Conducted 3 studies with 3 experienced Ss relating perception of motion to stimulus uncertainty. Generally, detectability declined when the observer was uncertain about the direction in which a target moved, but the visibility loss associated with direction uncertainty was attenuated if the observer had adequate practice. This attenuation depended on the observer's ability to switch among directionally selective visual mechanisms in an adaptive fashion. Implications for models of motion detection are discussed. (22 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved). © 1981 American Psychological Association.