Introduction

When an object in the world moves relative to the eye, the image of the object moves across the retina. Motion that occurs on the retina is referred to as retinal motion. When objects move within our visual field we tend to move our eyes, head, and body to track them in order to keep them sharply focused on the fovea, the region of the retina with the highest spatial resolution. When the eyes move to track the object, there is no retinal motion if the tracking is perfect (Figure 10.1), yet we still perceive object motion. Retinal motion is therefore not the only signal required for motion perception. In this chapter, we discuss the problem of how retinal motion and eye movements are integrated for motion perception. After introducing the problem of representing position and motion in three-dimensional space, we will concentrate specifically on the topic of how retinal and eye-movement signals contribute to the perception of motion in depth. To conclude, we discuss what we have learned about how the combination of eye movements and retinal motion differs between the perception of frontoparallel motion and the perception of motion in depth.

A headcentric framework for motion perception

Position (and motion) in the physical three-dimensional world can be described in a number of different ways. For example, it can be described in Cartesian coordinates (x, y, z) or in terms of angles and distances with respect to a certain origin.