Copyright © 1999 Massachusetts Institute of Technology
Psychological studies of human MEMORY have traditionally been concerned with conscious recollection or explicit memory for specific facts and episodes. During recent years, there has been growing interest in a nonconscious form of memory, referred to as implicit memory (Graf and Schacter 1985; Schacter 1987), that does not require explicit recollection for specific episodes. Numerous experimental investigations have revealed dramatic differences between implicit and explicit memory, which have had a major impact on psychological theories of the processes and systems involved in human memory (cf. Roediger 1990; Schacter and Tulving 1994; Ratcliff and McKoon 1997).
The hallmark of implicit memory is a change in performance -- attributable to information acquired during a specific prior episode -- on a test that does not require conscious recollection of the episode. This change is often referred to as direct or repetition priming. One example of a test used to assess priming is known as stem completion, where people are asked to complete word stems (e.g., TAB) with the first word that comes to mind (e.g., TABLE); priming is inferred from an enhanced tendency to complete the stems with previously studied words relative to nonstudied words (for reviews, see Roediger and McDermott 1993; Schacter and Buckner 1998; Schacter, Chiu, and Ochsner 1993). Priming is not the only type of implicit memory. For instance, tasks in which people learn to perform motor or cognitive skills may involve implicit memory, because skill acquisition does not require explicit recollection of a specific previous episode (for review, see Salmon and Butters 1995).
Implicit memory can be separated or dissociated from explicit memory through experimental manipulations that affect implicit and explicit memory differently (for methodological considerations, see Jacoby 1991; Schacter, Bowers, and Booker 1989), and neurological conditions in which explicit memory is impaired while implicit memory is spared. For example, it has been well established that performance on explicit recall and recognition tests is higher following semantic than following nonsemantic study of an item -- the well-known levels of processing effect. In contrast, however, the magnitude of priming on tasks that involve completing word stems or identifying briefly flashed words is often less affected, and sometimes unaffected, by semantic versus nonsemantic study (see reviews by Roediger and McDermott 1993; Schacter, Chin, and Ochsner 1993).
Perhaps the most dramatic dissociation between implicit and explicit memory has been provided by studies of brain-damaged patients with organic amnesia. Amnesic patients are characterized by a severe impairment in explicit memory for recent events, despite relatively normal intelligence, perception, and language. This memory deficit is typically produced by lesions to either medial temporal or diencephalic brain regions. In contrast, a number of studies have demonstrated that amnesic patients show intact implicit memory on tests of priming and skill learning. These observations suggest that implicit memory is supported by different brain systems than is explicit memory (Squire 1992).
The evidence also shows that various forms of implicit memory can be dissociated from one another. A number of studies point toward a distinction between perceptual priming and conceptual priming. Perceptual priming is little affected by semantic versus nonsemantic study processing. It is also modality specific (i.e., priming is enhanced when the sensory modality of study and test are the same), and in some instances may be specific to the precise physical format of stimuli at study and test (cf. Church and Schacter 1994; Curran, Schacter, and Bessenoff 1996; Graf and Ryan 1990; Tenpenny 1995). Conceptual priming, in contrast, is not tied to a particular sensory modality and is increased by semantic study processing; it is observed most clearly on tests that require semantic analysis, such as producing category exemplars in response to a category cue (Hamman 1990). Other evidence indicates that priming and skill learning are dissociable forms of implicit memory. For instance, studies of patients suffering from different forms of dementia indicate that patients with Alzheimer's dementia often show impaired priming on stem completion tasks, yet show normal learning of motor skills; in contrast, patients with Huntington's disease (which affects the motor system) show normal stem completion priming together with impaired learning of motor skills (Salmon and Butters 1995).
Recent studies using newly developed brain imaging techniques, such as POSITRON EMISSION TOMOGRAPHY (PET) and functional MAGNETIC RESONANCE IMAGING (fMRI) have demonstrated that visual priming on such tests as stem completion is accompanied by decreased blood flow in regions of visual cortex (Squire et al. 1992). Various other neuroimaging studies have produced similar priming-related blood flow decreases (see Schacter and Buckner 1998). These observations are consistent with the idea that visual priming effects depend on a perceptual representation system that includes posterior cortical regions that are involved in perceptual analysis (Tulving and Schacter 1990); priming may produce more efficient processing of test cues, perhaps resulting in decreased neural activity. Other studies have shown that conceptual priming is associated with blood flow reductions in regions of left inferior frontal cortex that are known to be involved in semantic processing (for review, see Schacter and Buckner 1998). In contrast, neuroimaging studies of motor skill learning have shown that the development of skill across many sessions of practice is accompanied by increased activity in regions involved in motor processing, such as motor cortex (Karni et al. 1995). Neuroimaging studies are also beginning to illuminate the networks of structures involved in explicit remembering of recent episodes, including the regions within the prefrontal cortex and medial temporal lobes (e.g., Buckner et al. 1995; Schacter et al. 1996; Tulving et al. 1994; for review, see Cabeza and Nyberg 1997) .
The exploration of implicit memory has opened up new vistas for memory research, providing a vivid reminder that many aspects of memory are expressed through means other than conscious, explicit recollection of past experiences. Nonetheless, a great deal remains to be learned about the cognitive and neural mechanisms of implicit memory, and it seems likely that further empirical study and theoretical analysis will continue to pay handsome dividends in the future .
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