Brain Mechanisms Underlying Visuo-Orthographic Deficits in Children With Developmental Dyslexia

Abstract

Multiple hypotheses have been proposed to explain the reading difficulty caused by developmental dyslexia (DD). The current study examined visuo-orthographic processing in children with dyslexia to determine whether orthographic deficits are explainable based solely on visual deficits. To identify orthographic-specific, visual perception-specific, and overlapping deficits, we included two tasks (lexical and perceptual) in three Chinese subject groups: children with DD, age-matched controls (AC), and reading matched controls (RC) using functional magnetic resonance imaging (fMRI). We found that the left precuneus showed decreased activation across both tasks for the DD group compared to the two control groups, thus reflecting visual processing deficits in children with DD, which also affects orthographic processing. Furthermore, we found that the functional connectivity between left middle occipital gyrus (LMOG) and left inferior frontal gyrus (IFG) was decreased in the DD group compared to AC and RC for only the lexical task. This suggests a weaker association between orthography and phonology for children with DD. In addition, the children with DD showed decreased functional connectivity between the LMOG and right parahippocampal gyrus for only the visual perceptual task, thereby indicating a weaker association between visual regions for DD during visual symbol processing. Taken together, our findings suggest that the observed orthographic processing deficit in DD might be driven by both a basic visual deficit, and a linguistic deficit.

Keywords: PPI; dyslexia; fMRI; orthographic deficit; visual deficit.

FIGURE 1

(A) Mean accuracy by group for the lexical and perceptual tasks, error bars are standard error. There were no significant group differences on accuracy. (B) Mean reaction time by group for the lexical and perceptual tasks, error bars are standard error. DD had a faster reaction time than AC for the lexical and perceptual tasks. ^∗p < 0.01.

FIGURE 2

Conjunction analyses of group comparisons in brain activation for the lexical and perceptual tasks. (A) Conjunction of AC > DD for the lexical task, RC > DD for the lexical task, AC > DD for the perceptual task, and RC > DD for the perceptual task (left precuneus, in Red). (B) Conjunction of DD > AC for the lexical task, DD > RC for the lexical task, DD > AC for the perceptual task, and DD > RC for the perceptual task (right pre/post central gurus, in Blue).

FIGURE 3

Group × task interaction effect on functional connectivity between the right pre/postcentral gyrus and the right anterior cingulate cortex (ACC). DD had weaker functional connectivity than AC and RC only for the perceptual task. There was no group difference in the lexical task. ^∗∗P < 0.01.

FIGURE 4

Group × task interaction effects on functional connectivity of the seed regions left middle occipital gyrus (LMOG) and right middle occipital gyrus (RMOG). (A) Group × task interaction effect on functional connectivity between the LMOG and the left inferior frontal gyrus (IFG) (in Blue). DD had weaker functional connectivity than AC and RC for the lexical task, and stronger connectivity than AC for the perceptual task. (B,C) Group × task interaction effect on functional connectivity between the two seed regions (LMOG and RMOG) and the right parahippocampal gyrus (RPHIP), respectively. From LMOG to RPHIP (in Green), DD had weaker functional connectivity than AC but not RC for the perceptual task. There was no group difference for the lexical task. From the RMOG to the RPHIP (in Red), DD had weaker functional connectivity than RC and AC for the perceptual task. There was no group difference for the lexical task. The interaction effect for the LMOG and the RMOG overlapped at the RPHIP (in purple). ^∗P < 0.05, ^∗∗P < 0.01.