Scott Watamaniuk^1,2, Devashish Singh², Stephen Heinen²; ¹Wright State University, ²The Smith-Kettlewell Eye Research Institute

During fixation, a covered eye can deviate either inward or outward from its viewing position, an eye movement known as a phoria. Phorias occur in most people and are usually present in both eyes, but occasionally become pathological in which case the deviation progresses in magnitude to eventually manifest as strabismus. Because covering an eye in normals reveals the phoria, we hypothesize that when uncovered, the oculomotor system must continuously generate an independent signal to align that eye with a target. If so, models that drive binocular eye movements using unitary neural signals (conjugate or vergence) could be incorrect. To reveal if independent signals persist during normal saccade generation, we measured both eyes as observers repeatedly made horizontal or vertical saccades during binocular and monocular viewing. The target was a small (.2 deg) white dot presented on a uniform grey background. Each trial began with a randomized (1-1.5 sec) fixation period during which the static phoria was assessed. For monocular viewing, an eye was occluded with an infrared pass filter to allow binocular eye movement recording (Eyelink 1000+ eye tracker). Under binocular viewing, both eyes had the same trajectory as expected. Under monocular viewing for horizontal saccades, the fixation-phoria in the covered eye was maintained during the saccade. However, saccade magnitudes in the eyes were the same. For vertical saccades the covered eye was also displaced horizontally while vertical alignment was maintained. The results suggest that a unitary signal drives both eyes during saccades, but independent control signals driven by vision correct each eye’s phoria during binocular viewing.

Acknowledgements: NIH 1R01EY034626-01