Figure 710

Speed of question answering in three conditions (repeated old scenes; manipulated old scenes; new scenes). Data from Whitlow et al. (1995).

memory, which is intact in amnesic patients. However, normal controls responded faster to repeated old scenes than to manipulated old scenes, whereas the amnesic patients did not (see Figure 7.10). These findings suggest that amnesic patients did not store information about the relative positions of the objects in the scenes, and so derived no benefit from having the scene repeated. This conclusion was supported by the eye-movement data. The normal controls had numerous eye movements directed to the parts of manipulated old scenes that had changed, whereas the amnesics showed no tendency to fixate on these altered aspects. The failure of amnesics to show intact implicit memory in terms of speed of question answering to repeated old scenes or eye movements to manipulated old scenes cannot readily be accounted for on the explicit/ implicit memory distinction.

Additional support for the notion that amnesic patients have great difficulties in storing integrated information was reported by Kroll et al. (1996). They studied conjunction errors, which occur when new objects formed out of conjunctions or combinations of objects seen previously are mistakenly recognised as old. Amnesic patients made numerous conjunction errors, presumably because they remembered having seen the elements of the new objects but did not realise that the combination of elements was novel.

Cohen et al. (1994) used fMRI to identify the brain regions involved in the integration of information. Seven normal participants were presented with three kinds of information at the same time (faces; names; and occupations). On some trials, they were told to learn the associations among these kinds of information, a task involving integrative processes. On other trials, the participants simply made gender decisions about each face, a task not requiring the integration of information. All the participants had more activation in the hippocampus on the task requiring the integration of information. This suggests that the hippocampus plays a central role in processes of association or integration.

In spite of the important role played by the hippocampus, it cannot be regarded as the seat of consciousness. Damage to the hippocampus still allows conscious access to many memories formed before the onset of amnesia, and large lesions of the hippocampus do not affect consciousness. According to Cohen et al., 1997, p. 148), "The hippocampal system plays only an indirect role in consciousness—it organises the database, so to speak, on which other brain systems may operate and, in so doing, determines the structure and range of conscious recollection."

Curran and Schacter (1997, p. 45) related some of the basic ideas discussed in this section to the implicit/ explicit distinction:

Implicit memory reflects primarily the bottom-up, nonconscious effects of prior experience on single brain subsystems, and may also involve interactions between a limited number of brain subsystems. Explicit memory reflects the top-down, simultaneous retrieval of information from multiple information-processing brain mechanisms. This massive integration of information (e.g., perceptual, semantic, temporal, spatial, etc.) may be necessary to support conscious recollection of previous experiences.

According to this viewpoint, information processing typically proceeds through two stages: (1) specific forms of processing in several brain subsystems; (2) integration of information from these brain subsystems. The processing of amnesic patients is essentially normal at the first stage, but severely impaired at the second stage. The various theories we have considered in this chapter are broadly compatible with that viewpoint:

• The conscious recollection of the past involved in episodic memory may depend on the second or integrative stage of processing.

• The context processing deficit theory focuses on the inability of amnesic patients to integrate contextual information with to-be-remembered information, which presumably occurs at the second stage of processing.

• The transfer appropriate processing theory focuses on the problems that amnesics have with conceptual processes, and these conceptual processes generally occur at the second stage of processing.

• As we have seen, Cohen et al. (1997) argued that declarative memory is basically concerned with the integration of information.

Final thoughts

Neuroimaging has begun to transform research on the brain systems involved in long-term memory. The change this has produced was well expressed by Gabrieli (1998, p. 108): "For nearly a quarter of a century, our understanding of the normal brain organisation depended upon studies of diseased memory. Now, functional neuroimaging studies of healthy brains can begin to illuminate how and why injuries to specific memory systems result in various diseases of memory."

In spite of the successes of the neuroimaging approach, it does not always shed much light on what is happening. For example, Shallice et al. (1994, p. 635) carried out a PET study on normals who learned or retrieved verbal material, and came to the following conclusion: "In common with nearly all relevant functional imaging studies, our study has failed to show selective activation of medial brain structures (apart from the thalamus), damage to which causes amnesia." The fact that the major role of the hippocampus in declarative memory does not always emerge from neuroimaging studies shows very clearly the importance of using a variety of techniques to study human memory.

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