Abstract
We show that human ability to discriminate the wavelength of monochromatic light can be understood as maximum likelihood decoding of the cone absorptions, with a signal processing efficiency that is independent of the wavelength. This work is built on the framework of ideal observer analysis of visual discrimination used in many previous works. A distinctive aspect of our work is that we highlight a perceptual confound that observers should confuse a change in input light wavelength with a change in input intensity. Hence a simple ideal observer model which assumes that an observer has a full knowledge of input intensity should over-estimate human ability in discriminating wavelengths of two inputs of unequal intensity. This confound also makes it difficult to consistently measure human ability in wavelength discrimination by asking observers to distinguish two input colors while matching their brightness. We argue that the best experimental method for reliable measurement of discrimination thresholds is the one of Pokorny and Smith, in which observers only need to distinguish two inputs, regardless of whether they differ in hue or brightness. We mathematically formulate wavelength discrimination under this wavelength-intensity confound and show a good agreement between our theoretical prediction and the behavioral data. Our analysis explains why the discrimination threshold varies with the input wavelength, and shows how sensitively the threshold depends on the relative densities of the three types of cones in the retina (and in particular predict discriminations in dichromats). Our mathematical formulation and solution can be applied to general problems of sensory discrimination when there is a perceptual confound from other sensory feature dimensions.
Highlights
In a classical wavelength discrimination experiment, the observer views a bipartite field, one half filled with light of a standard wavelength and the other with light of a comparison wavelength
It is well known that human performance does not approach that of a photon noise limited ideal observer[3, 4, 5, 6], and our primary aim here is to determine how well the shape of the human wavelength discrimination function is explained by the ideal observer, regardless of its overall amplitude
We show that our theoretical predictions from the augmented ideal observer analysis to accommodate the perceptual confound can give a better account of the behavioral data
Summary
In a classical wavelength discrimination experiment, the observer views a bipartite field, one half filled with light of a standard wavelength and the other with light of a comparison wavelength. Wavelength discrimination threshold is reached when the observer reports that the two fields always appear different, regardless of the radiance of the comparison [1]. This discrimination threshold in humans is a ‘‘w’’ shaped function of the wavelength of the light: it has a central peak at around wavelength l~540 nanometers (nm), minima at l~490 and l~580 nm, and rises up sharply for lw650 nm and for very short wavelengths[1]; similar results hold for the macaque monkey and presumably other old world primates[2]. If the shape were perfectly explained, it would imply that the neural mechanisms following the cones are efficient for different wavelengths
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