Abstract
Reading is an activity based on complex sequences of binocular saccades and fixations. During saccades, the eyes do not move together perfectly: saccades could end with a misalignment, compromising fused vision. During fixations, small disconjugate drift can partly reduce this misalignment. We hypothesized that maintaining eye alignment during reading involves active monitoring from posterior parietal cortex (PPC); this goes against traditional views considering only downstream binocular control. Nine young adults read a text; transcranial magnetic stimulation (TMS) was applied over the PPC every 5 ± 0.2 s. Eye movements were recorded binocularly with Eyelink II. Stimulation had three major effects: (1) disturbance of eye alignment during fixation; (2) increase of saccade disconjugacy leading to eye misalignment; (3) decrease of eye alignment reduction during fixation drift. The effects depend on the side; the right PPC was more involved in maintaining alignment over the motor sequence. Thus, the PPC is actively involved in the control of binocular eye alignment during reading, allowing clear vision. Cortical activation during reading is related to linguistic processes and motor control per se. The study might be of interest for the understanding of deficits of binocular coordination, encountered in several populations, e.g., in children with dyslexia.
Highlights
IntroductionBinocular motor coordination helps to obtain fused vision during fixations, by allowing the vergence angle (angle between the lines of sight of the two eyes) to be stable and appropriate for the distance of fixation
transcranial magnetic stimulation (TMS) disturbed the binocular coordination of reading fixations This study shows, for the first time, that the posterior parietal cortex (PPC) is involved in the coordination of the eyes during post-saccadic fixation drift, whereas no such effect was found for fixations following single saccades (Vernet et al, 2008)
In conclusion, this study shows that binocular motor control during reading, an automatic motor activity, involves complex 3D processes, aiming to keep saccade and drift disconjugacy small and to tailor drift disconjugacy appropriately to reduce misalignment of the eyes left at the end of the saccade
Summary
Binocular motor coordination helps to obtain fused vision during fixations, by allowing the vergence angle (angle between the lines of sight of the two eyes) to be stable and appropriate for the distance of fixation. Saccades could end with a convergent (lines of sight crossing closer) or null (lines of sight crossing at the same distance) vergence error (Yang and Kapoula, 2003). This transient saccade disconjugacy is traditionally believed to reflect peripheral asymmetries (premotor circuits and visco-elastic properties of the ocular muscles, see, e.g., Collewijn et al, 1988; Zee et al, 1992). The aim of the present study is to investigate the cortical basis of such 3D control during reading
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