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D.I. A. MacLeod, J. Judson; Blur in the Blue . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2667.
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Only a few percent of the cones in the human retina are blue–sensitive, and their distribution is correspondingly sparse. This would be a sensible arrangement if, as has long been thought, chromatic defocus filters out the high spatial frequencies from the image received by the blue cone array. But recently it has been pointed out that real eyes with substantial monochromatic aberrations may tolerate chromatic defocus much better than ideal eyes, and this might allow the blue cones to receive nearly the full range of spatial frequencies. This suggestion needs to be tested using experimental measures of the contrast available to the blue cones.
Since neural filtering precludes detection of high spatial frequencies using the blue cone system, we presented briefly pulsed grating test patterns to the red– and green–sensitive cones instead. The test pulse was blue with an energy spectrum chosen so that the spectral distribution of the light absorbed from it by the long–wavelength cones roughly mimicked the distribution that would be received by the blue cones from the relatively flat spectra typical of natural stimuli. A steady black on white target was provided to guide accommodation. Contrast sensitivity was measured for a range of optical distances of the test grating.
Contrast sensitivity was reduced when the blue test stimulus was at the distance of the accommodation stimulus, implying a loss of effective contrast in the image received by the blue cones. The loss was less than that calculated for an ideal eye, but was nevertheless enough to be functionally important.
The blue cones do receive a blurred image, for which their reduced sampling density may be roughly appropriate. Chromatic defocus not only reduces the contrast of high spatial frequencies in the blue cone image, but may also invert their spatial phase; removal of those frequencies by post–receptoral neural filtering can improve the fidelity of the neural representation, and there is no need for the blue cone array to represent those frequencies if they are to be excluded by post–receptoral filtering
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