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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Box DThe Importance of Context in Color Perception

Seeing the luminance of objects (that is, their brightness) can presumably be signaled by simply increasing or decreasing the overall firing rate of the relevant retinal ganglion cells, properly adapted to the overall level of ambient light (see text and Box E). Seeing color, however, logically demands that retinal responses to different wavelengths in some way be compared. The discovery of three human cone types and their different absorption spectra is correctly regarded, therefore, as the basis for human color vision. Nevertheless, how the three human cone types and the higher-order neurons they contact (see Chapter 12) produce the sensations of color is still unclear. Indeed, this issue has been debated by some of the greatest minds in science (Hering, Helmholtz, Maxwell, Schroedinger, Mach, and Land, to name only a few) since Thomas Young first proposed that humans must have three different receptive “particles,” i.e., cone types. A fundamental problem has been that, although the relative activities of three cone types can more or less explain the colors perceived in color matching experiments performed in the laboratory, the perception of color is strongly influenced by context. For example, a patch returning the exact same spectrum of wavelengths to the eye can appear quite different depending on its surround, a phenomenon called color contrast (see figure). Moreover, test patches returning different spectra to the eye can appear to be the same color, an effect called color constancy. Although these phenomena were well known in the nineteenth century, they were not accorded a central place in color vision theory until Edwin Land's work in the 1950s. In his most famous demonstration, Land (who among other achievements founded the Polaroid company and became a billionaire) used a collage of colored papers that have been referred to as “the Land Mondrians” because of their similarity to the work of the Dutch artist Piet Mondrian. Using a telemetric photometer and three adjustable illuminators generating short, middle, and long wavelength light, Land showed that two patches that in white light appeared quite different in color (e. g., green and brown) continued to look their respective colors even when the three illuminators were adjusted so that the light being returned from the “green” surfaces produced exactly the same readings on the three telephotometers as had previously come from the “brown” surface—a striking demonstration of color constancy!

The phenomena of color contrast and color constancy have led to a heated debate about how color percepts are generated that now spans several decades. For Land, the answer lay in a series of ratiometric equations that could integrate the spectral returns of different regions over the entire scene. It was recognized even before Land's death in 1991, however, that his so-called retinex theory did not work in all circumstances and was in any event a description rather than an explanation. An alternative explanation of these contextual aspects of color vision is that color, like brightness, is generated empirically according to what spectral stimuli have typically signified (see Box E).

Image ch11fbd1.jpg

The brown tile at the center of the illuminated upper face of the cube and the orange tile at the center of the shadowed face are actually returning the same spectral light to the eye (as is the tan tile lying on the ground-plane in the foreground). Readers who find this hard to believe can convince themselves by cutting holes in a sheet of paper such that the rest of the scene is masked out, in which case the two tiles on the faces of the cube look identical in both color and brightness. This illustration provides a dramatic example of the influence of context on the color perceived. (From Lotto and Purves, 1999.)

References

  1. Land E. Recent advances in Retinex theory. Vis. Res. (1986);26:7–21. [PubMed: 3716215]
  2. Lotto R. B. , Purves D. The effects of color on brightness. Nature Neurosci. (1999);2(11):1010–1014. [PubMed: 10526341]

From: Cones and Color Vision

Copyright © 2001, Sinauer Associates, Inc.

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