Abstract
A temporal forced-choice procedure was used to measure the contrast threshold for a sinusoidal test grating (spatial frequency = f) superimposed upon a sinusoidal background or masking grating (spatial frequency = 3f). The spatial contrast of the background grating was varied, and threshold measurements were made at each of a number of background contrasts to describe a threshold versus masking contrast ( tvc) function. Tvc functions were obtained when the background and test grating contrasts were, independently of each other, held steady (0 Hz) or modulated at 5 Hz. The changes of temporal modulation frequency affected the slopes of the tvc functions. In some cases the tvc functions for steady and flickering test gratings crossed one another. The changes of slope suggest, and the crossovers imply, that some steady and flickering patterns are detected by separate visual mechanisms.
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