An eye for an eye: the speed of sight

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Many of us have seen the spokes of the wheels of super-fast cars revolve backwards. Why doesn't it simply go forward?

Our eyes have millions of receptors called rods and cones. The color vision is mostly thought to involve a comparison of the signals received from different types of cones -photoreceptor cells in the eye that are sensitive to color. Rods, a different type of photoreceptor cell, that are thought to be used only for colorless vision in very dim light. The optic nerve, a cable–like grouping of nerve fibers, connects and transmits visual information from the eye to the brain; and finally, humans can see objects in three-dimension which is the courtesy of the crossing over of optic nerve fibers at the optic chiasm. So if we have all the correct connections in place and the images we perceive are at the speed of light, why are we affected by such an optical illusion?


The answer is rather simple (though it got me perplexed for quite some time), the optic nerve - or nerves in general - have to send the information to the brain. So even though the receiver (our eyes) may receive the light at light-speed(obviously!), in Operating System terms, the time for the signals to reach the software for processing does not happen at the same speed. A neuron can only fire once every few milliseconds[1]. As the cones and rods are spread about the eye, not all of the ~125 million receptors are firing signals together or all the time. It is done in a random fashion, where most of them are firing signals every now and then.

Even a firing rate of 100Hz (which is an overestimate) may cause such distortions in our vision if something exceeds this speed. When a car is going super fast or the blades of a helicopter are moving, the speed of revolutions can exceed the rate of transmission by our eyes. The effect this has - is nothing but aliasing, i.e, we are clicking pictures at a slower rate. When the revolution of the spoke starts, our eye transmits one image and by the time the next image is transmitted the spoke almost completes its revolution but is a little behind a full 360 degree completion. This process continues and the video produced by our brain makes it seem that the spoke is going backward, when it actually is not.

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