JUDGING OBJECT VELOCITY DURING SMOOTH-PURSUIT EYE-MOVEMENTS

Our tendency to constantly shift our gaze and to pursue moving objects with our eyes introduces obvious problems for judging objects' veloci ties. The present study examines how we deal with these problems. Spec ifically, we examined when information on rotations (such as eye movem ents) is obtained from retinal, and when from extra-retinal sources. S ubjects were presented with a target moving across a textured backgrou nd. Moving the background allowed us to manipulate the retinal informa tion on rotation independently of the extra-retinal information. The s ubjects were instructed to pursue the target with their eyes. At some time during the presentation the target's velocity could change. We de termined how various factors influence a subject's perception of such changes in velocity. Under more or less natural conditions, there was no change in perceived target velocity as long as the relative motion between target and background was maintained. However, experiments usi ng conditions that are less likely to occur outside the laboratory rev eal how extra-retinal signals are involved in velocity judgements.