Retinal optic flow during natural locomotion

Abstract

Here 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.

Look where you go: characterizing eye movements toward optic flow

“…The same areas are also involved in the control of smooth pursuit eye movements (Lisberger, 2015) and saccades to moving targets (Newsome, Wurtz, Dusteler, & Mikami, 1985). In the absence of explicit instruction or other tasks, looking at the FOE might generate a radial flow of visual information that is in line with what we experience in everyday life (where the retinal FOE is centered to straight ahead, Matthis, Muller, Bonnen, & Hayhoe, 2020).…”

“…Our work shows that human observers track FOE changes even when they are not explicitly instructed to do so, suggesting that this tracking behavior is intuitive. The ability to keep the eyes aligned with a shifting FOE might serve as an important gaze stabilization strategy during self-motion, facilitating the control of the body during natural locomotion, and compensating for the unstable flow experienced when the head moves rhythmically during a gait cycle (Matthis, Muller, Bonnen, & Hayhoe, 2020). Furthermore, we show that this intuitive tracking behavior shares many characteristics (e.g., overall alignment and sensitivity to motionsignal-strength manipulation) with instructed performance, supporting the use of intuitive eye movements as sensitive indicators of visual motion processing.…”

Look where you go: Characterizing eye movements toward optic flow

“…Our work shows that human observers track FOE changes even when they are not explicitly instructed to do so, suggesting that this tracking behavior is intuitive. The ability to keep the eyes aligned with a shifting FOE might serve as an important gaze stabilization strategy during self-motion, facilitating control of the body during natural locomotion and compensating for the unstable flow experienced when the head moves rhythmically during a gait cycle ( Matthis et al, 2020 ). Furthermore, we have shown that this intuitive tracking behavior shares many characteristics (e.g., overall alignment and sensitivity to motion-signal-strength manipulation) with instructed performance, supporting the use of intuitive eye movements as sensitive indicators of visual motion processing.…”

“…The same areas are also involved in the control of smooth-pursuit eye movements (Lisberger, 2015) and saccades to moving targets (Newsome, Wurtz, Dürsteler, & Mikami, 1985). In the absence of explicit instructions or other tasks, looking at the FOE might generate a radial flow of visual information that is in line with what we experience in everyday life, where the retinal FOE is centered to straight ahead (Matthis, Muller, Bonnen, & Hayhoe, 2020).…”

Visual effort moderates postural cascade dynamics

“…Between optic flow and mechanical layout, standing still relies on synergies spanning the limbs and joints at smaller scales and subtle postural perturbations, triggering reflex arcs and muscle twitches, at yet smaller scales. The task of standing still and visually fixating laces together activities at these multiple scales, entailing a close connection among events of widely varying scales, from whole-body postural fluctuations at large scales, to head sway and eye movements at medium scales, and photoreceptor activity at the finest scales [1–3]. Consequently, the postural center of pressure (CoP) fluctuations—which directly relate to sway—exhibit similar statistical signatures of cascade dynamics as found in fluid flow [4].…”