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Volume 17, Number 13, Issue of July 1, 1997 pp. 5155-5166
Copyright ©1997 Society for Neuroscience

Brain Aging: Changes in the Nature of Information Coding by the Hippocampus

Received Feb. 10, 1997; revised April 9, 1997; accepted April 11, 1997.

Heikki Tanila¹, Matthew Shapiro², Michela Gallagher³, and Howard Eichenbaum⁴

¹ Department of Neuroscience and Neurology, University of Kuopio, 70211 Kuopio, Finland, ² Department of Psychology, McGill University, Montreal, Quebec QC H3A 1B1, Canada, ³ Department of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, and ⁴ Department of Psychology, Boston University, Boston, Massachusetts 02215

Advanced age in rats is associated with a decline in spatial memory capacities dependent on hippocampal processing. As yet, however, little is known about the nature of age-related alterations in the information encoded by the hippocampus. Young rats and aged rats identified as intact or impaired in spatial learning capacity were trained on a radial arm maze task, and then multiple parameters of the environmental cues were manipulated to characterize the changes in firing patterns of hippocampal neurons corresponding to the presence of particular cues or the spatial relationships among them. The scope of information encoded by the hippocampus was reduced in memory-impaired aged subjects, even though the number of neurons responsive to salient environmental cues was not different from that in young rats. Furthermore, after repeated 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 testing. Thus changes in hippocampal memory representation associated with aging and memory loss can be characterized as a rigid encoding of only part of the available information.

Key words: spatial learning; spatial memory; place field; electrophysiology; encoding; representation; rat; age

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