Icy road ahead—rapid adjustments of gaze–gait interactions during perturbed naturalistic walking

Abstract

Most humans can walk effortlessly across uniform terrain even when they do not pay much attention to it. However, most natural terrain is far from uniform, and we need visual information to maintain stable gait. Recent advances in mobile eye-tracking technology have made it possible to study, in natural environments, how terrain affects gaze and thus the sampling of visual information. However, natural environments provide only limited experimental control, and some conditions cannot safely be tested. Typical laboratory setups, in contrast, are far from natural settings for walking. We used a setup consisting of a dual-belt treadmill, 240 projection screen, floor projection, three-dimensional optical motion tracking, and mobile eye tracking to investigate eye, head, and body movements during perturbed and unperturbed walking in a controlled yet naturalistic environment. In two experiments ( N = 22 each), we simulated terrain difficulty by repeatedly inducing slipping through accelerating either of the two belts rapidly and unpredictably (Experiment 1) or sometimes following visual cues (Experiment 2). We quantified the distinct roles of eye and head movements for adjusting gaze on different time scales. While motor perturbations mainly influenced head movements, eye movements were primarily affected by the presence of visual cues. This was true both immediately following slips and—to a lesser extent—over the course of entire 5-min blocks. We find adapted gaze parameters already after the first perturbation in each block, with little transfer between blocks. In conclusion, gaze–gait interactions in experimentally perturbed yet naturalistic walking are adaptive, flexible, and effector specific.

Slipping while counting - gaze-gait interactions during perturbed walking under dual-task conditions

“…All participants reported being sufficiently rested and focused in a questionnaire prior to the experiment, were naïve to the hypotheses and debriefed after the experiment. We aimed for a power of 80 % (Cohen, 1988) which, given 𝛼 = .05 and Cohen's 𝑓 = 0.25 (a realistic estimate based on previous work, Kopiske et al, 2021), required a sample size of 𝑁 = 24. A total of 27 participated, as after inspecting data quality, but prior to any hypothesis-related analysis, data of three participants had to be excluded due to a high proportion of missing eyetracking data (>20 % missing values, same cut-off as used in Kopiske et al, 2021).…”

“…On each time scale, we analyzed eye, head, and body movements to look at persistent changes. Previously, we had shown that participants respond to such perturbations by adapting their gaze both directly and long-term, and differently depending on whether there were visual cues to give advance notice of the perturbation or not (Kopiske et al, 2021). If the adverse effects of increased cognitive load are not offset by a more cautious gait mode, one would predict a stronger reaction of gaze and gait parameters to the perturbation while performing a cognitive task than without secondary task.…”

“…Especially in difficult terrain, this sensory information is helpful because it is usually available at a distance, providing important information about potential threats to stability early on (Fajen & Warren, 2003;Gibson, 1958) and so enabling preemptive gait adjustments (Warren et al, 1986) to prevent potential damage. Such obstacles or sudden hazards humans have to respond to are in experimental environments often simulated by induced motor perturbations (Kopiske et al, 2021). For example, participants can be made to slip or stumble to increase the difficulty and complexity of the experimental situations.…”

“…To do this, we asked participants to walk on a dual-belt treadmill (while secured by a safety harness) in a naturalistic virtual setting and repeatedly perturbed their gait to induce slipping (using a procedure established previously by Kopiske et al, 2021). As critical experimental manipulation, in the present study we combined in some conditions this perturbed walking with a cognitive task and quantified the effects on the relevant gaze and gait parameters.…”

Seeing does not mean processing: Where we look and the visual information we rely on change independently as we learn to walk.

“…Perry and colleagues (2020) found similar results during reaching, with fixation distance increasing as participant performance improved. During walking, healthy adults also look farther ahead as they practice and become more skilled at both obstacle avoidance (Kopiske et al 2021) and target stepping (Cates and Gordon 2022). This suggests that as people learn to perform a task, the role of vision shifts towards informing feedforward motor control strategies.…”