Anisotropic speed discrimination sensitivity for expanding and contracting optic flow.

Retinal optic flow provides critical visual cues for locomotion and self-navigation. Forward self-motion, such as walking or driving, typically induces an expanding radial flow pattern on the retina. Despite the predominance of expanding optic flow in everyday self-motion, several studies have reported higher sensitivity for contracting flow that arises under relatively rare conditions, such as walking backward or falling onto one's back. The underlying basis of this anisotropy remains elusive. In this study, we compared discrimination sensitivity to speed for expanding versus contracting optic flow, given that speed is a fundamental attribute of motion and that sensitivity to it provides insight into underlying visual mechanisms. We found that sensitivity in speed discrimination was comparable between expansion and contraction at low and high speeds but was significantly higher for contraction at an intermediate speed corresponding to ecologically relevant self-motion, such as walking. This anisotropic pattern was observed for both two-dimensional planar (Experiment 1) and simulated three-dimensional (Experiment 2) optic flow stimuli. Together, these findings suggest that anisotropic speed discrimination sensitivity to expanding and contracting optic flow at ecologically typical egomotion speeds may reflect an adaptive visual mechanism that supports postural stability and balance during relatively rare backward self-motion.