Bhagya L Marella¹ (), Stephanie A Biehn¹, T Rowan Candy¹; ¹Indiana University School of Optometry, Indiana University, IN, USA

Purpose: Humans experience asymmetric vergence demands during natural viewing and must calibrate responses during development. The roles of disjunctive saccades and slower vergence in habitual responses are not well understood in typical and atypical development. This study determined contributions of the two movements in 7-yr-olds and adults for different stimulus ratios. Methods: A 16deg radius spatially 1/f dynamic (120z) was presented dichoptically using a PROPixx projector. A 2deg fixation target was presented monocularly to the left eye. Step changes in disparity (1.5s) were introduced asymmetrically to stimulate disjunctive saccades and vergence (combinations of +3 to -3deg symmetric lateral and depth). An EyeLink 1000 (SR Research) made binocular recordings. Vergence and saccade responses were identified using velocity criteria and assigned to pre-saccade, intra-saccade and post-saccade periods. Results: For both children (7yrs) and adults, much of the vergence response amplitude was driven by post-saccadic vergence (median (SE): 40% (2.0) & 49% (1.8)), in children and adults, respectively), followed by pre-saccadic vergence (median (SE): 36% (2.1) & 25% (1.6)) and then intra-saccadic vergence. Duration of the disjunctive saccadic component (median: 22ms in both children and adults) was shorter than slower vergence during the pre- and post-saccadic periods. Median (SE) latency of the disjunctive saccade was longer in children 429ms (18ms) than adults 324ms (14ms) (lmer: p<0.001). Conclusions: Seven-year-olds produced comparable asymmetric vergence responses to adults for disparity only stimuli (except for the latency of the disjunctive saccade), consistent with a study of larger amplitude full cue stimuli (Yang et al, 2002). Our data suggest that both disjunctive saccades and slower vergence responses can be driven by disparity alone by 7 years. Given the separate neural pathways driving these components, the role of disjunctive saccades in human developmental disorders of eye alignment needs further investigation (Walton et al, 2017; Das, 2016).