A Model of Visual Motion Processing in Area MT of Primates

Motion perception requires the visual system to satisfy two conflicting demands: first, spatial integration of signals from neighboring regions of the visual field to overcome noisy signals, and second, sensitivity to small velocity differences to segment regions corresponding to different objects (Braddick, 1993). We have developed a computational model for the visual processing of motion in area MT that accounts for these conflicting demands. The model has two types of units, similar to those found in area MT. One type of unit in the model integrates information about the direction motion to estimate the local velocity; these local velocity units compete among themselves to determine the most likely local velocity. A second type of unit selects regions of the visual field where the velocity estimates are most reliable; these selection units have nonclassical receptive field surrounds by virtue of competition with pools of similar units across the visual field. The output of the model is a distributed segmentation of the image into patches that support distinct objects moving with a common velocity. T h e processing of motion in the primate's visual cortex begins in area V1, where cells with reliable selec-tivity for direction of motion are found (Maunsell and Newsome, 1987); however, these cells d o not detect true velocity but instead are tuned to a limited range of spatiotemporal frequencies and exhibit spatially restricted receptive fields so that they can report only the perpendicular component of the velocity for straight edges. This so-called aperture problem is illustrated in figure 27.1. T o overcome these limitations and compute true local velocity measurements, it is necessary to integrate motion responses from cells with a variety of directions and spatiotemporal frequency tunings over a Neurons that respond selectively to velocity over a wide range of spatial frequencies are found in visual area M T , which receives a direct projection from area A class of cells in M T , the "pattern cells" of Movshon and colleagues (1985), respond to the direction of overall motion of plaid patterns composed of two differently oriented gratings rather than to the direction of the individual components. Psychophysical studies suggest that the perceived velocity of such patterns generally is close to the velocity that is uniquely consistent with the constraints imposed by the individual component's motions (Adelson and Movshon, 1982), although other possibilities have been suggested (Wilson et al., 1992; Rubin and Hochstein, 1993). In …