discrimination. On the first day of each experiment, subjects either received presentations of A and B that were intermixed within a session (mixed exposure) or presentations of A in one session and of B in another (separate exposure). For half of the subjects in each of the exposure conditions, A and B differed in both colour and form; for the remainder A and B differed in form alone. On the second day of the experiments, the chicks were placed into a cool test apparatus and given training in which approaching A was rewarded by the delivery of a stream of warm air, but approaching B was not. Acquisition of this discrimination was more rapid when A and B differed in two respects than when they differed in form alone. When A and B differed in both colour and form, the heat-reinforced discrimination was acquired more rapidly after separate exposure than after mixed exposure; but when A and B differed in form alone, discrimination learning was more rapid following mixed exposure than separate exposure. The latter finding, that the opportunity to compare stimuli differing in only one dimension facilitates subsequent discrimination learning, is consistent with earlier suggestions (Gibson, 1969) regarding the conditions that promote perceptual learning.