The Magnitude of Motion Parallax Affects Control of Egospeed

Perceiving the speed of self-motion, or egospeed, from optical flow can be important for vehicular control. McDevitt, Eggleston, & Dyre (1999) found that judgments of egospeed are affected by the magnitude of motion parallax present in optical flow. The present study examined whether this motion parallax effect generalizes to control of egospeed. Participants were shown simulations of forward translation parallel to a set of three transparent planes defined by white dots and separated by a constant interplane distance. After a preview period, the motion parallax produced by the set of three planes was instantaneously changed by changing the interplane distance. Participants then reduced their egospeed to one-half that experienced during the preview period. Decreases in motion parallax resulted in higher adjusted speeds, while increases in motion parallax resulted in lower adjusted speeds. These results suggest that control of egospeed is affected by motion parallax in the same manner as judgments.