The anatomical bases of semantic retrieval deficits in early Alzheimer's disease

Semantic abilities deteriorate early in patients with Alzheimer's disease (AD) and their residual language is characterised by strong lexical effects such as the age of acquisition of words and their typicality. The anatomical bases of this early semantic degradation have not been fully explored. To clarify which neural structures, when atrophic, alter lexical–semantic function in patients with very mild AD, this study correlated the lexical attributes of words produced in a semantic fluency task with grey matter density values from 3D MRI scans of mild AD patients. The voxel-based analyses showed a significant correlation between the lexical attributes characterising residual linguistic production in early AD patients and the integrity of regions of the medial temporal lobes, especially in areas of the perirhinal and parahippocampal cortex. This correlation was present in both hemispheres. There were no correlations within these structures with scores on neuropsychological tests not involving semantic or episodic memory. The results have implications for the role of medial temporal structures in episodic and semantic retrieval and argue against a unitary function of these structures in respect of episodic and semantic memory processes. This evidence suggests that specialised regions within the hippocampal complex engage in processes of encoding and retrieval for both semantic and episodic memories.