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The consolidation theory

This job considers the theory that amnesia results from a consolidation deficit.

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Squire's (1992) view is that the hippocampus, and related structures, are necessary to establish declarative memories in a lasting manner in the neocortex. New associations are possibly formed through long-term potentiation (LTP), a form of synaptic plasticity. When a fact is associated with its semantic context, a new concept is formed. LTP binds together the regions that converge on the hippocampus, and possibly adjacent regions. These support the storage of information in the neocortex. Once memories are stored there, the hippocampus, eventually, becomes unnecessary for their organisation or retrieval. It is predicted that correct recognition, recall, and the associated confidence ratings, will be similarly impaired by damage to this region, but non-declarative memory will be preserved.

Hirst, Johnson, Phelps, & Volpe, (1988) tested free recall and recognition in amnesic patients. They showed a deficit for free recall but the same pattern of confidence in their recognition decisions as controls. This is not predicted by the consolidation deficit hypothesis. Haist, Shimamura, & Squire, (1992) were unable to replicate Hirst et al.'s (1988) finding. They considered the involvement of frontal lobe damage was unclear in Hirst et al.'s (1988) study. When patient scores for recognition were matched with those of controls by comparing them with control scores for a longer retention period, Haist et al. (1992) found that patient recall and confidence scores also matched those of controls at this period. This was considered consistent with declarative memory that is little affected by non-declarative processes.

Faster forgetting was tested by Isaac and Mayes (1999a, ; 1999b). In tests for free recall, cued recall, and recognition of prose, they found that global amnesic patients showed accelerated forgetting over delays of up to 10min, but normal recognition over delays of an hour. They also found accelerated forgetting in free recall for semantically related, but not unrelated, word lists. They explained their results with a dual-deficit hypothesis. In this, damage to the extended hippocampal system disrupts consolidation of complex item-context associations resulting in a decline in free recall. Damage to the perirhinal cortex-DM system impairs immediate storage of item information and simple associations. This causes reduced recall and recognition at short delays. This is inconsistent with a single deficit that underlies all recognition and recall performance. Hence, the view that amnesia results from a global consolidation deficit has to be adjusted to account for the evidence (but see Shimamura, 2002). However, the different findings of different experiments highlights the difficulties with measurement when matching performance of those with amnesia with those of controls.

Shimamura (2002) proposes that differences in the extent of retrograde amnesia for different forms of declarative memory are not due to qualitative differences in the way new representations are integrated with existing representations in the association cortex by hippocampal-cortical connections. The difference is due to typically greater reactivation of semantic than episodic representations. He also cites neuroimaging studies that support the role of the medial temporal lobe during retrieval of both remote and recent memories.

References:
Haist, F., Shimamura, A. P., & Squire, L. R. (1992). On the relationship between recall and recognition memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 18(4), 691-702.

Hirst, W., Johnson, M. K., Phelps, E. A., & Volpe, B. T. (1988). More on recognition and recall in amnesics. Journal of Experimental Psychology: Learning, Memory, and Cognition, 14(4), 758-762.

Isaac, C. L., & Mayes, A. R. (1999a). Rate of forgetting in amnesia: I. Recall and recognition of prose. Journal of Experimental Psychology: Learning, Memory, and Cognition., 25(4), 942-962.

Isaac, C. L., & Mayes, A. R. (1999b). Rate of forgetting in amnesia: II. Recall and recognition of word lists at differentlevels of organization. Journal of Experimental Psychology: Learning, Memory, and Cognition., 25(4), 963-977

Shimamura, A.P. (2002). Relational Binding Theory and the Role of Consolidation in Memory Retrieval. In L. R. Squire & D. L. Schacter. Neuropsychology of Memory, (3rd ed.). New York: The Guildford Press.

Squire, L. R. (1992). Memory and the hippocampus: A synthesis from findings with rats, monkeys, and humans. Psychological-Review, 99(2), 195-231.

For an overview of amnesia, see briefing notes on: http://ess.ntu.ac.uk/miller/cognitive/amnchima.htm