The roles of optic flow and proprioception in the perception of active and passive linear self-motio

This thesis examined how visual optic flow and the proprioceptive and efferent copy cues that accompany active leg motion contribute to humans' perception of the distance of their linear self-motion. Subjects sat on a stationary bicycle and wore a virtual reality helmet that displayed a simulation of a corridor. Forward motion along the virtual corridor was driven by rotation of the bicycle's rear wheel. Subjects were shown a virtual target at various distances down the corridor. The target then disappeared and subjects either (i) moved only visually along the corridor at a constant acceleration: "vision-only, passive condition", (ii) pedalled actively at a trained constant acceleration along the virtual visual corridor: "vision-plus-pedalling, active condition", or, (iii) pedalled actively at a trained constant acceleration in the dark: "pedalling-only, active condition". Their task was to push a button when they perceived that they had reached the position previously occupied by the target. For the constant acceleration vision-only condition, as the constant acceleration of optic flow decreased, subjects had to travel increasingly less to feel as though they moved through the target distance. Optic flow is a reliable cue for judging distance of self-motion (up to 32 m) provided the constant acceleration of flow is within the range 0.2--0.8 m/s2. Below this range, subjects overestimate their self-motion. Relative to the passive, vision-only condition, when actively pedalling both WITH and WITHOUT vision, subjects had to travel significantly further to match the visual target distances. When active pedalling with vision, subjects rely more on their proprioceptive and efferent copy cues to judge the distance of their self-motion, essentially ignoring the visual cues.