Opponent color coding: A mechanistic model and a new metric for color space.

Abstract

A mechanistic model of opponent color coding in the human retina is described. This description is based on the concept of the scaling-ensemble (Koenderink et al., 1971). All receptors that are a member of the same scaling-ensemble adapt together, regardless of their type. If the adaptation is the Weber-type adaptation, then it is shown that the sum of the outputs of receptors of an ensemble displays typical on-cell characteristics. In contradistinction with the phasic response of the sum signal, difference signals are of a typical tonic type. With three kinds of cones, one can essentially form two opponent signals and one non-opponent signal. The responses of the opponent and the non-opponent channels to spatiotemporal modulations of the input signal are entirely different. In this model the interaction betweenthe opponent and the non-opponent channels is such that even large intensity modulations do not disturb the opponent responses very much. The opponent signals carry almost entirely information about the spectral distribution of the input signal. The results of computer simulation studies of the proposed model are compared to electrophysiologically determined responses of cells in the monkeys lateral geniculate body. In the second part of this paper we make use of the line element approach in order to be able to confront the model's predictions with psychophysical measurements. This line element is closely related to those proposed earlier by Helmholtz (1896) and Schrodinger (1920). It introduces a hyperbolic geometry in perceptual color space. This specific metric leads to the vector model for brightness addition. In addition it is possible to regard the Bezold-Brucke hue shift as the direct result of the curvature of color space, induced by our metric.