A critical period in the learning of motionless stimulus properties in chicks

Abstract

Three experiments were performed to explore the possibility that learning of motionless stimulus properties by chicks does not occur on the basis of conventional associative learning. In Experiment 1 a food box with a coloured interior was presented on each of the first five post-hatch days. A different colour was presented on each successive day. When an opportunity was provided on the sixth post-hatch day to choose among the five colours, the chicks tended to choose those colours most recently presented. In Experiment 2 five groups of subjects were presented with a coloured box on only one day, gray boxes on the remaining four days. Each group received the coloured food box on a different day. In the test situation learning was found only for the group exposed to a coloured food box on the fifth post-hatch day. These results, along with those of a previously published study, were interpreted as providing evidence against the occurrence of a critical period prior to the fourth or fifth post-hatch days. In Experiment 3 chicks were deprived of food for their first three post-hatch days and, beginning with the fourth day, were treated in a manner identical with the subjects of Experiment 1. When placed in the test situation, the chicks tended to choose colours presented on their fourth and fifth post-hatch days. These results were interpreted as suggesting the presence of a critical period in the area of the fifth post-hatch day for the learning of stimulus characteristics defining stationary visual complexes. In contrast to Experiments 1 and 2 no tendency for the subjects to choose the most recently presented colours was observed. Just as in Experiments 1 and 2, however, Day 5 received the largest number of scores. The results of these three experiments were discussed in terms of several theoretical approaches, especially the associative learning approach of modern behaviour theory, and it was concluded that the findings are most compatible with a critical period interpretation. The possibility that the results were a function of the testing method used was suggested as meriting further investigation. It was also found that the manifestation of this type of learning appears to be independent of the state of food deprivation at the time of testing. No significant tendency was found for the chicks to show colour preferences in the test situation.