Visual Illusions: Illusory Motion Reversal and the Entopic Phenomenon
I had an interesting conversation this weekend about visual
illusions. First, my friends and I were on a beach and we started
talking about what you can see when you stare at a bright sky.
One of my friends mentioned that the big, floating spots you can see
are impurities in your retina. These are the things that seem
suspended in your eyes and tend to float around when you move your
eyes. Apparently, they are called floaters.
I then pointed out that you can also see little white spots spinning
around very quickly when you look at the bright sky. My friend
had never noticed these before and we couldn’t figure out what they
were. I did a google search and it turns out this is called the
“entopic phenomenon.” It’s caused by white blood cells moving
through the capillaries in your retina. That’s sort of amazing
that you can actually observe your own white blood cells in motion
without any magnification. Here’s a link with more details.
Finally, my dad mentioned there was one visual illusion he’d never
figured out: when you stare at an accelerating, spinning wheel (i.e. on
a car), the spinning motion appears to reverse at a certain
point. We all agreed that this is weird and there is no obvious
explanation for it. Turns out the scientific community hasn’t
figured it out either. One theory
suggests that the eye sees the world in discrete snapshots like a movie
camera. When the wheel spins really fast, the snapshots don’t
keep up with the motion. Therefore, the wheel can turn almost all
the way around between snapshots, and your mind interprets those
successive snapshots as backward motion. Another theory focuses
on “perceptual rivalry.”
This is a little bit more complicated, but basically your brain
incorrectly interprets the motion it sees due to occasional stimulation
of reverse-motion detectors.
If you’ve never experienced these illusions before, try them sometime. They’re really cool.
Jim
November 12, 2006 @ 11:36 pm
I believe that your information concerning the spinning wheel may be incorrect and it has to do with a strob effect and is usually observed with a man made light source, which cycles at 60 times per second, contrasting with a moving wheel that cycles above 60 times per second and what we see is the stop frame of exact timing. Try looking at wheels in sunlight, natural light, they do not have this effect. However, do not do this through or on any electronic media, i.e. movies or television as they have frame sequenceing that cycles at a certain number of frames per second as well and you will see this phenom on movies and TV but not on the road.