Abstract
We examine the ways that the optic flow patterns experienced during natural locomotion are shaped by the movement of the observer through their environments. By recording body motion during locomotion in natural terrain, we demonstrate that head-centered optic flow is highly unstable regardless of whether the walker’s head (and eye) is directed towards a distant target or at the ground nearby to monitor foothold selection. In contrast, VOR-mediated retinal optic flow has stable, reliable features that may be valuable for the control of locomotion. In particular, we found that a walker can determine whether they will pass to the left or right of their fixation point by observing the sign and magnitude of the curl of the flow field at the fovea. In addition, the divergence map of the retinal flow field provides a cue for the walker’s overground velocity/momentum vector in retinotopic coordinates, which may be an essential part of the visual identification of footholds during locomotion over complex terrain. These findings casts doubt on the assumption that accurate perception of heading direction requires correction for the effects of eccentric gaze, as has long been assumed. The present analysis of retinal flow patterns during the gait cycle suggests an alternative interpretation of the way flow is used for both perception of heading and the control of locomotion in the natural world.
Clickable Video Links (Click here for a playlist of all videos) https://www.youtube.com/playlist?list=PLWxH2Ov17q5HRHVngfuMgMZn8qfOivMaf
Video 1. Gaze Laser Skeleton – Video (Full Speed) – Free Walking – Raw (See Figure 1)
Video 2. Gaze Laser Skeleton – Video (1/4 Speed) – Free Walking – Optic Flow Vectors
Video 3. Gaze Laser Skeleton – Video (1/4 Speed) – Free Walking – Optic Flow Streamlines
Video 4. Sim. Eye Trajectory – Sim. Retinal Flow – Fixation Aligned with Path
Video 5. Sim. Eye Trajectory – Sim. Retinal Flow – Fixation to Left of Path
Video 6. Sim. Eye Trajectory – Sim. Retinal Flow – Fixation to Right of Path
Video 7. Sim. Eye Trajectory – Sim. Retinal Flow – Vertical Sin Wave
Video 8. Sim. Eye Trajectory – Sim. Retinal Flow – Horizontal Sin Wave
Video 9. Sim. Eye Trajectory – Sim. Retinal Flow – Corckscrew
Video 10. Gaze Laser Skeleton – Sim. Retinal Flow – Ground Looking
Video 11. Gaze Laser Skeleton – Video (Full Speed) – Rocky Terrain – Raw
Video 12. Gaze Laser Skeleton – Video (1/4 Speed) – Rocky Terrain – Optic Flow Streamlines
Video 13. Gaze Laser Skeleton – Sim. Retinal Flow – Rocky Terrain
Video 14. Quadcopter Gimbal – Video (Full Speed) – Optic Flow Streamlines
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵* j.matthis{at}northeastern.edu
Minor changes, a few new citations, updated video links to point towards a YouTube playlist (instead of Dropbox)
https://www.youtube.com/playlist?list=PLWxH2Ov17q5HRHVngfuMgMZn8qfOivMaf