Contour-, surface-, and object-related coding in the visual cortex

At the early processing stages in visual cortex, information is laid out in the form of maps of the retinal image. However, contrary to intuition, uniform surfaces are not mapped by uniform distributions of neural activity. We can perceive the three-dimensional (3D) shape of a uniform surface, but stereoscopic neurons are activated only by the contours of the surface, not the uniform interior. We perceive a uniform color figure, but colorselective neurons respond about five times less to the interior of the figure than to its boundaries. How cortical neurons signal contour and surface features is well known, but we do not yet understand how the brain “organizes” these feature signals to represent surfaces and objects. In this chapter, I summarize studies showing that the visual cortex codes surface color, depth ordering of surfaces, and border ownership in the contour signals. The signaling of border ownership (the one-sided assignment of borders that determines perceptual figure-ground organization) is the key finding, because it reveals mechanisms of feature grouping. I discuss how these mechanisms might be used by the system to represent objects and compute surface attributes.