Episodic memory in semantic dementia: Implications for the roles played by the perirhinal and hippocampal memory systems in new learning

Abstract

IntroductionThis target article describes how medial temporal lobemedial diencephalic interactions contribute to episodic memory. Previous models have focussed on neural circuitry within the temporal lobe. This earlier focus on "temporal lobe memory systems" arose from a number of assumptions about amnesia and models for amnesia. This target article questions these assumptions, and from this emerges a different way of considering the neural substrates of episodic memory. At the centre of this revision is the notion that the link from the hippocampus to the mamillary bodies and anterior thalamic nuclei, via the fornix, is critical for normal episodic memory (Gaffan 1992a). Moreover, damage to this axis is responsible for the core deficits in anterograde amnesia, as was originally proposed by Delay and Brion (1969). To understand why this view became unpopular and why it has now reemerged, it is necessary to describe how a number of past findings have been interpreted.We will first describe the main features of the proposed model. This is followed by a section summarising relevant evidence from studies of amnesia and animal models of amnesia, describing the way this evidence has often been interpreted. Section 4 examines certain assumptions underlying previous interpretations and shows that existing evidence can be reinterpreted in a different way. Section 5 BEHAVIORAL AND BRAIN SCIENCES (1999) 22, Abstract: By utilizing new information from both clinical and experimental (lesion, electrophysiological, and gene-activation) studies with animals, the anatomy underlying anterograde amnesia has been reformulated. The distinction between temporal lobe and diencephalic amnesia is of limited value in that a common feature of anterograde amnesia is damage to part of an "extended hippocampal system" comprising the hippocampus, the fornix, the mamillary bodies, and the anterior thalamic nuclei. This view, which can be traced back to Delay and Brion (1969), differs from other recent models in placing critical importance on the efferents from the hippocampus via the fornix to the diencephalon. These are necessary for the encoding and, hence, the effective subsequent recall of episodic memory. An additional feature of this hippocampal-anterior thalamic axis is the presence of projections back from the diencephalon to the temporal cortex and hippocampus that also support episodic memory. In contrast, this hippocampal system is not required for tests of item recognition that primarily tax familiarity judgements. Familiarity judgements reflect an independent process that depends on a distinct system involving the perirhinal cortex of the temporal lobe and the medial dorsal nucleus of the thalamus. In the large majority of amnesic cases both the hippocampal-anterior thalamic and the perirhinal-medial dorsal thalamic systems are compromised, leading to severe deficits in both recall and recognition.Keywords: amnesia; fornix; hippocampus; memory; temporal cortex; thalamus John Aggleton has been Professor of Cognitive Neuroscience ...