Forgetting from Short-Term Memory in Delayed Matching to Sample: A Reinforcement Context Model

Abstract

Short-term memory in nonhuman animals is typically studied in delayed matching to sample, with variation in the retention interval or delay between the to-be-remembered sample and subsequently presented choice or comparison stimuli. The forgetting function, which relates the systematic decrease in discriminability to increasing delay, is well described by an exponential in the square root of time, with an intercept and slope that vary systematically with different conditions, such as sample-stimulus disparity, retention-interval conditions, and reward parameters. We argue that the rewards for accurate matching are relative to the reinforcement context, which includes rewards Ro for extraneous or other behaviors. Forgetting results from competition between Ro and rewards for the delayed matching task. We suggest that Ro acts to shift attention from the memory task to extraneous behavior, and that Ro grows as a linear function of time in the retention interval. By incorporating these assumptions in the model proposed by White and Wixted (1999), we accurately predict the time course of forgetting under a variety of different conditions for delayed matching.