Arne L. Ostergaard University of California, San Diego
Terry L. Jernigan San Diego VA Medical Center and University of California, San Diego
The explicit/implicit memory distinction is one of several hypothesized memory dichotomies. As first proposed, the distinction was between tasks that require conscious recollection of experience (explicit) and tasks in which performance is facilitated by previous experience but for which recollection of that experience is not required ( Graf & Schacter, 1985). As such, the distinction is neutral regarding the question of underlying memory systems. However, several authors have postulated that performance on implicit memory tasks is mediated by a different underlying memory system than performance on explicit memory tasks ( Gabrieli, Milberg, Keane, & Corkin, 1990; Schacter, 1990; Squire, 1987; Tulving & Schacter, 1990), and a central question in current memory research regards the extent to which the experimental evidence requires that independent systems subserve explicit and implicit memory.
This issue has relevance not only for purely psychological memory theories but for anatomical models as well. If independent memory systems exist, then it is important to identify those brain structures comprising each of the systems. On the other hand, if implicit and explicit memory measures reflect the operation of the same underlying memory mechanisms, then discrepancies between performance on these tasks must be attributed to differing contributions by brain structures subserving nonmemory functions. Three types of evidence are advanced in support of the multiple memory systems proposal: (a) findings of stochastic independence between performance on explicit and implicit memory tasks; (b) functional independence between explicit and implicit memory tasks; and (c) evidence from brain-damaged patients, notably patients with the amnesic syndrome, indicating that performance on explicit memory tasks may be selec-