Physiological correlates of brightness fractionation



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Brightness fractionation of white light was studied both psychophysically and physiologically. The psychophysical and physiological experiments were conducted simultaneously. The method of fractionation was used. Two flashes were presented to the subject alternately. The subject was instructed to adjust the intensity of one of the flashes, so it would appear to be half as bright as the other. The mean adjustment was used to evoke cortical responses from the visual area. It was found that the ratio of the amplitudes of the visually evoked cortical potentials correlated with the psychophysical fractionation. A psychophysical brightness scale was constructed. It confirms Stevens' postulation of a power function between the sensation magnitude and stimulus intensity. No systematic relationship was found between the stimulus intensity and the amplitude of the visually evoked cortical potentials. This result is extensively discussed. It was speculated that the amplitude of the visually evoked cortical potential is related to the intensity of the stimulus in a subtle way, that it reflects the difference between the intensities rather than the absolute intensity in its raw form.



Visual perception.