Hue constancy across changes in spectral purity and a functional theory of the Abney Effect

Abstract

Nearly a century ago Abney reported that the hue of most wavelengths change when they are mixed with white light. These hue changes, known as the Abney Effect, have been studied extensively because they reveal nonlinearities in how the photoreceptor signals are combined in the neural coding of color, but their potential function has remained obscure. We show that the hue shifts may reflect a novel form of color constancy that compensates color appearance for the limited visible spectrum afforded by the cone receptors. This limit differentially biases the cone responses to stimuli with narrow or broad wavelength spectra, and thus alters the ratio of cone signals as spectral bandwidth increases. Yet for such stimuli, the cone ratios observers choose when matching the hue of narrow and broad stimuli instead remain surprisingly stable. This correction for spectral bandwidth predicts curved hue loci when the same purity change is instead created by dilution with white, because the visual system then makes the “right” response to the “wrong” stimulus. Paradoxically, the failures of hue constancy suggested by the Abney Effect may thus arise from post-receptoral adjustments designed to preserve hue when spectral purity varies in ways expected by the visual system.

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