Intact but less accessible phonetic representations in adults with dyslexia

Abstract

Dyslexia is a severe and persistent reading and spelling disorder caused by impairment in the ability to manipulate speech sounds. We combined functional magnetic resonance brain imaging with multivoxel pattern analysis and functional and structural connectivity analysis in an effort to disentangle whether dyslexics' phonological deficits are caused by poor quality of the phonetic representations or by difficulties in accessing intact phonetic representations. We found that phonetic representations are hosted bilaterally in primary and secondary auditory cortices and that their neural quality (in terms of robustness and distinctness) is intact in adults with dyslexia. However, the functional and structural connectivity between the bilateral auditory cortices and the left inferior frontal gyrus (a region involved in higher-level phonological processing) is significantly hampered in dyslexics, suggesting deficient access to otherwise intact phonetic representations.

Fig. 1

Quality of phonetic representations as derived from multivoxel pattern analysis. Average correlations between the (normalized) activity patterns elicited by phonetically identical syllables, syllables differing in consonant, syllables differing in vowel, and syllables differing in both consonant and vowel for dyslexic (DR) and normal readers (NR) in each of the anatomical regions (error bars represent 1 SE). The larger the overall quality of the phonetic representations, the larger the differences between the baseline correlation (between phonetically identical syllables) and the other correlations. Correlations differing from this baseline correlation are indicated with blue asterisks (paired t-tests) and green asterisks (repeated-measures ANOVA, Tukey-corrected post-hoc t-tests) (* p < .05, ** p < .01, *** p < .001, **** p < .0001). On the left: a representation of the left hemisphere anatomical regions.

Fig. 2

Functional connectivity analysis. (A) Schematic representation of the predefined seed regions. The color-coding corresponds with the one used in Fig. 1. (B-C) Color-coded matrices represent functional connectivity (expressed in Z-scores) among the 13 seed regions in normal and dyslexic readers. Significant correlations (p < .001, FDR-corrected) are indicated by a black dot. (D) Statistical comparison of the functional connectivity between the groups. Significant group differences in functional connectivity are indicated by a black dot (independent t-test, p < .05, FDR-corrected). (E) Scatterplot of the association between reaction time on the phoneme discrimination task in the scanner (y-axis) and intrinsic functional connectivity between left STG and left IFG (x-axis). Dyslexic readers are depicted by red squares, normal readers by green triangles.