Because the intuition behind this assumption is that the effect of the surround is, in essence, to relocate attributes of color appearance within a tristimulus space, we will refer to it as the “relocation assumption.” Restricting attention to infields of some fixed luminance presented in a given surround, this ubiquitous assumption implies that the achromatic point and the point of convergence for lines of constant hue also should coincide in this case. This transformation is thought to preserve both the three-dimensionality and the above-mentioned structural properties of color space. The effect of introducing neighboring stimuli usually is modeled as a context-dependent transformation of the coordinates of the isolated light ( 6– 8). However, it is well known that the perceived color of a light spot may change radically if it is surrounded by a colored region ( 4, 5) (see Fig. In this simple case, any light corresponds uniquely to a point in three-dimensional color space. These concepts concerning the structure of color space and its relation to perceptual variables ultimately have their roots in an experimental paradigm that studies isolated light spots surrounded by complete darkness ( 3).

The figure illustrates the impossibility of making true achromatic settings and the best alternative choice (black rings).

The left half of each disk represents the color of the common background component, and the right half represents the color of the “contrast component.” A feature that is not clear from this figure is that the salience of the background component increases as the infield chromaticity approaches that of the surround. Left and Right correspond to the situation of a yellow and a blue surround, respectively.

( d) Schematic illustration of the scission account of our results outlined in Discussion. The polygon is the border of the monitor gamut at the luminance used in our experiments. ( Right) Chromaticities of the patches for a center C, which, in this case is equal energy white, and an inner circle radius of 0.03. The surrounds in the experiment were chromatic and equiluminant to the patches. ( c Left) Schematic picture of the display used in experiment 1 with a black surround. The effect is even stronger when center-surround stimuli are viewed in a surround of complete darkness. Although the two small rectangles are physically identical, they look different because of the different inducing effects of the surrounds. The straight lines emanating from any other point, such as the white, dashed ones, cannot all be of constant hue. ( Right) The straight lines emanating from the achromatic point are of approximately constant hue with increasing saturation. ( Left) If the small, black circle around achromatic point A is the smallest circle around this point containing all hues (albeit at low saturation), then the larger, white circle is the smallest circle around the chromaticity X that contains all hues, because, if it is made smaller, the greens will be missing. A plausible interpretation of this apparently paradoxical finding in terms of color scission is proposed.įigure 1 ( a) Two properties of the achromatic point shown in the uv chromaticity space. Our experiments with infields in chromatic surrounds revealed that the point in chromaticity space that appears gray is clearly different from the point on which lines of constant hue converge. The basic assumption that these two criteria coincide is clearly true for isolated spots of light presented in darkness, and it usually is taken for granted that this coincidence generalizes to more complex visual stimuli. A further criterion shared by most theories is the structural assumption that lines in chromaticity space of constant hue converge on the neutral point. One of these criteria is that it is perceived as achromatic. It generally is assumed that this point satisfies several criteria simultaneously. Although current theories of color vision differ in many respects, they all assume the existence of a uniquely defined neutral point in chromaticity space.

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