Why do herpes simplex encephalitis and semantic dementia show a different pattern of semantic impairment in spite of their main common involvement within the anterior temporal lobes?

A very challenging problem in the domain of the cognitive neurosciences is to explain why herpes simplex encephalitis and semantic dementia show, respectively, a category-specific semantic disorder for biological entities and an across-categories semantic disruption, despite highly overlapping areas of anterior temporal lobe damage. The aim of the present review consisted in trying to make a separate survey of anatomo-clinical investigations (single-case studies and group studies) and of activation studies, in order to analyse the factors that could explain these different patterns of semantic disruption. Factors taken into account in this review were laterality of lesions, disease aetiology, kind of brain pathology and locus of damage within the temporal lobes. Locus of damage within the temporal lobes and kind of brain pathology seemed to play the most important role, because in patients with herpes simplex encephalitis and category-specific semantic disorder for biological entities the lesions prevailed in the anteromedial temporal lobes. Furthermore, the neuropathology concerned both the anterior temporal cortices and the white matter pathways connecting these areas with the posterior visual areas, whereas in semantic dementia the inferior longitudinal fasciculus involvement was restricted to the rostral temporal lobe and did not extend into the cortically uninvolved occipital lobe.