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Paul R MacNeilage, Luan Nguyen, Christian Sinnott; Characterization of natural head and eye movements driving retinal flow. Journal of Vision 2019;19(10):147d. https://doi.org/10.1167/19.10.147d.
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In the absence of moving objects, retinal flow is determined by eye velocity relative to the environment as well as by the structure of the environment. Eye velocity in space is the sum of head-in-space and eye-in-head velocity. To gain a better understanding of head and eye velocity driving retinal flow, we developed a system to measure both head and eye velocity during everyday behaviors outside the lab. The system consists of a Pupil Labs eye tracker with an inertial measurement unit (IMU) rigidly attached to the world camera. Head velocity is reconstructed using a computer vision algorithm known as simultaneous localization and mapping (SLAM) which works by tracking features in the scene to determine frame-to-frame image deformation, then solving for the camera motion that generated that deformation. The SLAM estimate is supplemented by angular velocity, linear acceleration, and magnetometer data from the IMU. The result is measurement of six-degree-of-freedom (6DOF) head velocity at 20 Hz with eye velocity sampled at 120 Hz. Head and eye velocity were recorded for participants performing a range of activities around campus. Not surprisingly, participants tend to fixate features of the stationary environment, and robust oculomotor stabilization leads to retinal flow that is minimal near the fovea. Linear components of retinal flow are driven by linear velocity of the head. Angular components, however, do not depend strongly on angular head velocity because angular optic flow is largely cancelled by compensatory eye movements. Instead, angular components of retinal flow are driven by compensation for linear optic flow at fixation, which depends on fixation eccentricity relative to the heading direction as well as distance to the scene. Consequently, we observe that retinal flow is driven most strongly by three factors: 1) linear head velocity, 2) fixation direction and distance, and 3) the structure of the environment.
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