THE motion after-effect occurs after prolonged viewing of motion; a subsequent stationary scene is perceived as moving in the opposite direction(1,2). This illusion Is thought to arise because motion is represented by the differential activities of populations of cortical neurons toned to opposite directions; fatigue in one population leads to an imbalance that favours the opposite direction once the stimulus ceases(3). Following adaptation to multiple directions of motion, the after-effect is unidirectional(4-6), indicating that motion signals are integrated across all directions. Yet humans can perceive several directions of motion simultaneously(7-10). The question therefore arises as to how the visual system can perform both sharp segregation and global integration of motion signals, Here me show in computer simulations that this can occur if excitatory interactions between different directions are sharply tuned while inhibitory interactions are broadly tuned, Our model predicts that adaptation to simultaneous motion in opposite directions will lead to an orthogonal motion after-effect. This prediction was confirmed in psychophysical experiments, Thus, broadly tuned inhibitory interactions are likely to be important in the integration and segregation of motion signals. These interactions may occur in the cortical area MT, which contains motion-sensitive neurons with properties similar to those required by our model(11-14).