Hippocampal dysfunction during aging II: deficits on the radial-arm maze.

Middle-aged and aged rats received dorsal hippocampal lesions before performance was evaluated on the radial-arm maze. The maze task contained simultaneous spatial working memory and visually cued reference memory components. Both middle-aged and aged rats that received lesions committed more errors of both types than sham-operated rats. Moreover, an age-related deficit was found for working and reference memory errors. After 14 sessions of training, a probe session revealed that: (a) middle-aged sham rats relied on spatial cues, (b) middle-aged lesioned rats employed the visual cues at the ends of the maze arms, (c) aged sham rats relied predominately on spatial information, (d) aged lesioned rats could not use spatial information or the visual cues at the ends of the maze arms. The additive effect of lesion and age suggests continued reliance on the hippocampus despite age-related deficits in its functioning. These data are suggestive of reduction in flexible cue utilization during aging, resulting paradoxically in more dependence on the hippocampus for aged rats than younger animals.