THE ROLE OF STIMULUS COMPARISON IN PERCEPTUAL-LEARNING - AN INVESTIGATION WITH THE DOMESTIC CHICK

In two experiments an imprinting procedure was used to familiarize chi cks with two stimuli, A and B, that subsequently served as the discrim inanda in a simultaneous discrimination. On the first day of each expe riment, subjects either received presentations of A and B that were in termixed within a session (mixed exposure) or presentations of A in on e session and of B in another (separate exposure). For half of the sub jects in each of the exposure conditions, A and B differed in both col our 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 tes t apparatus and given training in which approaching A was rewarded by the delivery of a stream of warm air, but approaching B was not. Acqui sition 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 diffe red in both colour and form, the heat-reinforced discrimination was ac quired 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 on e dimension facilitates subsequent discrimination learning, is consist ent with earlier suggestions (Gibson, 1969) regarding the conditions t hat promote perceptual learning.