ABSTRACT
Have you noticed that when you slow down after driving at high speed for some time your car seems to be going much slower than your speedometer indicated? This is an after-effect caused by your adapting to the previous speed. A more startling visual after-effect of motion can occur when you stop a car after watching the road travel past: just after you have come to a stop the road can appear to move forwards, or the car to roll backwards. This brief apparent backward motion is a visual after-effect of the preceding period of adaptation to forward motion. If you have never noticed this effect watch for it next time you are in a car or train (especially if you are not driving, since the effect is more obvious if you fixate on the road). After fixating a spot in a swift stream or waterfall, you can also see a motion after-effect (MAE) by switching your gaze to the bank, which will appear to move back upstream. Indeed the MAE is often referred to as the waterfall effect.
