A covered eye follows a target on a tangent screen but doesn’t point to it

We previously showed asynchronous eye movements of an occluded eye during midline smooth pursuit (Chandna et al., 2021) which we attributed to a failure of yoking (ala Hering’s law) during vergence. Here we investigate the integrity of yoking during “conjugate” pursuit. Neurotypical observers pursued a small (.4 deg) spot on a tangent screen that moved horizontally with either a predictable sinusoidal profile, or an unpredictable combination of sinusoids. Peak target velocity was either 25 or 5 deg/sec. Binocular eye movements were recorded during binocular and monocular viewing with an EyeLink at 1000 Hz, and occlusion was implemented with an infra-red pass filter to allow recording. We found that during pursuit, the covered eye followed the trajectory of the target, but was horizontally displaced from it. This displacement is typically called phoria by clinicians, and can be a biomarker for strabismus, nevertheless it occurs in neurotypicals. However, it is unknown whether phoria is caused by the extraocular muscles “relaxing” under cover and returning to primary position or by a neural signal. If phoria is due to relaxed muscles, its magnitude should vary systematically with eye position, as the muscles should be tenser as the eyes stray further from primary position. If neural, it should remain constant or be modulated by task demands. Consistent with a neural signal driving phoria, its magnitude varied across conditions (range .8 - 4.8 deg), but usually remained relatively constant throughout a trial. Furthermore, systematic differences in the phoria between conditions occurred; the phoria was smaller during high-speed pursuit, and lower yet in unpredictable conditions. The results suggest the eyes are driven conjugately in a yoked fashion, but with added independent monocular signals that control phoria and are modulated by cognitive factors.