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Introduction
Color-vision sensitivity in humans
Color vision starts with the comparison of outputs from multiple cone types, each with its own opsin and associated absorption maximum (λmax ). In humans with normal color vision, there are three cone types with λmax located approximately at long (L)/561 nm, middle (M)/530 nm, and short (S)/430 nm wavelengths (Schnapf et al., 1988; Stockman & Sharpe, 1999). Wavelength-opponent pathways exploit differential absorptions between the cone types to produce color information that is represented in the red/green and blue/yellow color-opponent channels.
Increment threshold spectral-sensitivity functions (ITSS), based upon detection of large, long-duration stimuli added to a photopic white background, reflect detection by wavelength-opponent mechanisms (Guth et al., 1969). ITSS functions for trichromatic humans and macaque monkeys have three distinct peaks and are accurately described with models incorporating subtractive interactions between the L- and M-cones (Sperling & Harwerth, 1971)
In keeping with wavelength-opponent mechanisms mediating ITSS, King-Smith and Carden (1976) reported that normal human trichromats evidence high color-vision sensitivity, that is, they can discriminate the color of spectral increments at detection-threshold intensities under photopic conditions producing the three-peaked ITSS function. Chaparro et al. (1993) also demonstrated high color-vision sensitivity with cone contrast threshold measurements, reporting that the best detected color change was seen 3-9 times better than the best-detected luminance change. Loop and Crossman (2000) found that a trichromatic macaque monkey could also discriminate spectral increments from a white increment at detection thresholds. Additionally, Loop and Crossman (2000) found that detection thresholds for both macaque and normal humans were lower for a red increment than for a white increment, a threshold-level manifestation of the Helmholtz-Kohlrausch effect whereby color increases the apparent brightness of stimuli (Padgham & Saunders, 1975; Wyszecki & Stiles, 2000).
Human red/green dichromats have been shown to lack a photopigment, and thus lack one photoreceptor type. However, they exhibit an ITSS for large and long-duration photopic increments that can be modeled by a subtractive interaction between their two cone types (S vs . L or M), as do normal trichromats. Miyahara et al. (1996) reported that opponent (subtractive) models predict spectral sensitivities for dichromats whereas a luminance (additive) system does not. Schwartz (1994) also provided ITSS functions for human trichromats and dichromats that again indicate detection...