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Robert Ennis, Barry Lee, Qasim Zaidi; Physiological signature of time-varying color after-images. Journal of Vision 2011;11(11):378. doi: 10.1167/11.11.378.
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© ARVO (1962-2015); The Authors (2016-present)
At VSS 2010, we presented a new psychophysical method for measuring color afterimages. Colors of two halves of a bipartite disk were modulated sinusoidally (1/16 Hz to 2 Hz) from mid-gray to opposite ends of a color axis and back, e.g. grey > red > grey on one half and grey > green > grey on the other. The two halves appeared identical initially, increased in difference, then decreased to no difference, then increased again in opposite phase, so that when the physical modulation returned to grey, negative afterimages were perceived: the half modulated through red appeared green and vice versa. The physical contrast between the two halves, when they appeared identical, provided a Class A measurement of the after-image magnitude. Here, we present an early neural substrate for the afterimages by measuring the responses of retinal ganglion cells (RGC) to similar stimuli. Parafoveal RGCs were shown uniform circular patches modulating towards each of the poles of the preferred axis of the cell at 1/32 Hz and 1/16 Hz. The responses of Parvocellular and Koniocellular RGCs vigorously tracked modulations in their preferred direction, but decreased to base-rate 1–2 sec before physical modulations returned to mid-gray, dipped below base-rate and then recovered. Cell responses to modulation in the non-preferred direction, tracked the sinusoidal dip, but the response recovered faster than the stimulus, firing was significantly above base-rate when the stimulus reached grey, and the excitation persisted for a short time. Together, the excitation and inhibition of RGCs tuned to opposite directions along a color axis provide an early neural explanation for the afterimages: cells responding vigorously at mid-grey propagate an after-image signal to subsequent stages. RGC responses to both modulation frequencies were well described by a cone-opponent subtractive adaptation with a slow time constant of 5–10 seconds. Slow neural adaptation of the RGC population thus accounted for the after-image psychophysics.
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