The format of conceptual representations disrupted in semantic dementia: A position paper

To account for the selective, progressive and multimodal conceptual disruption observed in semantic dementia (SD), Patterson et al., 2007 have proposed that the neural network for semantic memory cannot consist of a 'distributed only' system, but requires a single convergence zone or 'hub' that supports the interactive activation of representations in all modalities and for all semantic categories. This 'semantic hub' should be bilaterally supported by anterior regions of the temporal lobes (ATL) and contain amodal representations. However, anatomo-clinical data show that it is only in the moderate to advanced stages of the disease, when atrophy affects the ATL bilaterally, that the semantic impairment is 'multi-modal', whereas in its early stages, when important asymmetries can be observed at the level of the ATL, the semantic impairment can be modality-specific. In these cases, it mainly affects the lexical-semantic knowledge when the left temporal lobe is more atrophic and the pictorial representations when the atrophy prevails on the right side. In different sections of our position paper we review: (a) the lines of research suggesting that in lateralized forms of SD the semantic impairment is modality-specific; (b) other data suggesting a prevalent involvement of the left TL in verbal and of the right TL in pictorial/sensory aspects of conceptual knowledge; (c) recent data supporting respectively the model of a bilateral amodal semantic hub and that of two lateralized, modality-specific semantic networks. Taken together, the surveyed data suggest that the semantic disorder observed in SD is due to the co-occurrence of verbal and non-verbal defects, resulting from left and right ATL atrophy and that the multimodal semantic impairment observed in advanced stages of SD is due to the joint disruption of pictorial and verbal representations, rather than to the loss of an amodal knowledge, bilaterally supported by the ATL. (C) 2011 Elsevier Srl. All rights reserved.