Age-related impairments of new memories reflect failures of learning, not retention

Abstract

Learning impairments and the instability of memory are defining characteristics of cognitive aging. However, it is unclear if deficits in the expression of new memories reflect an accelerated decay of the target memory or a consequence of inefficient learning. Here, aged mice (19-21-mo old) exhibited acquisition deficits (relative to 3-5-mo old mice) on three learning tasks, although these deficits were overcome with additional training. When tested after a 30-d retention interval, the performance of aged animals was impaired if initial learning had been incomplete. However, if trained to equivalent levels of competence, aged animals exhibited no retention deficits relative to their young counterparts. These results suggest that age-related "memory" impairments can be overcome through a more effective learning regimen.

Figure 1.

(Top) Errors to locate food in the Lashley III maze across four training trials and after 30 d of retention in both young and old mice. A significant deficit in 30-d retention was observed in old, relative to young, mice after four training trials. Brackets indicate standard errors. (Middle) Errors to locate food in the Lashley III maze across eight training trials and after 30 d of retention in both young and old mice. Both young and old animals exhibited comparable retention. Brackets indicate standard errors. (Bottom) Errors to locate food in the odor discrimination task. Both young and old animals exhibited comparable performance after four training trials, and no differences in retention between young and old animals were observed after 30 d. Brackets indicate standard errors.

Figure 2.

(Top left) Latency to locate hidden platform in the spatial water maze across six training trials and 30 d of retention. A significant impairment in acquisition was observed among aged animals (an effect also seen in the search strategy exhibited during a short-term probe test, top right). A significant deficit in 30-d retention was observed in old relative to young mice after six training trials. Brackets indicate standard errors. (Bottom left) Latency to locate hidden platform in the spatial water maze across 11 training trials and 30 d of retention. Both young and old animals exhibited comparable performance after 11 trials (a pattern also seen in the search strategy exhibited during a short-term probe test, bottom right). After 11 training trials, no differences in retention between young and old animals were observed after 30 d. Brackets indicate standard errors.