PEAK PROCEDURE PERFORMANCE IN YOUNG-ADULT AND AGED RATS - ACQUISITIONAND ADAPTATION TO A CHANGING TEMPORAL CRITERION

Twenty-four-month-old and 4-month-old rats were trained on a peak-inte rval procedure, where the time of reinforcement was varied twice betwe en 20 and 40 sec. Peak times from the old rats were consistently longe r than the reinforcement time, whereas those from younger animals trac ked the 20- and 40-sec durations more closely. Different measures of p erformance suggested that the old rats were either (1) systematically misremembering the time of reinforcement or (2) using an internal cloc k with a substantially greater latency to start and stop timing than t he younger animals. Old rats also adjusted more slowly to the first tr ansition from 20 to 40 sec than did the younger ones, but not to later transitions. Correlations between measures derived from within-trial patterns of responding conformed in general to detailed predictions de rived from scalar expectancy theory. However, some correlation values more closely resembled those derived from a study of peak-interval per formance in humans and a theoretical model developed by Cheng and West wood (1993), than those obtained in previous work with animals, for re asons that are at present unclear.