Impaired capacity for familiarity after hippocampal damage

Abstract

Recognition memory is thought to consist of two components: recollection and familiarity. Whereas it is widely agreed that the hippocampus supports recollection (remembering the episode in which an item was learned), there is uncertainty about whether it also supports familiarity (simply knowing that an item was encountered but without remembering the learning episode). We tested a counterintuitive prediction that follows from the idea that the hippocampus selectively supports recollection. Patients with hippocampal lesions should have strong experiences of familiarity as often as controls do; however, unlike controls, these experiences should not be accompanied by recollection. Accordingly, with methods that allow participants to report whether they remember an item as encountered previously or whether they simply know it is familiar, patients should express strong familiarity (in the absence of recollection) more often than controls. We indexed strong familiarity and recollection for previously studied words by obtaining confidence ratings together with Remember-Know judgments. The result was that patients provided fewer high-confidence Know responses than controls rather than more. Furthermore, the number of Know responses made by patients was substantially less than was predicted if recollection were impaired. This was true regardless of whether the prediction was based on the assumption that recollection and familiarity are independent or dependent processes. These results suggest that hippocampal lesions impair both recollection and familiarity. Unlike many previous studies of these constructs, the prediction (and the result) is independent of any particular theoretical model, and it holds even if Remember-Know judgments are not process-pure indicators of recollection and familiarity.