Contrast perception and electrophysiological correlates.

1. Previous findings show that the amplitude of the human visual potentials evoked by gratings alternating in phase at several c/s is linearly related to the logarithm of the grating contrast.2. Similar previous experiments on the cat have shown that both the amplitude of evoked potentials and the amplitude of the response of simple cortical cells to alternating gratings increase linearly with the logarithm of the contrast. No such relation is found for single neurones in the retina or in the lateral geniculate body.3. In this work the contrast of a grating is evaluated psychophysically by human subjects in a range of more than 10 times above threshold, using a binocular matching technique.4. The apparent contrast is found to increase linearly with the logarithm of the stimulus contrast.5. The conclusion is drawn that the linear relation between apparent contrast and the logarithm of the objective contrast might be the outcome of a process of elaboration that is carried out at the simple cell level.