Whereas semantic dementia and Alzheimer's disease are progressive disorders arising from intrinsic changes in brain cells, herpes simplex encephalitis (HSE) is an acute, nonprogressive illness. The viral agent attacks principally the frontotemporal regions, and although about one-third of patients make a full recovery, others are left with permanent residual deficits, reflecting damage to the medial temporal lobes, temporal neocortex and/or the frontal lobes. Semantic deficits occur in those patients with temporal neocortical damage (Pietrini et al., 1988; Sartori & Job, 1988; Warrington & Shallice, 1984). Those deficits may be relatively selective, although they are more usually accompanied by classical amnesia and/or frontal executive and behavioural changes. Hemispheric asymmetry, with greater damage to the left or right temporal lobe, is common. Herpes simplex encephalitis is theoretically important because it may give rise to circumscribed semantic deficits in which certain categories of knowledge appear to be impaired more than others.
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