Effects of Percentage of Reinforcement upon Behavioral Contrast in the Rat

Abstract

A large amount of data has been reported in recent years concerning the interaction of a given condition of reinforcement and previous or contemporary conditions of reinforcement. This interaction has been commonly referred to as “behavioral contrast”. In this paper, it was tested whether the contrast effect was obtained in the discrete trial situation in terms of running behavior and behavior variability.Of two experiments reported here, Exp. I was concerned with simultaneous contrast effect, while Exp. II with successive contrast effect. The apparatus used was one pair of T maze. Except a start box and goal boxes, one maze was painted white (S1) and the other black (S2). A start box and goal boxes were painted gray in both mazes.In Exp. I, Ss were randomly assigned to five groups of 10, each. The groups, defined in terms of the percentage of reinforcement administered in their positive and negative mazes, respectively, were; Group I (100%-100%), Group II (100%- 50%), Group III (100%-0%), Group IV (50%-50%), Group V (50%-0%). After exploration and pretraining, all Ss were given 21 successive free-choice trials per day in two T mazes for 10 days.In Exp. II, Ss were divided into four groups of 10, each. The groups were; Group I (100% control), Group II (100% downshift), Group III (30% control), Group IV (30% upshift). During first 5 days of training, Groups II and IV received identical treatment with Groups I, III, respectively. Subsequently in Group II, percentage of reinforcement of S2 were shifted from 100% to 30% during Days 6 to 15. On the contrary, Group IV received upshift of S2 from 30% to 100%.Three response measures were employed in both experiments, i. e., starting latency, running time and choice direction.The results were as follows : 1) In Exp. I, the result of starting latency indicated a significant evidence for the positive contrast effect. That is, the starting latency in the 100% maze was shortest for Group III, intermediate for Group II and longest for Group I (Fig. 1). Though there was statistically no difference among 3 groups in running time, the similar tendency was found.2) The choice behavior significantly differed among 5 groups. Although Groups I and IV tended to chose the same side with about equal frequency at the end of training, the Ss subjected to the differential conditioning (Groups II, III and V) preferred one side in 100%, maze and avoided that side in, the other maze (Fig. 2). This fact would suggest that some difference in percentage of reinforcement tends to fixate the choice behavior and these tendencies result from behavioral contrast effect.3) Unlike Exp. I, Exp. II showed no evidence of contrast effect. The successive shift of percentage reinforcement of S2 did not result in any change in response time (Fig. 3) and choice behavior. It would be shown from the present experiments that behavioral contrast effect was observed in the simultaneous shift of percentage of reinforcement, but not in the successive shift.