Figure

Recognition memory for pictures in Korsakoff patients and normal controls. Data from Huppert and Piercy (1976).

Evaluation

It is not clear within context processing deficit theory exactly why amnesic patients are able to store information about the to-be-remembered stimulus, but cannot store relevant contextual information. After all, what is regarded as the to-be-remembered stimulus and what is regarded as the context is often rather arbitrary and dependent on the researcher's whim. Mayes (1988) suggested that amnesic patients have reduced processing resources, and so can only process to-be-remembered information adequately by ignoring contextual information. However, it is necessary to obtain independent evidence that amnesic patients do actually have reduced processing resources.

At the experimental level, the context processing deficit theory has some problems in accounting for amnesics' poor recognition memory. As we saw in Chapter 6, contextual information is generally less important in recognition memory than in recall, and yet amnesic patients perform poorly on both kinds of memory test. An additional problem is that there is clearer evidence of deficits in contextual processing in amnesic patients with damage to the diencephalon than in those with damage to the temporal lobes (Parkin & Hunkin, 1997).

It could be argued that many of the tasks performed normally by amnesic patients (e.g., motor skills; repetition-priming tasks) share the characteristic that contextual information is not required for successful performance. However, there are substantial differences among these tasks in other ways. Thus, the notion that one should distinguish between memory tasks on which contextual information is important and those on which it is not is an oversimplification.

Explicit versus implicit memory

The notion that memory performance always depends on conscious awareness has been disproved in studies on normals. There is also compelling evidence from amnesic patients that conscious recollection is often not needed to produce good memory performance. A hackneyed anecdote related by Claparede (1911) illustrates the point. He hid a pin in his hand before shaking hands with one of his amnesic patients. After that, she was understandably reluctant to shake hands with him, but was unable to explain why. The patient's behaviour revealed clearly that there was long-term memory for what had happened, but this occurred without any conscious recollection of the incident.

Schacter (1987) argued that amnesic patients are at a severe disadvantage when tests of explicit memory (requiring conscious recollection) are used, but that they perform at normal levels on tests of implicit memory (not requiring conscious recollection). As would be predicted on this theory, most amnesic patients display impaired performance on tests of recently acquired episodic and semantic memories. Most studies on motor skills and on the various repetition-priming effects are also consistent with Schacter's theoretical perspective, in that they are basically implicit memory tasks on which amnesic patients perform normally or nearly so.

Particularly striking findings were reported by Graf et al. (1984), in a study that has already been mentioned. One of the tests of explicit memory was cued recall. The first three letters of list words were presented, and the participants retrieved the appropriate list word. The test of implicit memory was word completion. The same initial three letters were presented, but participants were told simply to write down the first word they thought of starting with those letters. The amnesic patients performed as well as normals on the implicit memory test (word completion), but much worse on the explicit memory test (cued recall) (see Figure 7.7).

Another example of intact perceptual priming by amnesic patients was reported by Schacter and Church (1995). In this study, the participants initially heard a series of words spoken in the same voice. After that, they tried to identify the same words passed through an auditory filter; the words were either spoken in the same voice or in an unfamiliar voice. The findings are shown in Figure 7.9 (a). Amnesic patients and normal controls both showed perceptual priming, in that word-identification performance was better when the words were spoken in the same voice.

The notion that perceptual priming depends on different brain systems from those involved in explicit memory would be strengthened if it were possible to obtain a double dissociation. In other words, it would be useful to find patients who had intact explicit memory but impaired perceptual priming. This was achieved by Gabrieli et al. (1995). They studied a patient, MS, who had a right occipital lobe lesion. MS had normal levels of performance on the explicit memory tests of recognition and cued recall, but he had impaired performance on perceptual priming. Gabrieli et al. also tested amnesic patients, and confirmed that they showed the opposite pattern of impaired explicit memory but intact perceptual priming.

Evaluation

Most of the tasks on which amnesic patients show impaired performance involve explicit memory, and most of those on which they show intact performance involve implicit memory. An important finding that does not fit is the intact (or nearly intact) short-term memory shown by most amnesic patients, as tests of short-term memory typically involve explicit rather than implicit memory. However, the distinction between explicit and implicit memory is of great value in distinguishing between tests of long-term memory on which amnesic patients do and do not perform poorly.

In spite of the usefulness of the explicit/ implicit distinction, the notion that amnesic patients have deficient explicit memory does not in and of itself provide an explanation of their memory impairments. As Schacter (1987, p. 501) pointed out, implicit and explicit memory "are descriptive concepts that are primarily concerned with a person's psychological experience at the time of retrieval."

Business Correspondence

Business Correspondence

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