Many people notice that the spinning rims of a car shown in a commercial or film appear to spin backward even though the car is clearly moving forwards. It is especially noticeable when the car is just beginning to speed up or slow down. During an extended shot of the car driving down an open highway, its spinning rims may even appear to stand still and then rotate backward. There are actually several different optical illusions at work which cause spinning rims to appear to spin in reverse.
There is a fundamental difference between real motion and the sort of animation used to create the illusion of motion on film or video. When a real car's wheels start to move forward, our eyes can usually follow the motion of individual spokes until the car reaches a certain speed and they become blurred. After that, our eyes can only catch glimpses of certain spokes if they are illuminated by an outside light source. This provides our brain with enough information to recognize the forward motion of real spinning rims.
Animation, however, works on a different principle. The actual car's motion is only represented by a series of still images moving at 24 or possibly 30 frames a second. Our eyes see the same spinning rims, but now the information is entering the brain at a different rate. If an individual spoke in a series of animated images appears in a different position, the brain interprets it as moving in a certain direction. If a spoke appears lower in subsequent frames, it will appear to move backward in our mind's eye. Even if the actual wheel is clearly moving forward, the individual spokes of the spinning rim may appear to move in reverse.
Besides the brain's interpretation of the moving spokes, our eyes can be fooled by an optical illusion called the stroboscopic effect. A stroboscope is a device which can be calibrated to send out pulses of light at a designated rate per second. If a stroboscope's light is focused on a fast moving object, such as a car's spinning rim, individual elements of that object could appear to stand still, move forward or move backward. In a car commercial, an outside light source such as the sun or movie lamps could create a stroboscopic effect as individual spokes are illuminated. Even though the actual tires are clearly moving forward, the stroboscopic effect can create an illusion of backward motion or even a complete standstill, much like dancers can appear motionless when illuminated by a strobe light.
When the information is limited, the brain often makes assumptions based on what it knows. In the case of spinning rims, the relative position of the individual spokes and the light source which makes them visible can combine to create an optical illusion of backward motion.
