Dynamic contrast sensitivity during human locomotion.

Locomotion poses a challenge to clear and stable vision. Reflexive head and eye movements act to stabilize the retinal image, but these do not act perfectly, so retinal image motion is increased during walking compared with standing. We nevertheless perceive the world as clear and stable during locomotion, suggesting that the visual system is well-adapted to meet the challenges posed by locomotion. To better understand these processes, we assessed dynamic contrast sensitivity during locomotion by presenting brief (24 ms) foveal Gabor targets (6°, 11 cpd) at threshold contrast to observers walking on a treadmill in an otherwise darkened room. Head and ankle motion were tracked, and presentation time was randomized, which allowed post hoc binning of responses according to stride-cycle timing to investigate how sensitivity is impacted by head motion and stride-cycle timing. Contrast sensitivity was improved during walking compared with standing over large portions of the stride cycle, except for epochs aligned with heel strikes, which drive large and unpredictable perturbations. This resulted in periodicity in contrast sensitivity at two cycles per stride, with additional oscillations observed at four and six cycles per stride. Pupil size was found to be moderately larger during walking compared with standing and also exhibited periodic fluctuations that were phase-locked to the stride cycle. Perceptual oscillations reflect the entrainment of visual processing by active behaviors. Robust contrast sensitivity during walking may be supported by action-contingent effects of locomotion on visual cortical activity that have been observed in several animal models.