Visual Field Distortion

April 10, 2004 10:07 AM

Pizza induced visual field distortion : When I chew - as on a piece of pizza (or whatever) and stare at a computer screen, especially from it distance, I perceive a "ripple" (or a "flicker") on the screen which is congruent with my chewing action. (more inside)

Now, I know why this occurs, sort of : when I chew, it increases the pressure in my skull a little and this in turn squeezes or squishes my optic nerve, or maybe my eyeball, or whatever. OK, fine.

But this effect only occurs when I chew and stare at a screen. Why don't I get the same effect when I chew and stare at the wall, or out the window, etc. - in all other cases?

Has anyone else noticed this effect? I know that there's some brain algorithm driving the effect, and I assume that there's an evolutionary reason for this sort of "Steady-Cam" mechanism - and I can guess as to the reasons. But why does the "Steady-Cam" turn off when I chew my pizza and stare at a powered-on CRT or LCD screen ?

posted by troutfishing to health & fitness (17 comments total)

It might be similar to the "chopstick & TV" phenomenon. The inimitable Cecil Adams talked about it here. (Scroll down to "When Will I Learn?")
posted by Johnny Assay at 10:19 AM on April 10, 2004

This is a SWAG (Silly, Wild axx Guess)- Screens have a refresh rate. Normally it's pretty invisible.

But your eye is being jittered just enough to fall out of sync.
posted by filmgeek at 11:03 AM on April 10, 2004

troutfishing - i've seen this; probably due to neuron firing pattern, rapid movement, visual tracking, and screen refresh rate.

try it with a monitor with a wide array of refresh rates - does it distort more at 60 hz than at 80 or 120? my guess is yes; the faster the refresh the less noticable this oght to be.

other things won't do this wiggling dance when you look at them because they aren't presented in digital (that is, individual still frames linked to fake movement) but in analog - which is why spoked wheels only appear to spin backwards on TV, and not when you see a spoked wheel tunring in real life. real life is analog, and the visual tracking system we all have evolved in an analog world. when we view digital we trick the brain into filling in the gaps; if the frame sequence is interrupted or not fast enough, it looks distorted or jerky because the visual fill-in system can't keep up.

(probably also why low-frame rate animation pisses me off so much...)
posted by caution live frogs at 11:03 AM on April 10, 2004

Something like this happens to me when looking not at a screen but at some (and only some) LED displays. The clock on my dad's microwave, for instance. Chewing or pressing my jaws together causes the display to shake. Very strange.
posted by kenko at 11:10 AM on April 10, 2004

Do you wear glasses? I experience the same effect more often when I'm wearing glasses, and most often when I wear a specific pair that sit lower on my face and are jiggled by my chubby cheeks while I chew (I literally can't eat while driving when I wear those glasses).
posted by wendell at 12:37 PM on April 10, 2004

When you're waiting in line at an ATM, put the ATM card halfway into your mouth, bite down on it and then flick the end up and down to get that "twang twang twang" noise while you look at the little ATM screen. Not only will the person in front of you hurry up, you'll see that same flicker.
posted by jessamyn at 1:19 PM on April 10, 2004

See the 2-11 entry here.
posted by notsnot at 1:23 PM on April 10, 2004

You should try eating crunchy foods while watching the monitor, then. A bowl of Cap'n Crunch, and you'll be tripping out on the weird sync/eye-jiggle patterns.
posted by five fresh fish at 2:43 PM on April 10, 2004

Like CRTs, some LED and VFD clocks actually flicker on and off at a very high speed. When you look directly at one, the flicker is too fast to notice, and it looks like a constant brightness.

When you click your teeth or chew on something crunchy, your skull vibrates, and so your eyes will move around a bit, too. They're not fast enough to compensate, so the point at which your eyes are looking will move also. It's like shaking a camera while taking a picture with a long shutter speed.

Most things, like a wall or an incandescent light, are constantly reflecting/emitting light, so they just blur as your eyes move like that. But if something is flashing quickly, it won't have a smooth blur, and will show up in your vision at discrete points. Imagine taking a photo with a 15 second shutter of a blinking light, but moving the camera while the shutter is open. So while things that aren't flickering have a smooth motion blur, the flickering light source is superimposed over it only at certain points (when it was on), and it looks like its jumping around.

LED clocks that flicker like this sometimes light up segments individually, and so individual segments are "on" at different times. When you get the jittery effect with LED clocks, you'll see that the different segments actually move around relative to one another, instead of jumping around together.

Here's an example (if ImageShack doesn't eat my photo like it did last time). This is a photo of my LED clock with plenty of motion blur taken at 5:57. First, notice that the background is entirely blurred, but the LED segments aren't. Then, look at either 5. See how it's never a complete 5 all in the same place? The 7, on the other hand, seems to always be lit all at once. It looks like it's wired to only light three segments at a time.
posted by whatnotever at 3:21 PM on April 10, 2004

There was a My Favorite Martian episode that kinda, sorta addressed this phenomenon. Sorta.

It's good to see better answers than I had pinned down -- I'd been going with "well, the frequency of the sound, and ergo the vibration of my head, is close to that of the strobing Whatever, and I'm seeing interference patterns, I guess, maybe.."

Never thought about why it happened with LEDs, though. Probably 60 or 120 times a second, because of AC power?
posted by cortex at 6:15 PM on April 10, 2004

This happens for me with almost all LEDs. It's more than likely refresh rate, as people have said. Try plucking a (guitar) string in front of a computer screen, you can see it wave.
posted by abcde at 6:54 PM on April 10, 2004

cortex, LEDs in clocks strobe because it simplifies the digital circuitry running them. Take a look at the circuit here. There are four 7-segment displays, for a total of 28 LEDs, but only 12 wires going into them to control them. The four wires on the bottom are used to choose which 7-segment section is active at any given time, and the 8 going in on the left set the LEDs in that section on or off. So at any one time, just one of the four 7-segment displays will be lit, and the circuit quickly cycles through them.

The other way to do it would have more than twice as many control wires, one for each individual LED, meaning the controller microchip would need more pins, which makes it more expensive. Your keyboard works in the same way. There isn't a single wire for each key, but instead there is a grid of wires, each wire shared between multiple keys. A circuit scans through the grid fast enough that even though it's not continuously sensing every key, it's fast enough to pick up any keypresses, which are relatively slow compared to the speed of scanning.
posted by whatnotever at 7:06 PM on April 10, 2004

A similar visual effect happens when you quickly wave your hand in front of a monitor versus waving it in front of a light.
posted by the biscuit man at 8:06 PM on April 10, 2004

Or in front of a rotating fan.
posted by five fresh fish at 9:11 PM on April 10, 2004

Blowing a raspberry also causes the same effect when looking at any light source with a slow enough refresh rate.

If you want to eliminate it, simply get an LCD monitor or a laptop.
posted by kindall at 12:11 AM on April 11, 2004

You can see the same effect if you look "sideways" at your monitor rather than directly at it. With two monitors, I see a continuous flicker out of the corner of my eye in the monitor that I am not focussed on.
posted by dg at 6:32 PM on April 11, 2004

Wow - thanks, everyone, for all this informed speculation. I have an awful lot to chew on here.

[ orders pizza ]
posted by troutfishing at 8:00 PM on April 11, 2004