Well we’ve all seen it before: a object, that we know is spinning, appears to be turning backwards when we know for a fact that it’s doing exactly the opposite. Why does that clockwise motion appear to be counterclockwise (or vice versa) to our eyes?
The answer (or answers) to this quandary begins on the silver screen, since the effect is extremely simple to produce with an ordinary film camera. Because movie cameras capture video as a series of still frames, and the wheel it’s filming moves continuously without such discrete states, some artifacts are bound to pop up in the interaction. In the case of a video camera, or in real life with strobed lighting, the periods of visibility broken by periods of invisibility will interact with frequency of the rotation of the wheel. This so-called Wagon Wheel Effect can, but does not always, produce the illusion that a car’s wheel (or other object) is stationary, or even rotating in the wrong direction.
Imagine if one spoke of an old wooden wagon wheel is a twelve o’clock in the first frame of a movie, and the wheel is rotating clockwise. In the time it takes two switch to and take the second frame, that spoke has moved, say, three quarters of a full revolution, and is now at 9 o’clock. In the third frame, it will be at 6 o’clock, then 3. Over these four frames the spoke seems to have moved counter-clockwise, from 12 to 9 to 6 to 3, but in reality it has moved clockwise continuously, with no particular stops at any point in the rotation. If the frequencies had lined up perfectly, and the spoke returned to 12 for each frame, the wheel would have looked totally still.
However, the human eye is not subject to this framed effect — or is it? As mentioned, a strobed light-source can do the trick, as can a vibration in the eyeball itself. One researcher found the wagon wheel effect could be produced simply by humming a particular tone to vibrate his eye. More advanced theory puts the blame on the brain itself, which may process visual information as a series of data-points not precisely like the frames of a movie — but not all that unlike them, either.
The major competing explanation is more abstract. Essentially, the brain seems to create explanations for ambiguous input, applying some reasonable physical understanding to input that might not have an obvious meaning. One study found that two identical rotating wheels can be seen as rotating in opposite directions even when displayed side by side — those they are exactly the same, the brain might latch on to one explanation for one, and another for the other.
The classic example of such conceptualization of physical space by the brain is the Necker cube, which can pop from one configuration to another. It’s not just that you can see either shape in the lines, but that once you see one the other becomes unthinkable — until you see that one, which then seems just as obviously the correct interpretation. These two theories have varying levels of support throughout the scientific community at present, but each likely has something to do with the effect, as we perceive it.
Ever Wondered What Makes Wheels Appear to Spin Backward?
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