Anomalous properties of hippocampal lesion-induced retrograde amnesia

Abstract

Retrograde amnesia can result from transient or permanent insults to the central nervous system and is typically manifest as a temporally graded memory loss. This temporal gradient of retrograde amnesia has been considered evidence for memory consolidation, since newly acquired information is vulnerable to amnestic treatments, whereas older information is not. Although investigations of transient insult-induced retrograde amnesia have provided evidence against a consolidation interpretation, hippocampal lesion-induced retrograde amnesia is still considered to represent a consolidation (or storage) failure. In order to investigate the consolidation interpretation of hippocampal lesion-induced retrograde amnesia, two experiments were undertaken. In the first, rats were reminded of “old” memories prior to hippocampal lesions, a procedure that produced retrograde amnesia. This result is difficult to reconcile with current interpretations of retrograde amnesia, since consolidated memories are presumably not vulnerable to amnesia. The second experiment explored the permanence of hippocampal lesion-induced retrograde amnesia by presenting amnestic rats with a portion of the training treatment (reactivation) prior to testing. The reactivation treatment successfully reversed retrograde amnesia. Taken together, the results from these experiments indicate that hippocampal retrograde amnesia may not in all cases arise from storage failure and illuminate new circumstances under which damage to the hippocampus may affect memory.