Is a single ‘hub’, with lots of spokes, an accurate description of the neural architecture of action semantics?: Comment on “Action semantics: A unifying conceptual framework for the selective use of multimodal and modality-specific object knowledge” by van Elk, van Schie and Bekkering

Abstract

How is knowledge about tools and their associated actions represented in the brain? In their otherwise excellentreview of the behavioral, developmental, and neuropsychological literatures, Van Elk, van Schie, and Bekkering [16]argue that the so-called ‘hub and spoke’ architecture [12] provides a good answer. Semantics, in this model, is “rep-resented across modality-specific ‘spokes’ that converge in a multimodal association area (i.e. the ‘semantic hub’)that provides an amodal representation of semantic information”. Exactly what type of information is contained inan ‘amodal’ object representation, and how this representation is created from the convergence of information frommultiple modalities, is left unsaid.Leaving aside these thorny theory-laden concerns, in this comment we highlight several problems with the ‘huband spoke’ architecture. The semantic ‘hub’ is claimed to reside in a specific region of the brain, the anterior temporallobes, bilaterally (ATL). This claim is largely based on the notion that the clinical disorder of semantic dementia iscaused solely by damage limited to the ATL. This is not the case. Semantic dementia is a progressive disorder thattypically involves a broad expanse of temporal, and often frontal, cortex, as well as subcortical regions – especiallyduring the later stages when conceptual knowledge is most degraded (e.g., [5]). More to the point, even if the dam-age associated with semantic dementia was limited to the ATL, the fact that this damage produces semantic deficitsinvolving multiple object categories and concepts does not constitute evidence for a semantic ‘hub’ any more so thanwould finding that damage to posterior temporal cortex – a brain region known to represent multiple object categoriesin spatially distinct regions – constitute evidence for an object perception ‘hub’.