Velocity blindness during shearing motion

When two subjects lie at different distances from a moving observer, there is a velocity step at the occluding boundary of the nearer object. When the differential motion is tangential to the boundary, the effect is as if a shearing is taking place. If all texture cues are removed by using similar random dot patterns on each side of the boundary, then 20% of the population cannot use this differential motion to locate the boundary when it is presented to the parafovea. These observers are thus abnormally insensitive to the differential rate of texture flow at boundaries undergoing shearing motion. As no such population differences were observed for differential motion perpendicular to the occluding boundary (occluding motion), we infer that independent mechanisms process shearing and occluding motion.