ABSTRACT
In contrast to the findings for frontal and parietal regions, only three of the studies reviewed here reported retrieval-related activation in the hippocampus or adjacent medial temporal cortex. Indeed, if the findings of Saykin et al. (1999) are discounted (on the grounds that the study confounded memory retrieval and oddball effects), the only studies to find hippocampal activation were those of Cabeza et al. (2001) and Eldridge et al. (2000). These findings were obtained for test items likely to have elicited strong episodic recollection. Thus, they are consistent with the proposal that retrieval-related hippocampal activity is associated specifically with this form of memory (Rugg et al., 1997; Schacter et al., 1996) and, more generally, with the view that the hippocampus proper forms part of a circuit specialised for episodic memory rather than memory based on non-episodic information such as item familiarity (e.g. Aggleton & Brown, 1998). These findings lend weight to the possibility that the failure to find hippocampal activation in other studies of yes/no recognition reflects the fact that, as noted in the earlier section “Episodic memory”, this task is “process impure”; specifically, old/new decisions can be made on the basis of an acontextual sense of familiarity in the absence of the (putatively hippocampally mediated) retrieval of a study episode (Aggleton & Brown, 1998; Yonelinas, 1994). However, this possibility seems unlikely to account fully for the inconsistent findings noted above for the medial temporal lobe. First, two other studies (Henson et al., 1999b; McDermott et al., 2000) also employed procedures that permitted responses to items eliciting episodic recollection to be contrasted with responses to new items, but in neither case was differential hippocampal activity reported. Second, it has been suggested that item familiarity, the “nonrecollective” basis for recognition, depends upon perirhinal cortex, a medial temporal region that lies ventral and anterior to the hippocampus (Aggleton & Brown, 1998). Thus, to the extent that recognition judgements are based upon familiarity rather than episodic recollection (as is thought to be the case for items accorded “Know” judgements, for example), one might expect to see retrieval related activation in anterior medial temporal cortex.
