The ability of observers to discriminate differences in global-motion-signal strength(that is the proportion of coherently moving dots in a field of randomly moving dots) was determined for both first and second-order stimuli. Observers could accurately discriminate differences in signal intensity for all reference signal levels tested; 20-100%. A similar pattern of performance was obtained for both types of stimuli. The observed first-order signal-discrimination performance is compatible with the results of electrophysiological studies that have investigated the dependence of the firing rate of V5 cells (also called the middle temporal area) upon global-motion signal intensity. (C) 1998 Elsevier Science Ltd. All rights reserved.