Differential ontogeny of working memory and reference memory in the rat.

In two experiments we examined the ability of 15-, 21-, and 27-day-old rats to perform two spatial working memory problems (delayed alternation and discrete-trials delayed alternation) and a reference memory problem (position habit) in a T-maze. In the delayed alternation problem, each animal was presented with a series of free-choice trials and was rewarded for regularly alternating responses to the left and right arms of the T-maze. In the discrete-trials delayed alternation problem, each animal was forced to one maze arm and rewarded (forced run) and was then placed back into the start box and given a choice of arms (choice run). The direction of forced runs followed an irregular, counterbalanced series, and animals were rewarded for choosing the alternate maze arm on choice runs. In the position habit problem, animals were rewarded for consistently choosing one of the two arms of the T-maze. Performance on these problems was assessed relative to control conditions in which reward was not contingent on choice behavior. At all ages, rat pups learned to perform the delayed alternation and position habit problems (Experiment 1). However, only 21- and 27-day-old rats were able to learn the discrete-trials delayed alternation problem. The 15-day-olds were unable to learn this task (Experiment 2). The results of these experiments show that reference memory capacity is present by at least 15 days of age in the rat and does not develop further at later ages.(ABSTRACT TRUNCATED AT 250 WORDS)