Humans can use optic flow to estimate distance of travel

We demonstrate that humans can use optic flow to estimate distance travelled when appropriate scaling information is provided. Eleven subjects were presented with visual targets in a virtual corridor. They were then provided with optic flow compatible with movement along the corridor and asked to indicate when they had reached the previously presented target position. Performance depended on the movement profile: for accelerations above 0.1 m/s2 performance was accurate. Slower optic-flow acceleration resulted in an overestimation of motion which was most pronounced for constant velocity motion when the overestimation reached 170%. The results are discussed in terms of the usual synergy between multiple sensory cues to motion and the factors that might contribute to such a pronounced miscalibration between optic flow and the resulting perception of motion.