The perception of relative speed of two bodies as a function of independent observer movement

Abstract

Various studies examined the perception of two moving objects from a static viewpoint or observer movement relative to a reference. However, the influence of observer movement on the perception of relative movement between two other bodies was not thoroughly examined yet. Participants watched two virtual characters running after each other from behind and judged whether the chaser was catching up or falling behind. We adapted the chaser’s speed within three staircases to fit a psychometric function targeting at the point of subjective equality of speeds of the characters (PSE) and the just noticeable difference of speeds (JND). This procedure was repeated for an observer who is static or moving with 50%, 100%, or 150% of the speed of the chased person which itself was constant. JNDs were comparable for all observer speeds. However, PSEs increased with the observer’s speed showing that observer movement influenced the perception of relative speed of two bodies. The slope of the increase is consistent with a strategy of keeping the partial occlusions of the two characters constant as well as with a strategy of keeping the distance proportion (of the chaser-chased distance on the overall observer-chased distance) constant