Neural activation and connectivity differences among semantic association, categorization and synonym judgment: an fMRI study

Abstract

The human capacity to understand the world depends on manipulating various concepts through language. To make sense of the complexities of a possibly infinite number of concepts, they must be linked and organised into a comprehensible structure. Semantic association and categorization are two of the various means for assigning relations between concepts. Semantic association refers to the temporal or spatial co-occurrence of concepts that are usually united by a theme, event or scene, while semantic categorization indicates the hierarchical organization of meaning. Despite the well-documented brain network of general semantic processing, less is known about how the semantic network is engaged to support association and categorization. This study compares the neural correlates and connectivity patterns of these two processes to synonym judgement, which measures general semantic processing, with a within-subject fMRI study. Twenty-three native Mandarin speakers were recruited in Beijing and scanned with a Siemens Prisma 3T scanner at Peking University. Association, categorization and synonym judgements tasks on Chinese characters pairs were devised to probe into the three processes respectively in different runs. A font-size judgement task was used as the baseline task. Activation maps of each semantic process were obtained by contrasting the semantic blocks with the baseline blocks. To compare the relative contributions of each region in the semantic network to the three processes, a region-of-interest (ROI) analysis was conducted at 7 locations according to published meta-analyses of the semantic network (Jackson, Hoffman, Pobric, & Lambon Ralph, 2015; Wu, Ho, & Chen, 2012). To further examine the directed information flow within the semantic network during Chinese reading, a Granger causality (GC) analysis among the chosen ROIs was conducted with the MVGC toolbox in Matlab (Barnett & Seth, 2014). The whole brain analysis results showed that the three tasks commonly recruited the left dorsal and ventral lateral prefrontal cortices, including the middle frontal gyrus, pars opercularis, pars triangularis and pars orbitalis, bilateral medial frontal gyrus; the left superior parietal lobule (BA 7), left fusiform gyrus (BA 37) and the right cerebellum, but not the anterior temporal lobe or the angular gyrus. ROI analysis revealed stronger activation in the posterior middle temporal gyrus (pMTG) and the inferior frontal gyrus during association, but weaker activation in the pMTG during categorization. Granger causality analysis revealed similar connectivity patterns for association and categorization versus synonym judgement on the left hemisphere, but only categorization exhibited significant connections between the right insula and the left hemisphere. We discuss how the results suggest that demands on semantic retrieval, degree of accurate semantic representation, perceptual experiences and world knowledge lead to observable differences in the three kinds of semantic processing. In particular, synonym judgement is the relatively simple task that requires basic semantic retrieval but also more top-down modulation for accurate semantic representation; semantic association requires the connection of lexical meaning to perceptual experiences and hence more embodied semantic representations, while semantic categorization needs the integration of word meaning and world knowledge about hierarchical or taxonomic relationships.