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

Semantic dementia is a recently documented syndrome associated with non-Alz heimer degenerative pathology of the polar and inferolateral temporal neoco rtex, with relative sparing (at least in the early stages) of the hippocamp al complex. Patients typically show progressive deterioration in their sema ntic 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 p rovide, therefore, a unique opportunity to investigate the organization of human long-term memory and, more specifically, to determine the relationshi p between semantic memory and other cognitive systems, such as episodic mem ory. In this review we summarize recent empirical findings from patients wi th semantic dementia and discuss whether the neuropsychological phenomena o f the disease are consistent with current cognitive and computational model s of human long-term memory and amnesia. Six specific issues are addressed: (i) the relative preservation of category-level (superordinate) compared w ith fine-graded (subordinate) semantic knowledge as the disease progresses; (ii) the better recall of recent autobiographical and semantic memories co mpared with those in the distant past; (iii) the preservation of new learni ng, as measured by recognition memory, early in the disease; (iv) the inter action between autobiographical experience and semantic knowledge in the cu rrent, but not the distant, time-period; (v) increased long-term forgetting of newly learned material; and (vi) impaired implicit memory. It is conclu ded that recent findings from semantic dementia offer strong support for th e view that memory consolidation in humans is dependent upon interactions b etween the hippocampal complex and neocortex, Furthermore, these investigat ions have provided computational modellers of human memory with a novel set of neuropsychological data to be simulated and tested.