The role of disconnection mechanisms in category-specific semantic disorders for living beings. A critical review

Abstract

This review reconsiders some debates raised by the discovery that herpes simplex encephalitis (HSE) can provoke category-specific semantic disorders (CSSD) for living entities, due to the greater weight that visual features have in the identification of members of this category. The main issues taken into account regard the interaction between two research domains. The first concerns the fractionation of the ‘living beings’ into the ‘animals’ and the ‘plant life’ categories, which has shown that the impairment of fruits and vegetables is provoked by lesions in the territory of the left posterior cerebral artery (PCA). The second concerns the role played by the etiology of lesions on CSSD for living beings. This issue stems from the fact that CSSD for living beings have been rarely reported in Semantic Dementia (SD), even if both HSE and SD involve the same anterior fronto-temporal structures. Our review strongly suggests that the impairment of plant life categories in patients with left PCA infarct may be due to a posterior disconnection mechanism, decoupling the verbal representations of fruits and vegetables from the processing of colors only through the right hemisphere. A disconnection of white matter tracts could also explain the discrepancy between HSE and SD lesions in the generation of CSSD for living beings, because SD affects only the cortical neurons, whereas the swelling lesions typical of HSE involve also the underlying white matter that plays a critical role in disconnection mechanisms.