Abstract
A unitary vergence command is believed to control the same muscle groups in the two eyes (e.g., medial rectus) to enact gaze shifts in depth (Hering, 1868). Hering also postulated that accommodation drives vergence, the basis for ocular control models that inform strabismus treatment. Last year we showed that during monocular smooth pursuit in depth, the covered eye often exhibits conjugate behavior despite appropriate accommodation (Heinen et al., VSS 2019). That result suggested that a conjugate command was contaminating the vergence signal, and implied a weak accommodation-vergence coupling. This year we provide additional evidence supporting both claims, and that also question the existence of a unitary vergence command. Observers pursued a motorized physical target (small letter “E”) moving periodically in depth on the midline, between 33.3cm (3.0 dpt) and 66.7cm (1.5 dpt) with a peak velocity of 30cm/s. Viewing was either binocular or monocular with either eye. A Plusoptix photorefractor measured eye movements and accommodation from both eyes. The temporal delay (phase lag) of each eye relative to target motion was computed using cross-correlation. As expected, viewing eyes followed the target with near zero delay. Covered eyes however, were usually desynchronized with target motion, delayed by up to 2.6 sec. In contrast, covered eye accommodation delays were minimal, and were uncorrelated with gaze delays. The results suggest that during monocular midline pursuit, the covered eye is not under unitary vergence control and that accommodation in the viewing eye does not drive vergence in covered eye.