Abstract
Despite the abundance of time we spend holding gaze on visible objects, mechanisms that generate miniature eye movements of fixation are largely unknown. A general principle of ocular control is Hering’s Law which states that equal innervation drives both eyes during vergence and conjugate eye movements. However, during fixation, while microsaccades are largely conjugate, ocular drift in the two eyes is less correlated. We investigated ocular fixation with traditional foveal fixation stimuli (a single .2 deg dot), or rings with either 3 or 6 deg radii. Observers were instructed to fixate the center of each stimulus. Results showed decorrelated drift, and a large degree of horizontal “microvergence” in which the eyes drifted with different velocities. However, vertical drift showed microvergence for a similar duration as did horizontal drift. In contrast, microsaccades were always conjugate. We previously introduced a model to describe midline smooth pursuit vergence that incorporates independent eye control signals along with a unitary conjugate command (Heinen et al., VSS 2022). Here we demonstrate that this model generates conjugate microsaccades. Merely adding independent band-limited white noise to individual eye control modules caused the model to generate slow drift eye movements that predominantly exhibited “microvergence.” The results suggest that while microsaccades are generated by a unitary conjugate mechanism, slow drift is generated by independent control of the two eyes, and independent noise sources contribute to slow drift expression.