A contrast effect in differential conditioning.

Contrary to the interests of parsimony in theory construction, an increasing number of experiments have shown that the reinforcing effect of a given amount of reward is not a static parameter; rather, the conception that emerges from these studies is that the reinforcing effect depends upon the context in which that reward quantity occurs; in particular, it depends upon the range of alternative rewards 5 has received in the experimental situation. The most direct evidence for this context effect comes from studies of visual discrimination learning. Meyer (1951) andSchrierand Harlow (1956), working with monkeys, and Lawson (1957), working with rats, found that rate of error elimination in a series of visual discrimination problems was not significantly related to the amount of reward when 5 received the same amount (large or small) on every problem. By comparison, if 5 received different amounts of reward for performing correctly on different problems of the learning series, learning rate on a given problem varied directly with the amount of reward for that problem. These prior studies investigated the context effect in selective learning situations in which the performance measure (percentage of correct responses) presumably reflects the resolution among several competing reac-