Imagery or meaning? Evidence for a semantic origin of category-specific brain activity in metabolic imaging - PubMed (original) (raw)

Comparative Study

Imagery or meaning? Evidence for a semantic origin of category-specific brain activity in metabolic imaging

Olaf Hauk et al. Eur J Neurosci. 2008 Apr.

Abstract

Category-specific brain activation distinguishing between semantic word types has imposed challenges on theories of semantic representations and processes. However, existing metabolic imaging data are still ambiguous about whether these category-specific activations reflect processes involved in accessing the semantic representation of the stimuli, or secondary processes such as deliberate mental imagery. Further information about the response characteristics of category-specific activation is still required. Our study for the first time investigated the differential impact of word frequency on functional magnetic resonance imaging (fMRI) responses to action-related words and visually related words, respectively. First, we corroborated previous results showing that action-relatedness modulates neural responses in action-related areas, while word imageability modulates activation in object processing areas. Second, we provide novel results showing that activation negatively correlated with word frequency in the left fusiform gyrus was specific for visually related words, while in the left middle temporal gyrus word frequency effects emerged only for action-related words. Following the dominant view in the literature that effects of word frequency mainly reflect access to lexico-semantic information, we suggest that category-specific brain activation reflects distributed neuronal ensembles, which ground language and concepts in perception-action systems of the human brain. Our approach can be applied to any event-related data using single-stimulus presentation, and allows a detailed characterization of the functional role of category-specific activation patterns.

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Figures

Fig. 1

Brain activation modulated by semantic variables. (A) Activation peaks for the variables Action-relatedness (red) as well as Imageability (blue), projected on the surface (top) and on cortical slices (bottom) of a standard brain. Encircled activation spots were significant after SVC, with volumes of interest defined based on previous publications. Activation is displayed at a statistical threshold P < 0.001 uncorrected. Colour coding reflects _t_-values. Coordinates and statistics are provided in Table 1. (B) Parameter estimates (arbitrary units) for the two semantic variables in peak voxels of left temporal gyrus (LT) and left fusiform gyrus (LFF), as well as right superior temporal gyrus (RST) and right fusiform gyrus (RFF). The error bars represent within-subject standard errors.

Fig. 2

Category-specific effects of word frequency. (A) Brain areas significantly negatively correlated with Frequency for action-related words (red) and visually related words (blue) separately, projected on the surface (top) and on cortical slices (bottom) of a standard brain. Activation is displayed at a statistical threshold P < 0.001 uncorrected. The rendered image at the upper right represents the overlap of activations from this analysis with that presented in Fig. 1A at threshold P < 0.01. Colour coding reflects _t_-values. Coordinates and statistics are provided in Table 2. (B) The left diagram presents parameter estimates for the variable Frequency for action-related words and visually related words separately, for peak voxels in the left middle temporal gyrus (LMT) and left fusiform gyrus (LFF). The right diagram shows parameter estimates for the same voxels but for the variables Action-relatedness (Act.-rel.) and Imageability (Imag.) across all words independent of word category. The error bars represent within-subject standard errors.

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Imagery or meaning? Evidence for a semantic origin of category-specific brain activity in metabolic imaging - PubMed (original) (raw)