The effect of small brightness differences on the after-effect of seen movement

Two experiments are reported. In the first experiment the after-effect of seen movement has been shown to occur with a measurable duration when there was a small difference in brightness between adjacent parts of the stimulus pattern. In the second experiment the duration of the after-effect was observed under two conditions of brightness difference; one small and the other relatively large. The difference between the durations of the aftereffect for the two conditions of brightness difference was significant. Results from these experiments have been discussed together with data from an earlier study in which the effects of brightness differences of greater magnitude were investigated. The probable function of the duration of the after-effect with respect to brightness differences over the full range of differences has been considered.