Dynamics of pattern motion computation

Early in visual processing neurons with small receptive fields can only signal the component of motion perpendicular to the orientation of the contour that passes through them (the aperture problem). A moving visual pattern with differently oriented contours can thus elicit neuronal responses that convey conflicting motion cues. To recover the true direction of motion of such a pattern, later visual areas must integrate the different motion cues over space and time. There is extensive evidence which suggests that this integration is not instantaneous - instead it occurs over time and causes profound changes in the perception of direction of motion of some complex moving patterns. To account for such temporal dynamics, previous studies have focused on a two-pathway model of motion perception: a fast pathway to account for the early percept, and a slow one to account for the late percept. Neurons in macaque area MT are selective for the direction of motion of an object, and their responses appear to be connected directly to the perception of complex motion stimuli in the natural environment. In this chapter, we will discuss neurophysiological data from MT neurons which illustrate how the process of motion perception occurs dynamically. The responses of individual neurons in MT appear to reflect the process by which the primate visual system produces an initial estimate of motion direction and then refines it over time. We will argue that MT neuronal responses are consistent with a single pathway model of motion perception in which temporal dynamics emerge due to two factors: the contrast of elements in the pattern and the time required for the pattern computation.