The Neural Control of Vision

K. The Processing of Motion

The ability to extract motion information from the visual scene is one of the most important tasks of the visual system. Motion in the environment, much of it self-induced, is pervasive. Quite a number of different types of motion have been identified that include planar, circular, radial, as well as motion toward and away from the observer. It appears that motion analysis is performed predominantly by neurons that are direction or velocity selective or both. Direction-selective cells are already evident in the retina where they form several classes and project extensively to such structures as the terminal nuclei and the nucleus of the optic tract. In the geniculo-striate system direction selectivity emerges in full in the visual cortex. In V1, most of the simple cells and the majority of the complex cells are direction specific. Figure 48 shows spatiotemporal plots of several subclasses of simple cell plus one fully directional complex cell. The S1 cell shown has only one subfield that responds to only dark edges and does so only in one direction of motion. These cells, some of which respond to only light edges are quite common. The cells labeled S1, S2, S3, S4, all have similar subfields and would look the same were their receptive field plotted with just stationary spots (see Figure 11, center); specifying their directional selectivities discloses different organizations. Cells S5 and S7 demonstrate cells that do not exhibit direction selectivity.