Interpreting neural representations of aged animals

It is becoming more clear that normal age-related cognitive decline ischaracterized not so much by signicant cell loss in memory-related brainstructures like the hippocampus, but rather by a change in the functionalorganization of information processing of its neural elements. Conse-quently, investigations of the changes in neural plastic abilities of the agedbrain represent an approach that could reasonably be expected to result inimportant advances in our understanding of age-related cognitive decline.To this end, the recent ndings reported by Tanila et al. (1997) and Barneset al. (1997) move the eld forward in signicant respects. The datapresented in Tanila et al. (1997) suggest that hippocampal place elds ofaged rats are remarkably stable across different environments, while Barneset al. (1997) showed that a subset of hippocampal place elds of old ratsrepresent spatial locations less reliably when animals are repeatedly testedacross familiar environments. At rst glance, these results appear in conict.The recent commentary on this apparent discrepancy by Peter Rapp (1998)suggests the interesting possibility that in old rats there is greaterinterference between similar cues that were used in the different testsituations of Barnes et al. (1997), and that such interference was not presentin the Tanila et al. (1997) study. The results of both the Tanila and Barnesgroup suggest that aged animals show a change in hippocampal representa-tional plasticity.Additional issues deserve to be emphasized in the context of the abovedebate. The rst issue concerns the behavioral interpretation of theobserved unit correlates, and the second issue deals with age-relatedcompensation of function and its relationship to neural representationwithin and outside of hippocampus. Regarding the rst issue, there is evenmore converging behavioral and neurophysiological evidence that old ratsand their place cells have difficulty when using familiar spatial informationin different behavioral contexts. Barnes et al.’s data are entirely consistentwith recent behavioral data (Mizumori et al., 1996) showing that agedanimals are signicantly slower at acquiring a spatial working memory taskon a radial maze than young rats when acquisition training occurs in aspatial environment that is familiar to the rat (room A). In contrast, andparallel to the Tanila data, the