Torralba A, Alleysson D, Hérault J, 1998, "Spatio-chromatic processing in the human retina: towards an optimal trade-off between spatial resolution of luminance and range colour perception" Perception 27 ECVP Abstract Supplement
Spatio-chromatic processing in the human retina: towards an optimal trade-off between spatial resolution of luminance and range colour perception
A Torralba, D Alleysson, J Hérault
Colour vision is due to the sampling of incident light by three kinds of photoreceptors sensitive to different wavelengths (L, M, and S cones). As only one cone type is present at each spatial location, colour information is spatially multiplexed. By considering several models of cone distributions (regular, random, and intermediate situations) we can calculate the spatial correlation function of the colour multiplexed signal. The correlation function can be decomposed into two terms: a low-pass term obtained by averaging the cone outputs, luminance signal, and a spatial modulated term which contains the colour opponency, chromatic signal. A subsequent filtering stage allows one to separate the two signals (Hérault, 1996 Neurocomputing 12 2 - 3).
Because of spatial multiplexing, a discrepancy exists between luminance spatial resolution and colour spatial resolution. In foveal vision, the discrepancy is resolved by assigning the maximum acuity to luminance. We show that this is achieved by a regular centred hexagonal lattice with only L and M cones with densities having a ratio 2:1 (or 1:2). In such a specific sampling arrangement the chromatic signal is modulated at high spatial frequencies in order to occupy the spatial frequencies where no luminance signal is present. As a result, in such a sampling lattice, spatial resolution of the luminance signal is only limited by the space between photo- receptors. Thus, luminance has a maximum resolution of 60 cycles deg-1 and the colour signal a resolution up to 12 cycles deg-1, which is in good agreement with experimental data [De Valois, 1988 Spatial Vision (New York: Oxford University Press)]. As spatial resolution is not a strong requirement in parafoveal vision, the three kinds of photoreceptors are present and the sampling lattice accords more importance to colour representation than in the fovea.
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