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

The ability to generalize across symmetric views is an essential feature of the human visual system. Poor readers often make mirror errors and struggle to distinguish between symmetrical letters, such as "b" and "d". Unlearning the mirror generalization is therefore crucial for developing proficient reading skills. Here we investigated the effect of symmetry on visual word form perception in developmental dyslexic (DD) and chronological age (CA)-matched typical juveniles. We employed real-, non-, jiagu- (precursors of modern Hanzi), and scrambled characters in Hanzi with symmetric or asymmetric compositions as stimuli. In the behavioral experiment, both groups responded more accurately to the symmetric than the asymmetric compositions, namely the "symmetry effect". In a block-design functional magnetic resonance imaging (fMRI) experiment, the visual word form area (VWFA) in the left fusiform gyrus was identifiable for the CA group but remained elusive for the DD group. The CA group showed higher sensitivity to visual word forms in the lingual and fusiform areas. Notably, the dyslexic readers showed increased occipitotemporal activity during symmetry analysis, specifically in response to visual word forms. To conclude, whereas typical readers learn to suppress mirror generalization during literacy acquisition, persistent mirror generalization in dyslexic readers may hinder their reading performance.