BRAIN AGING - CHANGES IN THE NATURE OF INFORMATION CODING BY THE HIPPOCAMPUS

Advanced age in rats is associated with a decline in spatial memory ca pacities dependent on hippocampal processing. As yet, however, little is known about the nature of age-related alterations in the informatio n encoded by the hippocampus. Young rats and aged rats identified as i ntact or impaired in spatial learning capacity were trained on a radia l arm maze task, and then multiple parameters of the environmental cus s were manipulated to characterize the changes in firing patterns of h ippocampal neurons corresponding to the presence of particular cues or the spatial relationships among them. The scope of information encode d by the hippocampus was reduced in memory-impaired aged subjects, eve n though the number of neurons responsive to salient environmental cue s was not different from that in young rats. Furthermore, after repeat ed manipulations of the cues, memory-intact aged rats, like young rats , altered their spatial representations, whereas memory-impaired aged rats showed reduced plasticity of their representation throughout test ing. Thus changes in hippocampal memory representation associated with aging and memory loss can be characterized as a rigid encoding of onl y part of the available information.