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Andrew Stockman; A psychophysical dissection and model of the common pathway that encodes colour and brightness. Journal of Vision 2013;13(15):T30. doi: https://doi.org/10.1167/13.15.30.
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© ARVO (1962-2015); The Authors (2016-present)
Flickering monochromatic lights near 560 nm appear brighter, and those near 650 nm appear yellower, than steady lights of the same wavelength and mean intensity. Both effects are consistent with distortion within the visual pathway: brightness enhancement at an expansive nonlinearity and hue change at a compressive one. We have manipulated the distortion products generated by each nonlinearity to extract the temporal properties of the early (pre-nonlinearity) and late (post-nonlinearity) stages of the pathways signalling brightness or colour. We find that the attenuation characteristics of the two 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 both filter shapes and incorporates the two nonlinearities within a common, probably parvocellular, pathway that signals both hue and brightness. The early filter shape is consistent with antagonism between “centre” signals and more sluggish and delayed “surround” signals. The brightness change may result from the half-wave rectification and partition of signals into ON- and OFF-components. The hue shift is caused by the additional effects of a later smooth nonlinearity that compresses chromatic “red” and “green” signals generated at 650 nm more than the balanced ones generated at 560 nm. Plausible sites for the rectifying nonlinearity are soon after surround antagonism, but the compressive nonlinearity may be later.
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