Abstract
Patterns of motion on the retina during locomotion depend critically on both eye and body motion, and
these in turn depend on behavioral goals. In rocky terrain, walkers look close to the body to locate stable
footholds, whereas in smooth terrain, gaze is mostly on more distant regions (Matthis et al, Current Biology
2018). In order to describe the statistics of retinal motion over a range of different eye movement patterns,
we recorded eye and body movements during natural locomotion, together with video from a head-
mounted camera, using a Pupil Labs mobile eye tracker integrated with an IMU-based motion capture
suit. Eye movement data were analyzed to extract the fixations and then aligned relative to gaze
location. Using video-based optic flow estimation of the retinally-aligned images, we derived retinal
motion patterns during locomotion over a variety of terrains, during the periods of stable gaze.
Analysis of retinal motion inputs expressed as the magnitude of expansive and rotational motion at
each retinal location reveals complex spatial structure that depends both on the lateral angle of gaze
relative to translation direction, and also the gaze angle relative to gravity. More eccentric gaze
relative to body translation direction affects the rotational components of motion, with little effect on
the expansive components, which varies primarily as a function of distance from the subject.
Conversely, gaze angle relative to gravity affects primarily the expansion component with little effect
on the rotational components. At downward gaze angles, there is greater expansion in the upper
visual field compared to lower. At gaze angles closer to the horizon there is greater expansion in the
lower visual field. Since gaze varies systematically with gait patterns and terrain complexity, this
allows the selection of behaviorally-relevant representative motion stimuli for both physiological and
psychophysical experiments.