Acquisition and long-term retention of a two-lever DRL schedule: comparison between mature and aged rats.

The effects of ageing on temporal regulation, general activity and memory were analysed in a two-lever DRL schedule. The task consisted in pressing on lever A and then waiting a minimum of time before pressing lever B to get the reinforcer. Adult and senescent rats were submitted to preliminary training followed by 5 DRL 5-second training sessions and 3 retention testing sessions after a 21-day break. Results showed that, relative to adults, senescent rats were slower to reach the 5-second DRL criterion, emitted fewer temporally regulated A-B response sequences and an equivalent amount of repetitive superfluous A-A and B-B response sequences. The quality of temporal regulation was evaluated by the coefficient of variation (CV) and the median of the A-B interresponse-time distribution. In training, aged rats exhibited a higher CV only during the first 10-minute periods of the sessions, and emitted median IRTs similar to those of adults. The B-A intertrial-intervals were longer in aged than in adult rats. No age-related differences appeared for efficiency. Finally, long-term retention was not affected in either age group. The results favour an interpretation in terms of temporary recall memory deficit with a preservation of temporal regulation capacity, rather than age-related motor and motivational differences.