Surface errors without semantic impairment in acquired dyslexia: a voxel-based lesion-symptom mapping study

Abstract

Patients with surface dyslexia have disproportionate difficulty pronouncing irregularly spelled words (e.g. pint), suggesting impaired use of lexical-semantic information to mediate phonological retrieval. Patients with this deficit also make characteristic 'regularization' errors, in which an irregularly spelled word is mispronounced by incorrect application of regular spelling-sound correspondences (e.g. reading plaid as 'played'), indicating over-reliance on sublexical grapheme-phoneme correspondences. We examined the neuroanatomical correlates of this specific error type in 45 patients with left hemisphere chronic stroke. Voxel-based lesion-symptom mapping showed a strong positive relationship between the rate of regularization errors and damage to the posterior half of the left middle temporal gyrus. Semantic deficits on tests of single-word comprehension were generally mild, and these deficits were not correlated with the rate of regularization errors. Furthermore, the deep occipital-temporal white matter locus associated with these mild semantic deficits was distinct from the lesion site associated with regularization errors. Thus, in contrast to patients with surface dyslexia and semantic impairment from anterior temporal lobe degeneration, surface errors in our patients were not related to a semantic deficit. We propose that these patients have an inability to link intact semantic representations with phonological representations. The data provide novel evidence for a post-semantic mechanism mediating the production of surface errors, and suggest that the posterior middle temporal gyrus may compute an intermediate representation linking semantics with phonology.

Keywords: alexia; brain mapping; phonology; reading; semantics.

Patients with "surface" dyslexia characteristically mispronounce irregular words by over-reliance on pronunciation rules (e.g., reading plaid as "played"). Binder et al. identify neuroanatomical correlates of these "regularization" errors, and show that such errors can occur independent of damage to the semantic memory system.

Figure 1

Example lesion segmentation. Top row: T₁-weighted anatomical MRI showing the lesion in an example participant. Bottom row: Lesion segment, shown in red overlay, obtained using the semi-automated segmentation method.

Figure 2

VLSM results. (A) Lesion overlap across all 45 patients, thresholded to include only voxels that were lesioned in at least five patients. Colours indicate the degree of overlap. Subscript numbers indicate the x-axis location of the slice in standard space. The same locations are used for the other sagittal series. (B) VLSM analysis using rate of regularization errors as the dependent measure. Colours indicate z-scores. Subscripts below the coronal images indicate the y-axis location of the slices. Green lines on the axial image indicate the locations of orthogonal slices. (C) VLSM analysis using accuracy on the semantic picture matching task as the dependent measure.

Figure 3

Regularization errors and functional MRI consistency effect. Composite image showing the areas associated with regularization errors in the VLSM analysis and the location of spelling-sound consistency effects in a previous functional MRI (fMRI) study of healthy readers (Graves et al., 2010).