Advances in Photoreception Proceedings of a Symposium on Frontiers of Visual Science (1990) / Chapter Skim
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The Invisible Cone Mosaic
The Invisible Cone Mosaic
Pages 135-148
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From page 135... ...
The second has to do with the diameter of the cone aperture, d, which causes them to act as low-pass spatial filters. These two factors are potential sources of information loss that are quite different in nature, and I will discuss them in turn, explaining why neither has much impact on ordinary spatial vision.
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From page 136... ...
\ -2.\ ... ~ FIGURE 1 A psychophysicist's view of the foveal cone mosaic, characterized by the spacing between rows of cones, r, and cone diameter, d.
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From page 137... ...
Slightly larger pupils could produce slightly more contrast at higher spatial frequencies, but in general there is precious little modulation in the retinal image in the aliasing range. The situation is considerably different in the parafovea and periphery, where the MTF of the eye probably exceeds the cone Nyquist frequency.
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From page 138... ...
The high spatial frequency, high-contrast, sinusoidal gratings used in the laboratory are rare events in natural scenes. Natural scenes are typically complex, so that weak aliasing effects may be masked by the predominant spatial frequencies in the image that lie below the Nyquist frequency.
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From page 139... ...
Even with interference fringes, contrast sensitivity is clearly limited by postreceptoral factors in the peripheral retina, and acuity falls far short of the cone Nyquist frequency. So the scheme suggested by Hirsch and Hylton may
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From page 140... ...
-my- - - r -- rThis leaves us with another puzzle, since cones everywhere could benefit from being somewhat larger, which would allow them to catch more photons without producing appreciable contrast loss in the "visible" range of spatial frequencies. Why then are cones so small for a given spacing between them?
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From page 141... ...
measured contrast sensitivity at high spatial frequencies under conditions designed to isolate M or L cones. Neither study found a difference in performance at high spatial frequencies when only one submosaic mediated detection compared with both cone classes operating together.
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From page 142... ...
The contrasts in the M and L cones for each of these stimuli were calculated using Smith and Pokorny fundamentals (Boynton, 1979~. These conditions were chosen, based on the results of contrast sensitivity measurements, so that both fringes near the resolution limit and zebra stripes would be above contrast threshold for the favored cone mechanism but well below threshold for the unfavored cone mechanism.
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From page 143... ...
The left half of the image shows a regular triangular lattice sampling a grating whose period nearly equals the sample spacing. That is, the grating frequency is near a moire zero for the mosaic.
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From page 144... ...
So there is in fact no theoretical expectation that the spatial frequency yielding the coarsest zebra stripes should change when one submosaic is effectively removed. This does not imply that there are no differences in the aliasing effects predicted for a complete mosaic and the submosaics that comprise them.
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From page 145... ...
We are presently engaged in a search for chromatic aliasing, since the spatial frequencies and orientations of the gratings that produce them would specify the packing arrangement of M and L cones. We have identified a red-green zebra stripe pattern when viewing interference fringes near the foveal Nyquist frequency (Sekiguchi et al., in press)
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From page 146... ...
have shown that the ability to detect interference fringes above the cone Nyquist frequency must be due in part to cone aliasing. But is it possible that some of this aliases originates at sites deeper in the visual system?
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From page 147... ...
Estevez 1975 Contrast sensitivity of individual colour mechanisms of human vision. Journal of Physiology 248:649 662.
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From page 148... ...
1982 Spectral analysis of spatial sampling by photoreceptors: topological disorder prevents aliasing. Vision Research 22:1205-1210.
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