The Effect of aging on object recognition memory.
1. NSMA, Univ Arizona, Tucson, AZ, USA
2. Dept Aging Intervention, NILS, Oobu, Japan
Cognitive functions, such as learning and memory, in mammals show a gradual decline during normal aging. This decline is thought to involve anatomical and functional changes in the medial temporal lobe, including the hippocampus, entorhinal cortex and perirhinal cortex. While the hippocampus is crucially involved in spatial memory, the perirhinal cortex is necessary for recognition memory. It is therefore possible that age-related decline in recognition memory could involve deficits of neural function associated with the perirhinal cortex; however, little is known about the impact of aging on the functional integrity of perirhinal circuits. In the present study, we attempted to clarify whether perirhinal cortex-associated recognition memory is affected by the aging process. It was previously shown that aged rats were impaired at spatial recognition, but not object recognition with a 5 min delay between the sample and the test phase (Cavoy and Delacour, 1993). Recognition memory impairments, however, have been shown to be delay dependent with age-related deficits only emerging at longer delays (Dunnett et al., 1988). It is therefore possible that age-related recognition memory deficits can be observed when longer delays are used between the sample and the test phase. To examine this, young (9 months old) and aged (24 months old) rats were tested on a spontaneous object recognition task at delays of 2 min, 15 min, 2 hr and 24 hr. During the sample phase rats were placed in a chamber with two identical objects for 4 minutes and allowed to explore. After the delay, during the test phase, rats were placed in the same chamber for another 4 min but this time there was one object that was identical to the objects in the sample phase and a novel object. After the 2 min delay, both young and old rats showed a significant exploratory preference for the novel object. After 15 min, 2 hr and 24 hr delays, however, the aged rats did not explore the novel object significantly more than the familiar object while the young rats showed an exploratory preference for the novel object at both the 15 min and 24 hour delays. These findings suggest that the maintenance of object representations declines with age. Because performance on the spontaneous object recognition task is sensitive to dysfunction of the perirhinal cortex (e.g. Bussey et al., 1999), it is possible that functional alterations within the perirhinal cortex, that occur during normal aging, contribute to this decline.
Supported by AG003376, FNS054465A
Key words: Hippocampus, Perirhinal cortex, Spontaneous recognition