Plato's Cave: The Feature Contour System

The Feature Contour System (FCS) Model

The boundary completion mechanism outlined above can be used to account for both the Kanizsa type illusions as shown above, and the perceptual grouping phenomena shown below, because in all these cases the illusory boundary or grouping percept completes a collinear relation between spatially separated inducers. There is however a significant difference between these percepts, specifically, that in the Kanizsa illusions the illusory contour exhibits an actual brightness contrast across the illusory edge, whereas in the perceptual grouping phenomena all that is perceived is an invisible grouping relation.

Grossberg proposes a solution to this problem by suggesting that the perceptual system consists of two distinct but interacting mechanisms, the Boundary Contour System (BCS), which represents only the invisible grouping percept, and the Feature Contour System (FCS), which represents the visible brightness percept. The figure below shows a simplified schematic of the BCS / FCS interaction for the case of an original image, labeled ORIG in the figure, consisting of a white square on a black background. A plot of image intensity along a horizontal scan line through the center of the image is shown below the image. The retinal response to this input can be modeled approximately by an on-center off-surround receptive field filtering, as shown schematically in the figure, labeled DOC, where a light color represents a positive response, a dark color a negative response, and the neutral gray indicates a zero response. The scan line plot below the DOC image shows how this representation preserves the direction of contrast across the edge while showing no signal in the uniform dark or light regions. In the BCS these edge signals are abstracted to a contrast-insensitive edge representation, so that the dark/light edge at the left side of the square and the light/dark edge at the right side of the square both produce an equal positive response in the BCS image, as shown in the figure. The BCS signal therefore represents the edge information in the original, independent of direction of contrast. It is within this contrast-insensitive representation that boundary completion in the BCS or in the Directed Diffusion model are performed, as described above.

In the FCS representation, initially the signal is identical to the DOC image, but the nature of the FCS is a diffusion operation that allows the brightness and darkness signals to diffuse spatially across the image, except where gated by the presence of a BCS signal. This is indicated in the enlarged scan line plots to the right in the figure above, where the BCS plot exhibits two peaks along the vertical edges of the square, exactly between the dark and bright peaks of the FCS image. The brightness signal is free to diffuse inwards, and the darkness signal is free to diffuse outward, eventually filling the inner square with a uniform bright percept, and the outer background with a uniform dark percept, but the presence of the BCS edge along the perimeter of the square prevents the brightness and darkness signals from mixing across that line. The final percept at equilibrium therefore is an approximate reconstruction of the original image, as indicated by the dotted lines in the FCS scan line plot.

In the simple example presented in this case, the result of all this complex processing is simply a decomposition followed by a reconstruction of the original image. There are several interesting functions that are served by this BCS / FCS interaction. In the first place, the retinal on-center off-surround filtering has the effect of "discounting the illuminant", i.e. the global illumination value is lost, but the relative illumination at the edges is preserved, so that the reconstructed FCS image is actually insensitive to absolute illumination and represents a spatial percept based on only relative illumination. Land [] has shown that this is indeed a property of human color and brightness perception, and is the reason for the brightness contrast effect which is reproduced by the BCS / FCS model. Another important property of this model is that it accounts for the blind spot illusion, the fact that the retinal blind spot is not perceived as a perceptual hole, or missing information, but rather it takes on the brightness percept of the surrounding region. Visual stabilization experiments by Yarbus [ ] exhibit similar phenomena which are also explained by the BCS / FCS model.

The figure above shows schematically how the BCS / FCS model explains the Kanizsa illusion. The boundary completion operation of the BCS generates the missing sides of the triangle, but without the FCS this would result in only an invisible grouping percept. The influence of the BCS on the FCS signal however creates an enclosed boundary around the perimeter of the triangle which then captures a disproportionate concentration of the diffusing brightness signal due to the high contrast at the corners, resulting in the perception of a brighter triangle over a less bright background. In the case of the invisible grouping percepts shown near the top of this page there is no such imbalance in brightness signal, and therefore the percept is represented only in the BCS system, and thus remains invisible, although not imperceptible.

One of the greatest contributions of the BCS / FCS model is the introduction of this distinction between the visible and invisible, or boundary and surface components of perception, both of which clearly represent manifestations of [reification] as opposed to abstraction from the original image.

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