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Michele A Cox, Norick R Bowers, Janis Intoy, Martina Poletti, Michele Rucci; Decoupling eye movements from retinal image motion reveals active fixation control. Journal of Vision 2019;19(10):148. doi: https://doi.org/10.1167/19.10.148.
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Even when fixating steadily on an object, the eyes continually jitter in an ostensibly erratic manner. This movement, known as ocular drift, has an instantaneous speed of almost 1 deg/s and can cover an area as large as the entire foveola (1 deg2). Previous studies have demonstrated that the visual system is sensitive to the temporal modulations resulting from ocular drift on the retina and suggested that this motion is under active oculomotor control. To examine mechanisms of drift control, here we manipulated retinal image motion using a custom, high-resolution system for gaze-contingent display. If retinal image motion is actively controlled, we expect eye drift to be attenuated as retinal image motion increases and vice versa. Subjects (n=10) viewed natural scenes. During periods of ocular drift, the image either remained stationary on the display or moved proportionally to the subject’s measured drift. In the first condition, retinal motion naturally arose from subjects’ eye movements only. In the latter, retinal motion was a function of both eye drift and how the image moved on the display. In this way, retinal motion could be attenuated (i.e. stabilized or halved) or amplified (i.e. doubled or tripled) relative to drift. We found that when retinal motion was attenuated, both drift speed and area increased ~20% relative to normal vision. When retinal motion was amplified, drift became more curved by ~10%. Taken together, these results demonstrate that the dynamics of ocular drift actively change to compensate for varying amounts of retinal motion. The eye moves faster and curves less when retinal motion is lower than normal and moves slower when retinal motion is higher. These findings are consistent with mechanisms of drift control to maintain a specific amount of retinal image motion.
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