Threshold energy of foveal, 2-msec white test flashes was plotted as a function of their timing with respect to sinusoidal fluctuations of nearly achromatic background light. With frequencies of 3.1–10 Hz, modulation near 100%, and peak retinal illuminance of 1280 td, the resulting masking curves are nonsinusoidal. Threshold maxima precede background maxima, whereas threshold minima coincide with background minima. The phase lead of the threshold maxima increases with frequency, but tends to stabilize at about 90° above 5 Hz. Maximal and minimal threshold energies also increase with frequency, but their ratio is nearly constant between 3.1 and 8 Hz. Steady light added to the 1280-td fluctuation elevates threshold minima, but often lowers maxima, and introduces secondary maxima that accompany decrements of background luminance. Generally, thresholds are higher with fluctuating backgrounds than with peak-equivalent steady backgrounds. The data for 1280-td and 128-td fluctuations confirm that masking is a nonlinear function of modulated background luminance, and suggest the presence of unidirectional rate sensitivity. Of several possible physiological correlates discussed, ganglion-cell discharge patterns are most nearly compatible with the masking curves.
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