SMOOTH-PURSUIT EYE-MOVEMENTS ELICITED BY FIRST-ORDER AND 2ND-ORDER MOTION

The perception of the displacement of luminance-defined contours (i.e. , first-order motion) is an important and well-examined function of th e visual system. It can be explained, for example, by the operation of elementary motion detectors (EMDs), which cross-correlate the spatiot emporal luminance distribution. More recent studies using second-order motion stimuli, i.e., shifts of the distribution of features such as contrast, texture, flicker, or motion, extended classic concepts of mo tion perception by including nonlinear or hierarchical processing in t he EMD. Smooth-pursuit eye movements can be used as a direct behaviora l probe for motion processing. The ability of the visual system to ext ract motion signals from the spatiotemporal changes of the retinal ima ge can be addressed by analyzing the elicited eye movements. We measur ed the eye movement response to moving objects defined by two differen t types of first-order motion and two different types of second-order motion. Our results clearly showed that the direction of smooth-pursui t eye movements was always determined by the direction of object motio n. In particular, in the case of second-order motion stimuli, smooth-p ursuit did not follow the retinal image motion. The latency of the ini tial saccades during pursuit of second-order stimuli was slightly but significantly increased, compared with the latency of saccades elicite d by first-order motion. The processing of second-order motion in the peripheral visual field was less exact than the processing of first-or der motion in the peripheral field. Steady state smooth-pursuit eye sp eed did not reflect the velocity of second-order motion as precisely a s that of first-order motion, and the resulting retinal error was comp ensated by saccades. Interestingly, for slow second-order stimuli we o bserved that the eye could move faster than the target, leading to sma ll, corrective saccades in the opposite direction to the ongoing smoot h-pursuit eye movement. We conclude from our results that both visual perception and the control of smooth-pursuit eye movements have access to processing mechanisms extracting first- and second-order motion.