Hippocampal function: Working memory or cognitive mapping?

Abstract

The relative merits of two different descriptions of hippocampal function are evaluated in a series of experiments examining the performance of rats following lesions in the hippocampal system. Cognitive mapping and working memory are operationally defined, and experimental tasks are categorized as requiring one or both of these two functions. In test procedures that have working-memory components, rats with fimbria-fornix lesions performed at chance levels and showed no signs of recovery of function even after extended postoperative testing. In test procedures that did not have working-memory components, rats with fimbria-fornix lesions had, at most, a short-term impairment. Thus, the performance of rats with lesions was systematically influenced by the memory characteristics of the task. Altering the cognitive mapping characteristics of tasks had much less of an effect; an impairment in a task that required working memory was found, irrespective of whether rats did or did not use a mapping strategy, and only transitory impairments were found in tasks that did not require working memory, irrespective of whether rats did or did not use a cognitive mapping strategy. These results support the hypothesis that the memory characteristics of tasks are the major factors influencing the magnitude of the impairment exhibited by rats with damage to the hippocampal system.