Persistent Memory Retention of Reward Events and Proactive Interference in Reward Series Learning By Mice

Abstract

This study examines acquisition of a single alternating series of reward quantities in mice. Four male ICR mice, trained in a straight runway, showed deferential response to items in a 3-0-3-0-3-0-3 series, constructed from a varying number of 0.045 g food pellets under inter-trial intervals(ITI) of 30 s (Experiment 1) or 20 min (Experiment 2), by running more slowly to nonrewards than rewards. Although mice showed reliable item anticipation under 20 min ITIs, nonreward anticipation became poorer in later serial positions than in earlier positions. It is possible that gradual deterioration of nonreward anticipation in a series is caused by proactive interference from previous item memories, since the nonreward anticipation was improved when the target item was divided by a long 120 min interval from prior items that were a potential source of proactive interference (Experiment 3). In Experiment 4, mice learned to respond differentially to the second item of 5-0 and 0-5 series with an ITI of 180 min. These results suggest that mice can discriminate reward magnitudes by forming item-associations between adjacent items and retain information of a previous item for a long interval, and that proactive interference occurs among item memories in a series.