Human hippocampal-dependent tasks: is awareness necessary or sufficient?

The hippocampus has been shown to be required for the acquisition of declarative or explicit memory. Whether all hippocampal-dependent forms of learning and memory are explicit is an open question. Controversy has emerged about the existence of implicit hippocampal-dependent tasks. Two implicit tasks that may involve the hippocampusare a relational eye tracking task (Ryan et al. (2000) Psychol Sci 11:454-461) and transitive inference (Greene et al. (2006) J Cognit Neurosci 18:1156-1173; Greene et al. (2001) Mem Cognit 29:893-902). Recently, it was shown that both of these tasks may depend upon task awareness (Smith et al. (2006) J Neurosci 26:11304-11312; Smith and Squire (2005) J Neurosci 25:10138-10146). It is argued that in both cases, distinct, explicit versions of the tasks were created, which do not disprove the implicit nature of the original tasks. (c) 2007 Wiley-Liss, Inc.