Cortical activations for symmetry effect on visual word form perception in developmental dyslexia

Developmental reading difficulties may be associated with visual-spatial deficits. Poor readers often make mirror errors along the vertical axis and cannot discriminate between symmetrical patterns, such as d and b as different letters. These mirror errors, observed both in the alphabetical and orthographic languages, may contribute to the difficulties in analyzing the spatial relationships among strokes within a character. Here we investigated the effect of symmetry on visual word form perception in developmental dyslexic (DD) and chronological aged (CA)-matched typical juvenile (aged 12-17 years). We employed real-, non-, jiagu- and scrambled characters in hanzi with symmetric or asymmetric compositions as stimuli. In the behavioral experiment, observers had to judge whether the two stimuli presented on both sides of the central fixation were identical. Both groups responded more accurately in the symmetric compositions than the asymmetric ones. This symmetry effect was found for all character types except for the real-characters. In a block-design functional magnetic resonance imaging (fMRI) experiment, we measured blood oxygenation level dependent (BOLD) activations when observers were asked to match the presented character with that of the previous trial. The visual word form area (VWFA) in the left fusiform gyrus showed significantly differential cortical activations to real- versus scrambled characters for the CA group but not for the DD group. The CA group showed stronger BOLD activations in the lingual and fusiform areas while processing non-characters versus real-characters compared to the DD group. While analyzing symmetric versus asymmetric compositions, the DD group had a greater activation in the bilateral fusiform gyri which are sensitive to global configurations of a visual word form. Taken together, the behavioral and neural evidence revealed intact symmetry processing in the dyslexic readers, suggesting independent processes for spatial configuration and orthographic visual word forms.