The relationships between temporal lobe and diencephalic structures implicated in anterograde amnesia

The relationship between the anterograde amnesic syndromes associated with diencephalic and temporal lobe pathology is examined in the light of recent findings. It is proposed that a common feature of anterograde amnesia is damage to part of an "extended hippocampal system" comprising the hippocampus, the fornix, the mammillary bodies, and the anterior thalamic nuclei. Damage to this system results in deficits in the recall of episodic information, the core symptom of anterograde amnesia. In contrast, lesions in this system need not disrupt tests of recognition memory when they primarily tax familiarity judgements. It is assumed that familiarity judgements depend on other regions (e.g. the rhinal cortex in the case of temporal lobe amnesia) and that the extended hippocampal system is principally involved in those aspects of recognition that are retrieval-based rather than familiarity-based. These proposals arise from new evidence on the performance of delayed nonmatching-to-sample by animals, from a meta-analysis of the performance of amnesic subjects on a test of recognition memory, and from new research into the pattern of connections between the medial temporal lobe and the medial diencephalon in primates.