Distributed cell assemblies for general lexical and category‐specific semantic processing as revealed by fMRI cluster analysis

Abstract

Here, we ask whether frontotemporal cortex is functionally dissociated into distributed lexical and category-specific semantic networks. To this end, fMRI activation patterns elicited during the processing of words from different semantic categories were categorized using k-means cluster algorithms. Results showed a distributed pattern of inferiorfrontal, superiortemporal, and fusiform activation shared by different word categories. This shared activation contrasted with patterns of category-specific semantic activation in widely distributed neural systems. Clustering revealed congruent functional specificity of focal area activations in frontal and temporal cortex; thus suggesting a correspondence between functional partitionings of frontocentral mirror neuron systems and those of inferiortemporal lexical and semantic circuits. Action words related to the face, arms, and legs specifically activated the motor system in a somatotopic manner, whereas form-related words activated prefrontal areas. Similar functional specificity was evident in temporal cortex, where a different semantic topography emerged for form- and action-related words. Results were replicated in a separate data set, therefore recommending fMRI cluster analysis as a reliable method for scrutinizing the brain basis of lexical, semantic, and conceptual systems in humans. As focal modules do not explain the distributed character of functionally specific clusters and their distinct topographies are at variance with general distributed processing accounts, the functionally-homogenous distributed clusters specific to semantic types are best explained by specifically-distributed cortical circuits which, similar to Hebbian cell assemblies, represent functional units with specific roles in cognitive processing, especially in lexical and semantic access and memory.

Body-part-specific Representations of Semantic Noun Categories

“…Essentially, semantic processing has been reported in all parts of the left language-dominant hemisphere of right-handed study participants (for review, see Binder, Desai, Graves, & Conant, 2009;Vigneau et al, 2006;Pulvermüller, 1999). The more fine-grained approach to semantic categories put forward here might explain at least some of this local variability in terms of the semantic type of stimulus word under study (Pulvermüller, Kherif, Hauk, Mohr, & Nimmo-Smith, 2009). …”

Right fusiform response patterns reflect visual object identity rather than semantic similarity

“…Caution needs to be exercised when MVPA is applied to study category effects as subtle perceptual differences between classes of objects may be picked up by this highly sensitive method. Visuoperceptual similarity between entities from a same category is often intertwined with category membership, at least if stimuli are only presented through the visual input-modality and in particularly so when broad categories such as animate versus inanimate entities are compared (Devlin et al, 2002;Forde et al, 1997;Gale et al, 2001;Garrard et al, 2001;Mahon and Caramazza, 2009;Op de Beeck et al, 2008;Pilgrim et al, 2005;Pulvermüller et al, 2009;Rogers and McClelland, 2004;Sartori and Job, 1988). Some previous MVPA studies of semantic category effects (e.g.…”

Meaning and the brain: The neurosemantics of referential, interactive, and combinatorial knowledge

“…In addition to word-kind-specific fronto-central foci, differential activation was evident in inferior-temporal cortex (Fig. 5, Pulvermüller, Kherif, Hauk, Mohr, & Nimmo-Smith, 2009).…”