With sinusoidal modulation of the radiance of the stimulus as a function of time, amplitude thresholds are measured instead of the repetition-rate threshold usually obtained in flicker-fusion experiments. Controlling the modulation amplitude independently of the average radiance provides an additional degree of freedom, so that the observers adaptation level can be held constant while his amplitude sensitivity is measured as a function of the modulation frequency. With an edgeless flickering field, these amplitude sensitivity curves show a broad peak of maximum visual response, in the region from 10 to 20 cps at high photopic levels. Such classic relationships as the Ferry-Porter, Talbot-Plateau and Weber-Fechner laws are derivable from the present results, as descriptions of the behaviour of certain parts of the amplitude sensitivity curves as functions of adaptation level.
Abstract