MadSci Network: Biophysics
Query:

Re: Airport Landing Light 'Illusion...'

Date: Sat Jun 27 16:18:47 1998
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Biophysics
ID: 895506433.Bp
Message:

Hi Patrick!

Cool observation!

As an electronics student in 1980 I was writing software for the Center for Visual Science at the University of Rochester NY. Keep in mind that I am NOT an expert in all of this, so the stuff below is partly the informed opinion of a student research assistant.

The guy down the hall from me at CVS was doing research in exactly the area you mention. He was using a computer to sequentially flash a row of closely-spaced strobe lights (LEDs, actually), and a human volunteer would try to judge whether the apparent motion was left-to-right or the reverse. The researcher was continually varying the parameters of the experiment in order to draw graphs of the strength of the "motion illusion" versus spacing of the lights, speed of the "motion", pulse width of flashes, brightness of some flashes, dark stripes across the row, wiggly motion, etc. Study of the Motion Illusion falls under the part of science called Vision Psychophysics. Take a look at the CVS website above. Perhaps you can find more info about it by searching the Web for the words "temporal" and "optical illusion," or just "motion illusion." I vaguely recall that this was also called "temporal masking", but I'm not sure.

I am still irritated about one part of all this: some books will wrongly tell you that the motion illusion is called "persistence of vision." No. In truth, persistence of vision occurs when we cannot see the flashing of a rapidly-blinking light. Persistence of vision is mostly caused by the slow response of the cells of the retina. On the contrary, the motion illusion has little to do with the response speed of the retina, instead it is part of the sophisticated processing which takes place in poorly-understood sections of the brain. If a human being sees two identical pictures which are rapidly flashed in sequence, and if the position of one picture is slightly different from the other, then the person will not see two separate pictures. Instead the person will see a single picture which suddenly jerks sideways. However, if the pictures are flashed much more slowly, then the person will see two separate pictures with no jerks of motion. While persistence of vision can explain why we cannot see the fast flickering of movies, of TV screens, or the 120hz flashing of flourescent lighting, it does not explain why we percieve the sequential presentation of still photographs in movie film as being moving images.

The motion illusion arises in many places in the natural world, so it is sensible that humans and animals should have ways of correctly interpreting what is really occuring. For example, if a deer is running through the woods behind trees and bushes, we see this as a single deer which is being periodically occulted by opaque foreground objects. We do not see it as an unmoving row of deer which peek out from behind various objects, even though that could be the actual cause of the image. What if a bug is flying through the grass, won't a predator lizard want to correctly interpret that bug's path and know where it landed? Even if the grass is very dense, and the bug can be seen only in flashing glimpses? The brain cannot see the true path of the deer or the bug, but it somehow seems to build an internal model of a single moving target. We end up seeing this model, and not the reality. The reality is that the deer appears in glimpses from different spots. But instead we see it as a single moving deer. The brain fills in the invisible path of the deer as it moves behind the opaque bushes, and this is entirely illusory. It is a useful optical illusion being constructed by our brains in hyper-fast realtime parallel processing. No modern supercomputer has a hope of accomplishing something like this; it is a totally staggering feat of processing power.

Your aircraft strobes are a naturally-occuring version of that old experiment at CVS. When you are far from the strobe array, your brain decides to see them as a moving dot. When you are above them, their apparent spacing is much wider, the motion-illusion turns off, and your brain decides to see them as a row of dots which all flash simultaneously.

What causes your brain to switch? I suspect that this depends on what sorts of visual motion are important in your life. For example, if a deer is running at 100mph behind a row of trees, then that is an unnatural situation, and besides, you'd have no hope of catching the deer. In that case, there would be no reason for our brains to evolve to (or to learn to) interpret the situation correctly. We would probably see a row of deer which briefly flash into existence at many separate spots, rather than correctly seeing a single deer which travels at a ridiculous speed. But a lizard might be different. If that lizard has to predict the landing site of extremly fast grasshoppers which are seen in brief glimpses in heavy grass, then the lizard's "programming" might be very different from ours. The lizard might see your widely-spaced aircraft strobes as a single moving dot.

I'm not sure, but I suspect that the motion illusion probably responds to training, or even to conscious intent. If you spent days watching widely-spaced strobe lights and trying to judge the direction of the illusory "motion", then perhaps you would start to really see a single, fast-moving dot, rather than a row of lights which all flash nearly at the same time. Sounds like a personal project for a very bored pilot! :)


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