Adlam, Anna-Lynne Ruth; (2003) Dissociations in memory: A study of developmental amnesia. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Bilateral damage to the medial temporal lobes (MTL) is associated with declarative (explicit) memory deficits. However, there is some debate as to whether the MTL functions as a unified system supporting both episodic (event) and semantic (factual) memory (e.g. Squire and Zola, 1998), or as a dissociable system, such that different aspects of declarative memory are supported by distinct regions within the MTL (e.g. Mishkin et al., 1997; 1998; Aggleton and Brown, 1999). Previous studies of patients with bilateral hippocampal pathology sustained in childhood have reported a selective impairment in episodic memory, and a relative sparing of semantic memory (e.g. Vargha-Khadem et al., 1997; Gadian et al, 2000). This profile, termed developmental amnesia (DA), provides some support for the hypothesis of a dissociable MTL system. This thesis investigates that hypothesis further. The performance of twelve patients with bilateral hippocampal pathology sustained in childhood is compared to that of a group of twelve age-, sex-, and IQ-matched controls using a combination of standardised tests and experimental measures to investigate dissociations between episodic and semantic memory, recall and recognition, and recollection and familiarity. In addition, functions presumed to be subserved by the frontal lobes and the basal ganglia are assessed using measures of executive function and motor skill learning, respectively. Bilateral hippocampal pathology was confirmed using structural magnetic resonance imaging techniques, including hippocampal volumetries, and voxel-based morphometry (VBM). Analyses of VBM data revealed morphological abnormalities in additional regions, including the thalamus and the white matter of the temporal lobes and near striatal areas. There was no evidence of basal ganglia structural abnormality, consistent with preserved motor skill learning. The neuropsychological findings offer some support for the hypothesis that the MTL system is dissociable, in that episodic memory was more impaired than semantic memory, and recall was more impaired than recognition. However, recollection was not more impaired than familiarity-based recognition relative to controls. No impairments were found on measures of executive function, suggesting that the disproportionate episodic and recall deficits are not attributable to additional frontal lobe dysfunction.

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