Dissociate triggering of conjunctive and disjunctive eye movements

In natural behavior, our eyes must coordinate two types of movements when looking between points in space: conjunctive movements (where both eyes move together) and disjunctive movements (where the eyes move in opposite direction to change their convergence angle). Here we investigate how the initiation of these 2 different types of eye-movements is coordinated. We used the Size-Latency effect to modulate saccadic latencies. To elicit combined saccadic and vergence eye-movements, we displayed large ring targets at different vertical offsets and disparities relative to fixation. This allowed us to easily dissociate version and vergence eye-movements. We found that saccadic latencies were strongly modulated by the eccentricity of the targets as well as their hemifield, but not by the disparity of the targets. The opposite was true for vergence: vergence latencies were modulated by the disparity sign and amplitude of the targets, but not by their eccentricity or hemifield. We found a complete lack of correlation between saccadic and vergence latencies, both across and within conditions. Finally, we found that distributions of vergence latencies have a markedly reduced skewness as compared to distributions of saccadic latencies, a hallmark of evidence accumulation. Overall, our results demonstrate that the initiation mechanisms for these two types of eye-movements operate independently.