Neuroimaging the semantic system(s)

Abstract

Background: The organisation of the semantic system is controversially discussed among cognitive scientists. Whereas some authors assume that semantic processing is amodal, others propose modality-specific semantic systems. The amodal account holds that there is a single semantic system independent of the modality of stimulus presentation and nature of the concept. The multi-modal account, on the other hand, postulates that semantic knowledge is represented in modality-specific, distinct semantic systems. Attempts to integrate both accounts resulted in a hybrid model of semantic processing. Although an amodal semantic network is assumed in this view, input-dependent semantic specialisation coexists. Aims: The purpose of this study was to examine contrasting theories of the representation of the semantic knowledge with functional magnetic resonance imaging (fMRI). Methods & Procedures: Nine healthy subjects performed a semantic judgement task on graphemic words and pictures in the experimental condition, as well as a visual identity judgement task on pseudoword and three-dimensional geometric objects in the control condition. Outcomes & Results: A common activation pattern for words and pictures was found in the left inferior frontal gyrus and in the left middle temporal gyrus. Word-specific effects occurred in the left superior parietal, the left inferior occipital, the left anterior cingulate gyrus, and the right inferior frontal gyrus as well as bilaterally in the anteriomedial temporal area. Picture-specific areas involved the left supramarginal gyrus and the left cingulate gyrus. Further activation was found in the right middle frontal gyrus and bilaterally in the fusiform and inferior occipital gyrus. Conclusions: The results can best be accommodated in the framework of an integrative, hybrid model of semantic representation combining features of the amodal and the modality-specific account.