Abstract
Hering’s Law states that the eyes rotate as a unit, driven by a single command for both vergence and conjugate movements. Previously we presented evidence against the integrity of Hering’s Law for vergence during smooth pursuit of a midline target (Chandna et al., 2021). Here we provide evidence that the eyes are also not yoked during fixation. Binocular eye movements were recorded while eight observers fixated the center of a small “X” during either binocular viewing, or with one eye occluded with an infrared-pass filter. Results show greater position variability of the occluded eye than the viewing eye quantified with the bivariate contour ellipse area (BCEA). This difference is due to increased drift speed and excursion in the occluded eye. Microsaccades were all conjugate, and their rate and frequency did not differ between the eyes, nor between the binocular and monocular conditions. Surprisingly, the BCEA of the viewing eye during monocular viewing was also larger than the BCEA of either eye during binocular viewing. This result suggests that visual information from the covered and viewing eyes interact perceptually to guide the eyes when visual input is degraded. Overall, the results suggest that drift during fixation is controlled independently while microsaccades remain conjugate, consistent with our binocular control model comprising independent and conjugate channels.