J. Hyönä, D.P. Munoz, W. Heide and R. Radach (Eds.) Progress in Brain Research, Vol. 140 © 2002 Elsevier Science B.V. All rights reserved CHAPTER 15 Integration of motion cues for the initiation of smooth pursuit eye movements Richard T. Born * , Christopher C. Pack and Ruilin Zhao Department of Neurobiology, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115-5701, USA Abstract: To clearly see a moving object, an observer must rotate his or her eyes with a velocity that matches that of the object. Such rotations are called smooth pursuit eye movements, and they depend critically on the ability of the primate brain to integrate information about object velocity from various local motion signals. When the local motion signals are in conflict, it is possible to use smooth pursuit eye movements as a continuous read-out of the motion integration process. This review discusses the results of recent behavioral experiments that have taken this approach, along with relevant neurophysiological and computational studies. Keywords: Motion integration; Aperture problem; Eye movements; Smooth pursuit; Ocular following Introduction Smooth pursuit is an important oculomotor behavior that allows a moving object to be held relatively stationary on the fovea so that it may be analyzed with high spatial acuity; see Eckmiller and Bauswein (1986), Lisberger et al. (1987), Keller and Heinen (1991) and Ilg (1997) for general reviews of smooth pursuit eye movements. The neural system that ac- complishes this feat has been widely studied by both physiologists and psychologists because of the ease and precision with which its outputs (i.e. eye movements) can be measured, the ready manipula- bility of the inputs (i.e. visual stimuli), and, most significantly for this review, because of its intimate connection with visual motion processing. Because of this last fact, the behavior can serve as a con- venient read-out of the algorithms at work on the * Correspondence to: R.T. Born, Department of Neurobi- ology, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115-5701, USA. Tel.: +1-617-432-1307; Fax: +1-617-734-7557; E-mail: rborn@hms.harvard.edu sensory side. However, because the steady-state be- havior of any closed-loop system, such as pursuit, is dominated by the very presence of negative feedback (Carpenter, 1988), it has mainly been the initiation of pursuit (or other ‘open-loop’ conditions) that has served this purpose. The visual stimulus that drives the initiation of pursuit is target motion on the retina, or retinal slip (Rashbass, 1961). Such motion generates a number of visual ‘error signals’, including position, velocity and acceleration errors, all of which are involved in both the initiation and maintenance of the behavior, though retinal velocity error is the most important (Lisberger and Westbrook, 1985; Morris and Lis- berger, 1987). Traditionally, these basic properties of pursuit have been studied in the laboratory by mea- suring the movements of the eyes in response to a single moving target, usually a small spot, presented against a dark, featureless background. In the real world, however, the situation is more complicated. Often there are multiple possible tar- gets present simultaneously, any given target may have an irregular shape — thus generating ambigu- ous and conflicting local motion signals — and CICERO/GALAYAA B.V. / HYÖNÄ15: pp. 225-237