Initiation of smooth-pursuit eye movements to first-order and second-ordermotion stimuli

Since normal human subjects can perform smooth-pursuit eye movements only i n the presence of a moving target, the occurrence of these eye movements re presents an ideal behavioural probe to monitor the successful processing of visual motion. It has been shown previously that subjects can execute smoo th-pursuit eye movements to targets defined by luminance and colour, the fi rst-order stimulus attributes, as well as to targets defined by derived, se cond-order stimulus attributes such as contrast, flicker or motion. In cont rast to these earlier experiments focusing on steady-state pursuit, the pre sent study addressed the course of pre-saccadic pursuit initiation (less th an 100 ms), as this early time period is thought to represent open-loop pur suit, i.e. the eye movements are exclusively driven by visual inputs procee ding the onset of the eye movement itself. Eye movements of five human subj ects tracking first- and second-order motion stimuli had been measured. The analysis of the obtained eye traces revealed that smooth-pursuit eye movem ents could be initiated to first-order as well as second-order motion stimu li, even before the execution of the first initial saccade. In contrast to steady-state pursuit, the initiation of pursuit was not exclusively determi ned by the movement of the target, but rather due to an interaction between dominant first-order and less-weighted second-order motion components. Bas ed on our results, two conclusions may be drawn: first and specific for ini tiation of smooth-pursuit eye movements, we present evidence supporting the notion that initiation of pursuit reflects integration of all available vi sual motion information. Second and more general, our results further suppo rt the hypothesis that the visual system consists of more than one mechanis m for the extraction of first-order and second-order motion.