Proposed a model to account for the loss in visibility of moving targets that occurs when an observer is uncertain about the target's direction of motion. In response to uncertainty about 2 possible directions of motion, the observer is assumed to use a mechanism whose peak sensitivity is to a direction midway between the 2 possible directions. Seven experiments (observers were 7 naive Ss and the 1st author), using both reaction time and forced-choice data, demonstrate the predictive advantages of this midway model over competing single-band and multiple-band models. The experiments reveal several new properties of human motion perception: (a) direction and velocity information have orthogonal representations in the visual system; (b) although motion sensitivity does not vary with direction, the precision with which small changes in direction can be recognized does, reflecting differential breadth of tuning for directionally selective mechanisms sensitive to various directions; and (c) motion-analyzing mechanisms are broadly tuned for direction as well as speed. (47 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved). © 1980 American Psychological Association.