Contrast increment thresholds of rhesus monkeys

Increment contrast thresholds ware determined behaviourally as a function of background contrast for sinusoidal gratings with spatial frequencies between 0.5 and 8.0 c/deg. As has been previously reported for humans, the contrast discrimination functions ere characterized by a facilitation effect for low background contrasts and a masking effect for high background contrasts. The shapes of the functions varied with spatial frequency, but for all spatial frequencies, portion of the functions were adequately described by a power law. The shapes of the reaction time distributions obtained for near threshold stimuli suggested that at least two mechanisms with different spatio-temporal sensitivities were involved in detecting the stimulus and that the exact shape of the contrast discrimination function was dependent on which mechanism dominated detection.