Does binocular instability influence static body balance in adults with developmental dyslexia?

Abstract

The aim of the study was to investigate if body balance control deficits in dyslexia are present in dyslexic adults and if unstable binocular fixation relates to impaired body balance. Fifteen dyslexics adults and 15 age‑matched non‑dyslexics participated in the study. Posturography data were collected in two sessions: during quiet standing (single‑task) and while performing a mental task while standing on the platform (dual‑task). Each session was conducted under three distinct visual conditions: monocular fixation, binocular fixation, and eyes closed. Four parameters of the center of pressure (CoP) signal were analysed: medio‑lateral sway (XSD), antero‑posterior sway (YSD), sway area (Area95) and mean CoP velocity (Vavg). A psycho‑physical tests with Wesson card and a modified Mallett test were used to measure fixation disparity (FD). Slight underconvergence at the fixation point results in exo‑FD, and conversely, overconvergence results in eso‑FD. The results indicated that in dyslexics, the exo‑FD values were higher than in controls. In both groups, body stabilization was better with binocular fixation compared to eyes closed (lowest value of Vavg and CoP sway). Moreover, dyslexic adults demonstrated impaired body balance. The posturographic deficits remained unaltered by the viewing conditions, indicating that binocular fixation did not contribute to body instability, despite the higher incidence of fixation disparity in the dyslexic group. The existence of both posturographic deficits and the presence of FD may reflect deficits at the cerebellar level.