Effect of age on object exploration, habituation, and response to spatial and nonspatial change.

To measure their ability to detect novel arrangements in a given environment, young (6 months old) and senescent (22-24 months old) male F344 rats were repeatedly exposed to a given spatial configuration of objects contained in an open field. After the rats were habituated to the novel environment (1 trial with no objects, followed by 3 trials with 5 salient objects), the spatial arrangement of the objects was modified (2 trials), and object novelty was tested (2 trials) by substituting a familiar object with a new one at the same location (nonspatial change). The results indicated that the senescent rats explored old objects less than young rats, particularly on Trial 2. On the 1st trial with displaced objects (Trial 5), the senescent rats explored the displaced objects less than the young rats. However, when a new object was placed in the field (Trials 7-8), there were no age differences in new object exploration. These results suggest that senescent rats have decrements in the ability to build spatial representations of the environment and to use this information to detect such changes, even though object recognition is not impaired with age.