Abstract
Prior studies comparing human contrast sensitivity during walking and standing have reported poorer contrast sensitivity during walking (Benjamin et al 2017; Cao & Haendel 2019). Here we present preliminary evidence suggesting that human foveal contrast sensitivity may be improved during walking compared to standing despite increased retinal image motion of the target. Ten human subjects judged the orientation (+/-45 deg) of Gabor targets (~11 cpd, 4° diameter, 32 msec duration) presented in the center of a projection screen (ProPixx-Vpixx) against a grey background at a distance of ~1.75 m in an otherwise darkened room. The contrast of the Gabor targets was fixed at a level that was previously determined to yield ~79% correct responses for each subject during standing. The timing of target presentation was randomized relative to the previous response and there was no fixation point. In a counterbalanced design, the mean percent correct was significantly better (p=0.04) during walking on a treadmill at 1.3 m/s (83±7%) than during standing (75±9%). Eye, head, and heel movements were tracked (EyeLink 2, Optitrack) and used to estimate viewing distance, retinal image motion, and timing of stimulus presentation relative to the locomotor stride cycle. Average viewing distance was nearer (and spatial frequency lower) during walking than standing (1.79 vs 1.68 m; 11.2 vs 10.5 cpd), but mean retinal image motion was increased during walking (1.84±3.23°/s) compared to standing (0.23±1.33), with the greatest retinal image motion (and least percent correct) during the heel-strike phase. Pending experiments to better control for differences in viewing distance and pupil dilation between standing and walking, this result may indicate enhanced dynamic visual acuity during walking that allows overcoming visual perturbations introduced by active movement.