Semantic dementia: relevance to connectionist models of long-term memory

Semantic dementia is a recently documented syndrome associated with non-Alzheimer degenerative pathology of the polar and inferolateral temporal neocortex, with relative sparing (at least in the early stages) of the hippocampal complex. Patients typically show a progressive deterioration in their semantic knowledge about people, objects, facts and the meanings of words. Yet, at least clinically, they seem to possess relatively preserved day-to-day (episodic) memory. Neuropsychological investigations of semantic dementia provide, therefore, a unique opportunity to investigate the organization of human long-term memory and, more specifically, to determine the relationship between semantic memory and other cognitive systems, such as episodic memory. In this review, we summarize recent empirical findings from patients with semantic dementia and discuss whether the neuropsychological phenomena of the disease are consistent with current cognitive and computational models of human long-term memory and amnesia. Six specific issues are addressed: (i) the relative preservation of category-level (superordinate) compared with fine-graded (subordinate) semantic knowledge as the disease progresses; (ii) the better recall of recent autobiographical and semantic memories compared with those in the distant past; (iii) the preservation of new learning, as measured by recognition memory, early in the disease; (iv) the interaction between autobiographical experience and semantic knowledge in the current, but not the distant, time-period; (v) increased long-term forgetting of newly learned material; and (vi) impaired implicit memory. It is concluded that recent findings from semantic dementia offer strong support for the view that memory consolidation in humans is dependent upon interactions between the hippocampal complex and neocortex. Furthermore, these investigations have provided computational modellers of human memory with a novel set of neuropsychological data to be simulated and tested.