Saccades during symmetrical vergence

Abstract

When the eyes verge symmetrically along the median plane from far to near (convergence) or near to far (divergence), small saccades often occur. The current study aimed to: (i) characterize saccades during symmetrical vergence, (ii) describe the chronometry of saccade relative to that of vergence, and (iii) determine whether saccades mediate prefrontal control of vergence. In Experiment 1, we recorded horizontal saccades during vergence in ten normals using an infrared tracker. Transcranial magnetic stimulation (TMS) was delivered over dorsolateral prefrontal cortex to assess its contribution in saccade production. In Experiment 2, saccades during vergence were recorded in horizontal and vertical planes in two other normals using a video-based tracker. Horizontal saccades, either pure or oblique (Exp.1), occurred in 84.0% of trials, 58 ms after vergence onset, and averaged 1.5 degrees in amplitude. An idiosyncratic directional bias to the right was found for the rate and the latency of saccades. Prefrontal TMS had no influence on any saccade parameter. Pure horizontal, pure vertical and oblique saccades (Exp.2) occurred, respectively, in 60.4%, 72.4% and 25.4% of trials, 168 ms, 121 ms and 146 ms after vergence onset, averaging 1.1 degrees , 1.2 degrees and 1.5 degrees in amplitude. Chronometry of saccades during vergence suggests that they do not participate to vergence triggering, and that prefrontal control of vergence previously reported was not mediated by saccades.