In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in normal visual analysis. Classical demonstrations of perceptual filling-in involve images stabilized on the retina either by means of special lenses, or under certain conditions of steady fixation.
If a stimulus is entirely stabilized, its color and lightness fade until they are no longer seen and the area fills in with the color and lightness of the surrounding region. A famous example of fading under steady fixation is Troxler's fading. When steadily fixating on the central dot for many seconds, the peripheral annulus will fade and will be replaced by the color or texture of the background. Since the adapted region is actively filled-in with background color or texture, the phenomenon cannot be fully explained by local processes such as adaptation.
There is general agreement that edges play a central role in determining the apparent color and lightness of surfaces through similar filling-in mechanisms. However, the way in which their influence is performed is still unclear. Two different theories have been put forward to explain the filling-in completion phenomenon.
One theory, addressed as the "isomorphic filling-in theory" according to the definition of Von der Heydt, Friedman et al. In 2003, postulates that perception is based on an image representation held in a two dimensional array of neurons, typically arranged retinotopically, in which color signals spread in all directions except across borders formed by contour activity. The process is thought to be analogous to physical diffusion, with contours acting as diffusion barriers for the color and brightness signals. An alternative hypothesis is that image information is transformed at the cortical level into an oriented feature representation. Form and color would be derived at a subsequent stage, not as the result of an isomorphic filling-in process, but as an attribute of an object or proto-object. This theory is called the symbolic filling-in theory.
According to the isomorphic filling-in theory, color is represented by the activity of cells whose receptive fields point at the surface, but it is assumed that these cells receive additional activation through horizontal connections that keeps their activity level high despite mechanisms of lateral inhibition tending to suppress surface activity and despite the transient nature of the afferent signals. The lateral activation comes from receptive fields at contrast borders.
These signals are strong because receptive fields are exposed to contrast, and reliable because the border produces continuous light modulation even during fixation, due to small residual eye movements. In the alternative symbolic hypothesis, there is no spreading of activity, but all the information would be carried by the relevant features, that would be tagged with information on contrast polarity, color and lightness of the surfaces they enclose. Despite the many attempts to verify the two different models by psychophysical and physiological experiments, the mechanisms of color and lightness filling-in are still debated.