Performance on inclusion and exclusion memory tests as a function of whether attention at learning was divided or full. Adapted from Jacoby et al. (1993).

important. Many memory tests (e.g., cued recall; Jacoby et al., 1993) involve a mixture of explicit memory and implicit memory.

Various criticisms have been made of the process-dissociation procedure of Jacoby et al. (1993). Of particular concern is the assumption that implicit or automatic processes and explicit or controlled processes are totally independent of each other. If participants are instructed to complete word stems with the first word that comes to mind but to avoid words encountered previously, they are likely to use an implicit or automatic process followed by an explicit or controlled process. Such instructions are likely to lead to use of a generate-recognise strategy in which implicit and explicit processes are not independent of each other. Jacoby (1998, p. 10) studied the effects of such a strategy, and admitted that it produced problems: "Participants' reliance on a generate-recognise strategy violates assumptions of the estimation procedure."

Brain regions

Evidence that different brain regions are involved in explicit and implicit memory was reported by Schacter et al. (1996) in a PET study. When the participants performed an explicit memory task (recall of semantically processed words), there was much activation of the hippocampus. In contrast, when they performed an implicit memory task (word-stem completion), there was reduced blood flow in the bilateral occipital cortex, but the task did not affect hippocampal activation.

Theoretical considerations

Several theoretical accounts of the differences between explicit and implicit memory have been offered. Some theorists (e.g., Squire, Knowlton, & Musen, 1993) have focused on the underlying brain structures and their associated memory systems. Such theorists typically rely heavily on evidence from amnesic patients, and we will consider this evidence later in the chapter.

Varieties of implicit memory

Many researchers have discussed implicit memory as if it refers to a single memory system. However, the fact that there are numerous kinds of implicit memory tasks ranging from motor skills to word completion suggests that various memory systems and brain areas are involved. Evidence discussed later in the chapter indicates that different kinds of implicit memory tasks involve brain areas as diverse as the basal ganglia, the cerebellum, and the right parietal cortex.

It has been suggested by several researchers (e.g., Tulving & Schacter, 1990) that there are important differences between perceptual implicit tests and conceptual implicit tests. On most perceptual implicit tests, the stimulus presented at study is presented at test in a degraded form (e.g., word-fragment completion; word-stem completion; perceptual identification). On conceptual implicit tests, on the other hand, the test provides information conceptually related to the studied information, but there is no perceptual similarity between the study and test stimuli (e.g., general knowledge questions such as "What is the largest animal on earth?"; generation of category exemplars from a category such as "four-footed animals").

Different brain areas are involved in perceptual and conceptual priming. Patients with Alzheimer's disease (which involves progressive dementia or loss of mental powers) typically have intact perceptual priming but impaired conceptual priming. In contrast, patients with right occipital lesions have no perceptual priming on visual word-identication tasks but have normal conceptual priming (see Gabrieli, 1998). What we have here is a double dissociation, which is generally taken as evidence that separate processes and brain areas are involved in the two types of task.

Neuroimaging studies confirm that different brain areas are involved in perceptual and conceptual priming. As we have seen, PET studies on normals indicate that perceptual priming on visual word-stem completion tasks produces reduced activity in bilateral occipitotemporal areas (e.g., Schacter et al., 1996). In contrast, priming on conceptual priming tasks produces reduced activity in left frontal neocortex (e.g., Wagner et al., 1997). Why is brain activity reduced rather than increased? The most likely reason is because processing is more efficient when a stimulus is re-presented than on its original presentation.

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