Symbolic Filling-in
"Perceptual filling-in", in its simplest definition, is simply the filling-in of information that is not directly given to the sensory input. The missing information is inferred or extrapolated from visual data acquired in a different part of the visual field. Examples of filling-in phenomena include lightness assignment to surfaces from information of contrast across the edges and completion of features and textures across the blind spot, based on the features and textures that are detected in the visible part of the image. In this definition, it is clear that a filling-in process involves a rearrangement of visual information, in which activity in one region of the visual field (i.e. edges) is assigned to other regions (surfaces). In any event, the total amount of information available is not increased, being determined by the retinal input, and any rearrangement of information is useful only if it brings the information contained in the image into a form that is more easily analyzed by our brain.
Dennet and Kinsbourne (Dennet 1992; Dennet and Kinsbourne 1992) opposed to the idea that an active filling-in process would take place in our brain on philosophical grounds. They argued that such an idea would be the result of the false belief that in our brain there is a spectator, a sort of homunculus similar to ourselves, needing a filled-in image representation. From a scientific viewpoint, Dennet's homunculus may correspond to higher-order scene representation or decision-making mechanisms. The question is whether or not such mechanisms need a filled in, gap free representation of the image to function optimally (Ramachandran 2003).
The symbolic filling-in theory postulates that such a "homunculus" need not exist, and that image information is transformed at the cortical level into an oriented feature representation. Surface form and colour are not coded at this stage, but would be derived only at a symbolic level of representation, as attributes of objects or proto-objects.
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