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Andrew Stockman, Caterina Ripamonti; Red-green flicker is encoded by a peak detector and limited by slew rate. Journal of Vision 2014;14(10):592. doi: 10.1167/14.10.592.
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The appearance of L- or M-cone rapid-on (slowly-off) or rapid-off (slowly-on) sawtooth modulated flicker depends on temporal frequency. At low frequencies (0.5-4 Hz), the flicker is seen as a red-green hue change that approximately follows the waveform. However, at higher frequencies (5-13 Hz) the hue change becomes asymmetric and there is a change in mean hue in the direction of the slow phase of the sawtooth—consistent with the hue mechanism being limited by a maximum rate of change (a "slew" rate limitation). To investigate these phenomena, we presented only the 1st and 2nd harmonics of the sawtooth flicker and varied the phase of the 2nd harmonic. The phase-dependent changes in red or green hue were assessed by separately matching them against the corresponding phase of square-wave flicker of the same fundamental frequency and of variable modulation. Both L- and M-cone flicker was used and the results were essentially the same. We find that the phase-dependent hue changes from 0.5-4 Hz are consistent with a red-green hue mechanism that signals the peak excursions of the composite waveform. Thus, the maximum excursion in hue towards red or green occurs when the peaks or troughs of the two harmonics align, rather than when the rate of change of hue is minimum. However, at higher frequencies this relationship breaks down, the rate of hue change becomes important, and the hue excursions often reach only intermediate red-green hues. Our results suggest that red-green hue appearance is mediated by a mechanism that encodes the peak excursions towards red or green, but one that is inherently limited in the rate at which it can signal changes in hue. This slew rate limitation becomes evident when the frequency (of the 2nd harmonic) exceeds about 8 Hz. Hue is not only low-pass filtered but also slew-rate limited.
Meeting abstract presented at VSS 2014
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