Contrast-modulated grating patterns were used to compare evoked responses and psychophysical thresholds of contrast modulation. The stimulus consisted of the successive presentation of a grating pattern at a higher and lower contrast. At a modulation rate of 8 Hz it was found that there is correspondence between the two kinds of data for a 6 cycle/degree grating. The just noticeable difference was not constant, and data approximated a Weber's law of contrast discrimination. However, it was found that there is a slight departure from this law, in that the ratio decreases with mean contrast. Both psychophysical and electrophysiological data exclude a model of simple luminance detection and are consistent with the operation of spatial contrast detectors in the human visual system.
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