Visually induced gait deviations during different locomotion speeds.

Optic flow is essential for the perception of self motion and the control of path integration during locomotion. Inverting prisms oriented 15 degrees off vertical in the roll plane were used to experimentally distort optic flow during locomotion. Depending on the direction in which the prisms were rotated, optic flow was diagonally upward to the right or upward to the left. A reproducible deviation of gait toward the direction of perceived optic flow was found in ten healthy subjects. This deviation is explained to be a gait deviation that compensates for misleading perceived self motion induced by optic flow. The amount of deviation was dependent on locomotion speed. When walking slowly (about 1 m/s), mean deviation was 0.22+/-0.08 m/s to the right and -0.18+/-0.08 m/s to the left for right and left, respectively, diagonal prism orientation. Deviation was significantly less when running (about 3 m/s) with mean deviations of 0.05+/-0.03 m/s and -0.06+/-0.03 m/s, respectively (ANOVA, P<0.01). It is assumed that path integration during running is largely achieved by highly automated spinal programs operating independently of sensory control. In contrast, walking is more dependent on afferent and reafferent visual control. Thus, the experiments show that visual control of locomotion is direction specific and dependent on optic-flow-induced vection. It becomes less influential with increasing speed of locomotion, e.g., when walking in contrast to running.