A measure of decision time changes as a function of the interval between the last successful trial of associative learning and the first successful retrieval attempt.
1. Dept Psychology, Univ Arizona, Tucson AZ
2. ARL Division of Neural Systems, Memory and Aging, University of Arizona, Tucson AZ
One version of memory consolidation theory predicts that, unlike new memories which depend on indirect linkages via the hippocampal formation, consolidated memories are supported by direct cortico-cortical connections. Consolidated memories might, therefore, be subject to faster cued retrieval than unconsolidated ones. A conditional association task, which in monkeys depends on an intact hippocampus for initial acquisition, was used to measure the speed of retrieval during learning and after two retention intervals. Humans learned by trial and error an arbitrarily-determined association between a color scene and one of four locations on a computer screen by updating a cursor’s position with a joystick. After either 24 hours or 1 week, subjects were asked to retrieve the associations by moving a cursor to the correct location after randomized presentation of the stimuli. During the retention interval, stimuli were not seen and the associations were not practiced. Two separable components of retrieval time were computed: 1) a decision time, or the time from stimulus onset to the first movement of the joystick, and 2) the time taken to move the cursor to the target after the decision time. An analysis included each subject’s decision time on the first successful retrieval attempt of each association if the subject correctly retrieved that association on the last trial of the learning day. Subjects’ average post-retention interval decision times increased relative to the level reached on the last successful trial of learning (929ms vs. 731ms, F(1,54)=20.6, p<0.0001). The 24-hour and 1-week decision time distributions differed significantly (p=0.0075, KS test=0.1506), and the increase in median decision times was greater after 1 week (983ms vs. 720ms, ?=262ms) than after 24 hours (896ms vs. 752ms, ?=144ms). These effects were observed in separate studies in which learning for the set of stimuli did and did not reach ceiling. Contrary to the hypothesis, decision times increased with the age of the memory rather than decreased, at least over one week. These results are at variance with a similar study (Nieuwenhuis personal communication) in which a significant reduction in reaction time was observed in a task which associated faces with positions. Possible reasons for the different results will be discussed.
Supported by: MH046823, SBSRI Dissertation Research Grant