Performance on a variety of cognitive tasks has been reported to decline across the life span. The present research evaluated appetitive instrumental learning in young and mature rats. In Experiment 1, subjects were trained to criterion, placed on extinction training to criterion, and subsequently retrained for a total of three cycles. Results indicated that mature animals were impaired in the initial acquisition of the bar-press response but reacquired the response as quickly as young animals. Resistance to extinction was not significantly impaired in the mature group, both groups increased resistance by the third extinction period, despite the brevity of reinforcement. In Experiment 2, young and mature subjects underwent appetitive instrumental training that continued beyond the acquisition criterion for the first experiment. After the response had been established (to criterion), performance levels were equivalent for young and mature subjects. The number of responses were not significantly different between young and mature groups on the day criterion was met; comparison of number of responses for 4 days after criterion also indicated no significant differences over days of training or between age groups. Examination of the number of responses occurring early in training indicated no significant group difference; hence, the earlier acquisition by young animals in Experiment 1 does not appear to reflect greater activity level in younger animals resulting in earlier and greater exposure to reinforced responses. Results may reflect the contribution of use-induced plasticity, such as long-term potentiation, within brain systems involved in learning and memory. These findings are consistent with evidence of the effects of use and disuse on neurobiological and cognitive function.

These experiments examined the effects of long-term instrumental training on subsequent radial arm maze performance and synaptic transmission within the hippocampal formation. In the first experiment, young (3 mo) and aged (18 mo) male rats underwent 12 weeks of appetitive instrumental conditioning; half were continually reinforced and the other half alternated between reinforcement and extinction. Afterward, spatial cognition was evaluated using an eight-arm radial maze. Subjects undergoing instrumental training performed at rates superior to untrained (control) animals regardless of age or training condition; age-related differences did not exist in the trained groups. In the second experiment, subjects underwent 12 weeks of instrumental training with continuous reinforcement, and excitability of the hippocampus was examined by paired-impulse stimulation of the perforant path. Training enhanced maximal facilitation of population spikes evoked in the granule cell layer of the dentate gyrus of aged subjects to the degree that no statistical difference existed between young and aged animals. Data from untrained control animals indicated a robust decline in paired-impulse excitability in aged subjects. These findings suggest that learning-induced plasticity may attenuate both behavioral and neurobiological changes observed in aged subjects. It is postulated that disuse may underlie some of the cognitive changes exhibited across the life span.