Distinct neural signatures of cognitive subtypes of dyslexia with and without phonological deficits

Abstract

Developmental dyslexia can be distinguished as different cognitive subtypes with and without phonological deficits. However, despite some general agreement on the neurobiological basis of dyslexia, the neurofunctional mechanisms underlying these cognitive subtypes remain to be identified. The present BOLD fMRI study thus aimed at investigating by which distinct and/or shared neural activation patterns dyslexia subtypes are characterized. German dyslexic fourth graders with and without deficits in phonological awareness and age-matched normal readers performed a phonological decision task: does the auditory word contain the phoneme/a/? Both dyslexic subtypes showed increased activation in the right cerebellum (Lobule IV) compared to controls. Subtype-specific increased activation was systematically found for the phonological dyslexics as compared to those without this deficit and controls in the left inferior frontal gyrus (area 44: phonological segmentation), the left SMA (area 6), the left precentral gyrus (area 6) and the right insula. Non-phonological dyslexics revealed subtype-specific increased activation in the left supramarginal gyrus (area PFcm; phonological storage) and angular gyrus (area PGp). The study thus provides the first direct evidence for the neurobiological grounding of dyslexia subtypes. Moreover, the data contribute to a better understanding of the frequently encountered heterogeneous neuroimaging results in the field of dyslexia.

Keywords: Children; Cognitive subtypes; Dyslexia; Neuroimaging; Phonological deficit; Reading.

Fig. 1

Assignment of the participants to the phonological dyslexic group and the non-phonological dyslexic group on the basis of the results of the phonological awareness test (BAKO) and reading quotient (SLS). Notes: Phonological dyslexics are shown in red with BAKO T-values < 40 and an SLS reading quotient below 90. Non-phonological dyslexics are shown in blue with BAKO T-values > 40 and an SLS reading quotient below 90. The controls are shown in green with BAKO T-values > 40 and an SLS reading quotient above 90. The large triangles show the mean value of each group.

Fig. 2

Brain activation differences between the phonological task (Ptask) and the control task (Ctask). The top image above shows the phonological network for the total sample. Each group is presented separately below: controls (Con; green), phonological dyslexics (PhoDys; red) and non-phonological dyslexics (NonPhoDys; blue), cluster size k ≥ 10 voxel (local maxima significant at p < .001 uncorrected).

Fig. 3

Conjunction analysis of phonological dyslexics versus controls and non-phonological dyslexics versus controls (PhoDys > Con ∩ NonPhoDys > Con), with a cluster size k ≥ 10 voxel (local maxima significant at p < .001 uncorrected). The bar graphs represent the activation power (Beta) of the phonological task minus the control task in the right cerebellum for phonological dyslexics (PhoDys; red); non-phonological dyslexics (NonPhoDys; blue) and controls (Con; green).

Fig. 4

Subtype-specific brain activation profiles for phonological dyslexic (PhoDys; red) and non-phonological dyslexics (Non-PhoDys; blue). PhoDys compared to NonPhoDys (and reverse) were examined and masked inclusive for those regions that showed significant effects in PhoDys versus controls (Con; green). All clusters have an extent size of k ≥ 10 voxel and local maxima are significant at p < .001 uncorrected.