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 non-sinusoidal. 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.
G. M. SHICKMAN, "Visual Masking by Low-Frequency Sinusoidally Modulated Light," J. Opt. Soc. Am. 60, 107-117 (1970)