1'Vlotion sensitive mechanisms in human vision resemble other sensory mechanisms : they are independent of each other at threshold and inhibit one another above threshold. Several experiments show that 1) gratings moving in opposite directions are detected independently and 2) at suprathreshold levels there is inhibition between mechanisms sensitive to opposite directions of motion. A model is proposed which accounts for the co-existence of independence and inhibition. The model explains many perceptual phenomena including motion aftereffects and direction reversals in ambiguous moving stimuli.