1. the effect of the timing of stimuli on their perceived spatial location. For example, if three equidistant lights are flashed in succession, but the time interval between the first two is shorter than that between the second and third, then the first two lights are perceived to be closer together than the second and third.

2. the invariant that the time to contact between an object and an observer moving at a constant speed toward each other is inversely proportional to the rate of expansion of the observer’s retinal image of the object, regardless of the size of the object or the speed at which it travels. Research suggests that the tau effect is used in a variety of situations involving the control of movements, for example, by a ballplayer when preparing to catch a ball and by a diving gull in retracting its wings before hitting the surface of the water to catch a fish.