Understanding why spectra that are physically the same appear different in different contexts (color contrast), whereas spectra that are physically different appear similar (color constancy) presents a major challenge in vision research. Here, we show that the responses of biologically inspired neural networks evolved on the basis of accumulated experience with spectral stimuli automatically generate contrast and constancy. The results imply that these phenomena are signatures of a strategy that biological vision uses to circumvent the inverse optics problem as it pertains to light spectra, and that double-opponent neurons in early-level vision evolve to serve this purpose. This strategy provides a way of understanding the peculiar relationship between the objective world and subjective color experience, as well as rationalizing the relevant visual circuitry without invoking feature detection or image representation.empirical ranking | color vision | perception | simple networks | receptive field T he spectral properties of retinal images conflate illumination, surface reflectance, atmospheric transmittance, and a host of other factors. Nonetheless, biological visual systems routinely generate lightness and color percepts that lead to successful behavior in the real world. Thus, a major question in vision research is how the visual system contends with the fact that the physical parameters of the world are not available in retinal stimuli (the "inverse optics problem").A clue to the possible answer is the phenomenology of lightness and color perception. As shown in Fig. 1, the same contextual information can make regions of an image returning identical spectra to the eye appear different (lightness/color contrast) and regions returning different spectra look similar (lightness/color constancy). Based on psychophysics and analyses of natural images, we have argued that such perceptual effects are signatures of the strategy vision uses to circumvent the inverse problem (1-4). By depending on the frequency of occurrence of biologically determined stimulus patterns, perceptions of lightness and color track reproductive success rather than the physical qualities of objects and conditions in the world, abrogating the need for information about the physical world as such. Here, we show that simulated evolution driven by the frequency of occurrence of spectral stimulus patterns gives rise to the key receptive field characteristic in biological color vision and to responses whose perceptual counterparts are color contrast and constancy.
MethodsNetwork. The network and evolutionary paradigm are shown in Fig. 2. The input to the network was the output of 37 single-opponent neurons that responded to equiluminant spectral patterns along the blue-yellow color axis of natural images. Although the choice of blue-yellow input rather than red-green was arbitrary, the color gamut of dichromats is based primarily on this axis (5). The input thus simulates the information provided to cortical neurons by single-opponent blue-yellow b...