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Dyslexic Children Lack Word Selectivity Gradients in Occipito-Temporal and Inferior Frontal Cortex

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Dyslexic Children Lack Word Selectivity Gradients in Occipito-Temporal and Inferior Frontal Cortex

O A Olulade et al. Neuroimage Clin.

Abstract

fMRI studies using a region-of-interest approach have revealed that the ventral portion of the left occipito-temporal cortex, which is specialized for orthographic processing of visually presented words (and includes the so-called "visual word form area", VWFA), is characterized by a posterior-to-anterior gradient of increasing selectivity for words in typically reading adults, adolescents, and children (e.g. Brem et al., 2006, 2009). Similarly, the left inferior frontal cortex (IFC) has been shown to exhibit a medial-to-lateral gradient of print selectivity in typically reading adults (Vinckier et al., 2007). Functional brain imaging studies of dyslexia have reported relative underactivity in left hemisphere occipito-temporal and inferior frontal regions using whole-brain analyses during word processing tasks. Hence, the question arises whether gradient sensitivities in these regions are altered in dyslexia. Indeed, a region-of-interest analysis revealed the gradient-specific functional specialization in the occipito-temporal cortex to be disrupted in dyslexic children (van der Mark et al., 2009). Building on these studies, we here (1) investigate if a word-selective gradient exists in the inferior frontal cortex in addition to the occipito-temporal cortex in normally reading children, (2) compare typically reading with dyslexic children, and (3) examine functional connections between these regions in both groups. We replicated the previously reported anterior-to-posterior gradient of increasing selectivity for words in the left occipito-temporal cortex in typically reading children, and its absence in the dyslexic children. Our novel finding is the detection of a pattern of increasing selectivity for words along the medial-to-lateral axis of the left inferior frontal cortex in typically reading children and evidence of functional connectivity between the most lateral aspect of this area and the anterior aspects of the occipito-temporal cortex. We report absence of an IFC gradient and connectivity between the lateral aspect of the IFC and the anterior occipito-temporal cortex in the dyslexic children. Together, our results provide insights into the source of the anomalies reported in previous studies of dyslexia and add to the growing evidence of an orthographic role of IFC in reading.

Keywords: Dyslexia; Inferior Frontal Cortex; Occipito-temporal Cortex; Regions of Interest; Visual Word Form System; Word Selectivity; fMRI.

Figures

Fig. 1
Fig. 1
Whole-brain activation maps surface rendered on the standardized MNI SPM brain template. Within-group maps are displayed for the control group and the dyslexic groups: RW > Fix (top) and FF > Fix (middle) maps were thresholded at p < 0.001 corrected for cluster size. RW > FF (bottom) maps were thresholded at p < 0.001 (uncorrected; minimum k = 20). RW: real words; FF: false-fonts. Between-group comparisons are shown for controls > dyslexics (p < 0.001, uncorrected for all conditions; minimum k = 20).
Fig. 2
Fig. 2
Gradient images illustrating the layout of spatial sensitivity to real words (blue) and false-fonts (red) in occipito-temporal and inferior frontal regions. Differential activity (spmT maps) between real words and false-fonts was generated for each group using within-group whole-brain activation maps for each condition relative to fixation. fMRI t-statistic maps (p < 0.005; uncorrected) for FF vs. Fix were subtracted from the maps for RW vs. Fix. Resulting images were overlaid onto sectional slices using a standard MNI SPM brain template. These images are presented solely for the purpose of visualization of the gradients. (A) Maps illustrating differential activation between real words and false-fonts in the occipito-temporal cortex. Maps for the control group are shown at the top and for the dyslexic group at the bottom. (B) Gradient maps for control (top) and dyslexic (bottom) groups in the inferior frontal region. Images were masked to display activity contained within occipito-temporal and inferior frontal regions. L: Left hemisphere; R: right hemisphere; RW: real words; FF: false-fonts; x, y, and z coordinates represent locations in the standard MNI space. Color bar represents the gradient of sensitivity to each condition (t-statistic difference), with regions exhibiting greater activation for real words in red, and regions exhibiting greater activation for false-fonts in blue.
Fig. 3
Fig. 3
Region-of-interest analysis in left hemisphere occipito-temporal and inferior frontal regions. (A) Top left: ROIs in the left occipito-temporal region. ROI 1 represents the most posterior location, while ROI 6 is the most anterior location. ROI 5 (green) represents the location closest to the reported peak of the VWFA. (B) ROIs selected in the left inferior frontal region. ROI 1 represents the most medial location, while ROI 5 represents the most lateral location. (A & B) Top right: Mean percent signal change for real words minus false-fonts in the ROIs for the control (pink) and dyslexic (yellow) groups. Bottom: Mean percent signal change in each group for real words (red) and false-fonts (blue) separately. Error bars represent 1 S.E.M. L: Left hemisphere; R: right hemisphere.

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