Scope and Limits of Implicit Memory in Amnesia

Abstract One of the most striking examples of knowledge without awareness is seen in patients with global amnesia. Typically, these patients are unable to recall day-to-day experiences and they forget information encountered only minutes before. For many years now, these deficits have been studied experimentally, with the aim of elucidating the nature of the underlying processing deficits. In the context of this effort, Larry Weiskrantz and his colleague, Elizabeth Warrington, made the startling discovery that, given appropriate retrieval cues, amnesic patients can exhibit memory for the very information they cannot retrieve consciously (Warrington and Weiskrantz, 1968, 1970). This finding, counter-intuitive and controversial at the time, spawned three decades of research into the nature and the scope of preserved memory in amnesia. It is now well established that a number of different forms of memory are preserved in amnesia. These include skill learning, classical conditioning, and repetition priming, which refers to the bias or facilitation in processing a stimulus that results from prior exposure to that same or a related stimulus (Squire et al., 1993). These different forms of memory have in common the fact that knowledge can be expressed without awareness of the episode in which learning took place. By contrast, tasks which elicit severe impairments in amnesic patients, such as traditional tests of recall and recognition, do require awareness of the learning episode, as subjects are asked to retrieve information that occurred within a particular learning context. The distinction between forms of memory that are impaired and preserved in amnesia is captured by a number of taxonomies, each highlighting a different aspect of the dichotomy. Graf and Schacter (1985) introduced the terms implicit memory and explicit memory to emphasize the different ways in which memory is expressed. Focusing on the associated states of awareness, Jacoby and Witherspoon (1982) used the terms memory with awareness and memory without awareness. Finally, highlighting the nature of the information that is remembered, Squire (1992) introduced the terms declarative memory and non-declarative memory.