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Description
Spatial memory decline has been well documented in older humans and animals. The decline has been reported to stem from age-related changes in various brain regions including the hippocampus. Recent studies have revealed that the hippocampus is important for the detection of metric and topological changes in the allocentric relationship among stimuli. However, little is known about age-related differences in the ability to detect these changes. In the current study, 25 young (6 month) and 22 aged (24 month) Fischer-344/Brown- Norway rats were tested on a paradigm designed to assess object-place memory for metric and topological changes using an exploratory-based, novelty detection task. Performance on this task has been shown to be dependent on the hippocampus. Metric changes were made by altering the distance between two objects, while topological changes were made by changing the object configuration. The metric task involved four 5-min sessions, with a 3-min intersession interval between Sessions 1, 2, and 3, and a 10-min intersession interval between Sessions 3 and 4. Prior to testing, two different objects were positioned 68 cm apart in an open field. Sessions 1-3 allowed for habituation to the two objects. During Session 4, the objects were shifted in to a 38 cm separation for the first metric test and shifted out to 98 cm separation for the second test. The topological task involved five 5-min sessions, with a 3-min intersession interval after Sessions 1 and 3, and a 10-min intersession interval after Sessions 2 and 4. Prior to testing, four different objects were positioned 68 cm apart in a square formation in an open field. Sessions 1 and 2 allowed for habituation to the four objects. During Sessions 3 and 4, two of the objects were transposed. During Session 5, the remaining two objects were transposed. There was a 48-hr separation between metric and topological tests. Object exploration was recorded during all sessions and analyzed for age differences. In order to assess age group differences in exploration during the metric task, two separate 2 x 4 repeated measures ANOVAs were run for each condition, shift-in and shift-out, with Group (6 month, 24 month) as a between group variable and Session (1-4) as a within group variable. In order to assess age group differences in exploration during the topological task, a 2 x 2 repeated measures ANOVA was conducted with Group (6 month, 24 month) as a between group variable and Location (Front Switch, Back Switch) as a within group variable. Young rats spent significantly more time exploring displaced objects during the metric task compared to aged rats when the objects were shifted in. During the topological task, young rats spent significantly more time exploring objects when the front two objects were transposed compared to aged rats. The results of this study suggest that age-related changes in the hippocampus impair memory for metric and topological spatial representations. Impairments in the detection of metric and topological spatial change may be useful indicators of age-related memory decline.
