Visual word form processing deficits driven by severity of reading impairments in children with developmental dyslexia

Abstract

The visual word form area (VWFA) in the left ventral occipito-temporal (vOT) cortex is key to fluent reading in children and adults. Diminished VWFA activation during print processing tasks is a common finding in subjects with severe reading problems. Here, we report fMRI data from a multicentre study with 140 children in primary school (7.9-12.2 years; 55 children with dyslexia, 73 typical readers, 12 intermediate readers). All performed a semantic task on visually presented words and a matched control task on symbol strings. With this large group of children, including the entire spectrum from severely impaired to highly fluent readers, we aimed to clarify the association of reading fluency and left vOT activation during visual word processing. The results of this study confirm reduced word-sensitive activation within the left vOT in children with dyslexia. Interestingly, the association of reading skills and left vOT activation was especially strong and spatially extended in children with dyslexia. Thus, deficits in basic visual word form processing increase with the severity of reading disability but seem only weakly associated with fluency within the typical reading range suggesting a linear dependence of reading scores with VFWA activation only in the poorest readers.

Figure 1

Task. Word or symbol stimuli occurred in the middle of the screen. The captions “Tier”/“###” and “Sache”/“#*#” remained on the screen throughout the experiment, as a reminder of the task: the child was asked to press the left button if a word was an animal (in German “Tier”) or if a string was made up of the same symbols, and to press the right button if a word was not an animal (a thing: in German “Sache”) or if a string contained different symbols. Assignments of left and right buttons were counterbalanced across subjects. Example words are “Fisch” (fish) and “Kerze” (candle).

Figure 2

Word-symbol contrasts and regression with reading fluency. Word-sensitive activation in the vOT region was only found for the whole sample and the typical readers but not for children with dyslexia (but see Fig. 3 for direct group contrasts). Typical readers and children with dyslexia both show word-sensitive activation in the left inferior frontal gyrus. The analysis with reading fluency as regressor revealed increased word-sensitive activation the higher childrens’ reading fluency in the left fusiform gyrus in the whole sample on a cluster extent corrected threshold p_(FWEc) < 0.05 (p_(CDT) < 0.005) and on a voxel wise p_(FWEp) < 0.05 in the group of children with dyslexia. No significant correlation was found in the group of typical readers. Illustrated are one sagittal (MNI x = − 38) and five axial slices (MNI z =  −18, z =  − 12, z = 10, z = 30, z = 50). The contrast words > symbols is shown in green and the positive regression of the contrast words > symbols with reading fluency z-scores is shown in orange for each group (all children (top), typical readers (middle) and children with dyslexia (bottom)). Activation threshold for visualization: p_(unc) < 0.001 (corresponding to a t > 3.15 for All; t > 3.21 for typical readers and t > 3.25 for dyslexia). The activated clusters are superimposed onto the ch2better.nii template using mricron (https://www.nitrc.org/projects/mricron).

Figure 3

Group contrast children with dyslexia vs. children with typical reading skills. Higher word-sensitive (words—symbols) activation for children with dyslexia than for typical readers was found in bilateral precentral gyri. Activation threshold for visualization is set to p_(unc) < 0.001 (corresponding to t > 3.16). The activation clusters are superimposed onto the ch2better.nii template using mricron (https://www.nitrc.org/projects/mricron).

Figure 4

Regression plots showing the positive association between reading fluency and print sensitive activation (words-symbols) in the WFA ROI (top: F(1,137) = 15.09, p = 0.0002, r² = 0.0992)) and the LFA ROI (bottom: F(1,135) = 1.37, p = 0.2433, r² = 0.0101) of the entire sample (including gap group; n = 140). The blue shaded areas represent the 95% confidence limits, the blue dashed lines the 95% prediction limits.

Figure 5

Mean betavalues for ROI data. The plots illustrate the betavalues for children with dyslexia and typical reading children for each condition; Words (green), Symbols (red), and the Words-Symbols difference (blue). The ROIs WFA and LFA are shown separately in the left and right columns, respectively. Each 'raincloud' plot shows (from left to right) individual data points, boxplot with median and interquartile range, mean and 95% CI, and sample distribution.