|MadSci Network: Neuroscience|
This occurs because the fan blade is spinning faster than your photoreceptors can respond. As a result your visual system performs something called "aliasing" while you watch the blade. Aliasing occurs when the state of a stimulus is sampled at a rate much slower than the rate at which the stimulus changes. Imagine that your eyes "sample" a fan blade when it is straight up, at 12:00. Then imagine that your eyes can't sample again until the blade moves 270 degrees clockwise, at 9:00. Then your eyes sample again, when the blade is at 6:00. Now imagine that each successive sample occurs 270 degrees later. To your eyes it appears that the fan blade is moving "backwards," going slowly counterclockwise even though it is really moving clockwise at a much faster rate. The determining factor is called the "flicker frequency" of your photoreceptors. This is the frequency at which a light that is flickering on and off "fuses" into a continuous light to us. It occurs, depending on intensity, between 50 and 80 cycles per second (that's why fluorescent light appear to flicker -- they, indeed, flicker on & off at 60 Hz). The inverse of this number gives us a rough approximation of the "integration time" of the photoreceptors, the minimal time necessary for change in a stimulus to be noticed. That number is about 30 msec. Therefore, your eyes are only capable of recording differences in your visual field every 30 msec and they can't keep up with the fan. Altering the speed of the fan can, therefore speed up, slow down, or even "reverse" the "virtual fan's" direction.
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