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. 2013 Jun 27;7:303.
doi: 10.3389/fnhum.2013.00303. Print 2013.

Clustering the Lexicon in the Brain: A Meta-Analysis of the Neurofunctional Evidence on Noun and Verb Processing

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Free PMC article

Clustering the Lexicon in the Brain: A Meta-Analysis of the Neurofunctional Evidence on Noun and Verb Processing

Davide Crepaldi et al. Front Hum Neurosci. .
Free PMC article

Abstract

Although it is widely accepted that nouns and verbs are functionally independent linguistic entities, it is less clear whether their processing recruits different brain areas. This issue is particularly relevant for those theories of lexical semantics (and, more in general, of cognition) that suggest the embodiment of abstract concepts, i.e., based strongly on perceptual and motoric representations. This paper presents a formal meta-analysis of the neuroimaging evidence on noun and verb processing in order to address this dichotomy more effectively at the anatomical level. We used a hierarchical clustering algorithm that grouped fMRI/PET activation peaks solely on the basis of spatial proximity. Cluster specificity for grammatical class was then tested on the basis of the noun-verb distribution of the activation peaks included in each cluster. Thirty-two clusters were identified: three were associated with nouns across different tasks (in the right inferior temporal gyrus, the left angular gyrus, and the left inferior parietal gyrus); one with verbs across different tasks (in the posterior part of the right middle temporal gyrus); and three showed verb specificity in some tasks and noun specificity in others (in the left and right inferior frontal gyrus and the left insula). These results do not support the popular tenets that verb processing is predominantly based in the left frontal cortex and noun processing relies specifically on temporal regions; nor do they support the idea that verb lexical-semantic representations are heavily based on embodied motoric information. Our findings suggest instead that the cerebral circuits deputed to noun and verb processing lie in close spatial proximity in a wide network including frontal, parietal, and temporal regions. The data also indicate a predominant-but not exclusive-left lateralization of the network.

Keywords: clustering algorithm; left inferior frontal gyrus; meta-analysis; neuroimaging; noun-verb dissociation; task demand.

Figures

Figure 1
Figure 1
Example of dendrogram (tree) resulting from the hierarchical clustering procedure. The leaves at the bottom represent each individual activation coordinate. At each subsequent step, two clusters from the level immediately below are merged to form a new cluster. The number of clusters is thus decreased by one at each level, going from a total of N clusters at step 1 (where N is the number of input activation peaks) to one all-inclusive cluster at the last step.
Figure 2
Figure 2
Cluster distribution for the number of peaks included in each cluster (X-axis) and the number of studies contributing peaks to each cluster (Y-axis). The bimodal distribution of these variables is illustrated in the main panel, where each point represents a cluster (note that several points/clusters may overlap because of a same number of studies and peaks included). Unimodal distributions are represented through the histograms in the side panels. Dotted lines represent cut-off values.
Figure 3
Figure 3
Panel (A) illustrates the clusters that are associated either with noun (green) or verb (purple) processing. Panel (B) reports the clusters that are associated with task-by-grammatical class interaction (the brighter the color, the higher the number of activation peaks included in the cluster).

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