Motion perception in motion : how we perceive object motion during smooth pursuit eye movements

Abstract

Eye movements change the retinal image motion of objects in the visual field. When we make an eye movement, the image of a stationary object will move across the retinae, while the retinal image of an object that we follow with the eyes is approximately stationary. To enable us to perceive motion in the outside world accurately, our visual system therefore as to take eye movements into account when processing the retinal image motion. In other words, the visual system has to compensate for the effects of eye movements on the retinal image motion of objects in the visual field. In this thesis several aspects of motion perception during smooth pursuit eye movements, the type of eye movements we make to follow moving objects with the eyes, have been studied. In a number of studies we investigated whether our visual system compensates for the effects of smooth pursuit eye movements in the same way when we judge the motion of an object that is moving at an angle (other than 0 or 180 deg) relative to the eye movement (so-called noncollinear motion) as when the object is moving in the same direction as the eye (collinear motion). This turned out to be indeed the case. In addition, we extended a number of existing models of motion perception during smooth pursuit eye movements to two dimensions in the frontoparallel plane and compared their performance. According to the simplest (linear) model, our visual system estimates the retinal image velocity of a given object and the eye movement velocity independently. The sum of these two estimates then gives the perceived velocity of the object. This model turned out to describe most of the data well. A slightly more complicated model, incorporating an interaction between both signals produced even better fits to the data, but the differences between the model fits were quite small. We conclude that the better fits of the interaction model suggest that the two signals (retinal motion signal and eye movement signal) are not completely independent.