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Andrew Rider, Bruce Henning, Rhea Eskew Jr., Andrew Stockman; Illusory colors from harmonic combinations: an unexpected consequence of ON and OFF pathways. Journal of Vision 2018;18(10):361. doi: 10.1167/18.10.361.
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© ARVO (1962-2015); The Authors (2016-present)
We report an exciting new visual illusion that also provides a novel method of investigating early visual processing. When superimposed red and green lights are sinusoidally flickered in opposite phase at moderate temporal frequencies they take on a mean yellow appearance. However, adding second, third, or fourth harmonics to the flicker can dramatically change the mean colour appearance from yellow towards either red or green depending on the temporal alignment of the added harmonics, even though the mean chromaticity and luminance remain unchanged. Perceiving different mean colours in stimuli that have physically identical time-averaged characteristics points to the interposition of significant nonlinearities in the chromatic pathway. To investigate these colour shifts we varied the frequency and phases of different harmonic combinations of red/green flicker around the same physical mean. In spatial 2AFC tasks, observers indicated which appeared redder out of two polarity inverted waveforms presented in two semi-circular 5.7° fields. Measurements were made on carefully calibrated and linearized CRTs. Although the results appear complex, we can accurately predict the colour shifts using a relatively simple, yet physiologically plausible model, in which cone signals are first linearly filtered (presumably in the photoreceptors themselves), then half-wave rectified into ON and OFF pathways (at the cone-bipolar synapse). Signals in these two pathways are passed through later linear filters, then through saturating nonlinearities (presumably cortical saturating contrast response functions). Depending on the input phase alignments reaching the rectification stages, radically different signals are passed to the ON and OFF pathways, which are then differentially compressed by the late nonlinearity to produce an average imbalance in the signals and thus a colour shift. This phenomenon provides a powerful new tool for psychophysically dissecting the red-green chromatic pathway, and we expect other pathways.
Meeting abstract presented at VSS 2018
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