Colour and brightness encoded in a common L- and M-cone pathway? A Stockman, D Petrova, B Henning |
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Institute of Ophthalmology, University College London, United Kingdom
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| Flickering lights near 560 nm appear brighter than steady lights of the same mean intensity, whereas lights near 520 or 650 nm appear yellower. Both effects are consistent with the distortion of the representation of the input signal within the visual pathway: brightness enhancement at an expansive nonlinearity and the hue change at a compressive one. We have manipulated the distortion products produced by each nonlinearity to extract the temporal properties of the early (pre-nonlinearity) and late (post-nonlinearity) stages of the L- and M-cone pathways signalling brightness or colour. We find that the attenuation characteristics of both pathways are virtually identical both before and after the nonlinearity: the early temporal stage acts like a band-pass filter peaking at 10-15 Hz, while the late stage acts like a two-stage low-pass filter with a cut-off frequency near 3 Hz. We propose a physiologically-relevant model that accounts for the early and the late filter shapes and incorporates both types of nonlinearity within a common pathway. Modelling suggests that brightness enhancement is caused by rectification whereas the hue change is caused by a smoothly compressive nonlinearity. Plausible sites for the nonlinearities are after subtractive centre-surround antagonism possibly from horizontal cells. |
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