Short wavelength-sensitive cones and the processing of their signals.

Physiological studies of color have uncovered many puzzles. Here are four puzzles. First, neurons in the lateral geniculate nucleus (LGN; Fig. 1) are strongly color opponent, but the opponency does not correspond to Hering's fundamental opponent-color pairs (red–green, blue–yellow) (Webster et al., 2000); moreover, the color-tuning of the population of LGN cells is not balanced across color space but is rather strongly biased toward red–cyan and lavender–lime (e.g., few neurons show optimal tuning to blue , green , purple , or yellow). Second, the spatial opponency of LGN cells is confi gured in exactly the wrong way to subserve color contrast (Wiesel & Hubel, 1966): the optimal stimulus for a red-on cell is bright red on a dark red background—not red on green, as a mechanism for color contrast would have it And fourth, cone-opponent cells in V1 often respond to Scone isolating stimuli, and in these cells, the responses to Scone stimuli typically aligns with those to M-cone stimuli L input to account for the reddish quality of short-wavelength light (Ingling, 1977). Clearly, regions downstream of the LGN and V1 must be critically involved in the computation of hue and other color phenomena such as Hering's basic color categories One goal of contemporary neurophysiology is to identify these brain regions and then to address how they construct perceptual correlates from the cone signals they receive from the LGN, V1, and other areas. This goal is especially challenging since a given neural signal may not be exclusively involved in generating color percepts. It remains a conceptual hurdle to maintain a distinction between neural signals that may be involved in computing color on the one hand and the perceptual-cognitive output of the nervous system on the other hand. As shown by the title of this review, studies of color often slip into a shorthand that obscures this distinction (what constitutes a " color signal " in the brain?). Because there are very few S cones in the retina, and the eye is focused in the middle part of the spectrum (yellow), Scone signals are necessarily blurry. Scone signals are therefore unable to contribute to high acuity. What is their function? Many assume that it has something to do with color perception because Scone signals are necessary to establish perceptual color space. Thus neural responses to Scone activation are often thought to refl ect a neural color signal (Wandell et al., 1999). …