Answer Geek:How Chewing Affects TV Flicker

-- Q U E S T I O N: About the chewing flicker, it happens to me and I have braces. Maybe the magnetic field? — Ted Q U E S T I O N: In your screen flicker article about chewing and the screen doing funky things, I have noticed a similar phenomenon when I cough. Could it have something to do with pressure on the eyes? — Bill B. Q U E S T I O N: This isn’t a question, but the flickering effect seen when chewing gum can be simulated if you do a raspberry, like you would on a trumpet. I think you might have to be some distance away. I would try it now but I don’t want to look like an idiot at work. — Anthony

A N S W E R: When I added the little bit at the end of last week’s column dismissing the notion that there was any relationship between chewing and flickering computer monitors and television sets, I had no idea I was tapping into one of the great untold technology mysteries of the modern era.

Torrent of E-Mail

The question, and my rather flip response, unleashed a torrent of e-mail. It turns out that people all across the country are chomping on chips and wondering why it’s making the display unit they are looking at do strange things. An unprecedented number of you wrote to set me straight and share your theories — I received more messages in the first hour after that column went live than I usually get in a week, and more over the course of the week than I normally get in a month. (People also vigorously e-mailed ABCNEWS.com with comments and explanations.)

Not only are you chomping on chips (Doritos seem to be the favorite), you are chewing on ice, crunching on candy, clacking their teeth together, blowing raspberries, shaking your head, blinking quickly, and brushing your teeth with an electric toothbrush, all while watching TV or sitting in front of the monitor. Then there are the people who see the flicker while resting their head against a stereo speaker or while using one of those handheld electric massagers. A couple people wrote to say they can see the flicker if they hum very low notes. I even heard from someone who recommended sticking a Popsicle stick between my teeth and “boinging it like a diving board” in order to recreate the phenomenon.

Putting to Bed the Myths

So let’s start by getting some of the wackier theories out of the way. I can report with a great degree of confidence that it has nothing to do with electric fields produced by braces, but thanks for writing, Ted. Coughing is not the cause, either. Nor is it caused by astigmatism, wearing glasses, or hard contact lenses, or the color of your eyes, although I can’t entirely rule out the possibility that any of those situations may make it easier to perceive said flicker.

There were more plausible explanations as well. Junyong, who asked the original question, wrote in with this reasonable attempt to explain the phenomenon: “When your head quickly vibrates — from such actions as chewing brittle foods — it is as if any object you are observing is apparently vibrating when it is actually still. Assuming … that the monitor scans from top to bottom, as your head vibrates downward, it instantaneously and momentarily tracks the ‘dark line …’ and can possibly recognize it.”

(And just for the record, for those of you who e-mailed to scold me because they thought I was condescending or unkind in the way I answered the question, here is what Junyong had to say: “I took absolutely no offense to anything you wrote. On the contrary, I was very flattered you took up my inquiry with such attention.”)

Getting to the Bottom of It

To get to the bottom of this pressing mystery, I turned to three experts in the field of display technology and the interaction between human perception and computer monitors: Bob Myers, manager of the Hewlett-Packard Display Technology Center, Jeffrey Anshel, an optometrist and consultant, and Frances McManemin, an adjunct professor in the psychology department at the University of North Texas.

So what’s the answer?

Before we get to that, a correction and further explication to last week’s original question about the line that scrolls through a computer monitor when it is viewed on television is in order. This revision comes courtesy of Bob Myers and has been edited for the sake of brevity.

Let’s assume for simplicity that one pass of the beam across the entire screen takes 12.5 milliseconds. Oddly enough, none of the three phosphors (red, green, or blue) used in a modern color TV tube has persistence times anywhere close to that long; the shortest fades to under 10 percent of its original brightness in perhaps a few hundred microseconds.

Persistence of Vision Revisited

If one had fast enough vision, what the CRT screen “really” looks like is a very bright area where the beam has just written with the rest of the screen fading very rapidly to nearly black. Because we have “persistence of vision,” our eyes integrate the light over a given period of time, and the image appears to be stable and evenly lit. Put more simply, we see the average brightness of a given part of the screen.

So where does the “black bar” really come from? Well, real cameras aren’t infinitely fast, either. If the camera is operating at 60 Hz (normal TV field rate), most of that 16.67 ms frame time is spent collecting light — but there is a period during which those elements are NOT collecting light, but instead are being read. This is analogous to the “blanking time” of the CRT display. So the camera can be thought of as taking samples of the scene to be imaged, every 60th of a second, separated by some “dead time.”

When the subject is itself a periodically refreshed display, though, and the rates of display and camera don’t match, we run into a problem. There are some number of lines on the monitor screen that will not be refreshed while the camera is “watching,” so the camera will see those areas as dark for that frame. This is the black bar; how long it is, and at what rate it moves, depends on the exact nature of the mismatch between the camera and monitor rates.

Steady as She Goes

Now for the answer! Again, I quote from the experts. First up, Professor McManemin:

“When we watch TV, our eyes focus on the screen in a way that holds the images relatively steady although our eyes are always moving. When we are chewing, the head is moving slightly more and the focus of the eyes on the screen is slightly, or largely, disturbed by these movements, it depends on the chewing movements. This movement of the head and eyes must, on these older TV screens, bring the refresher rates into our conscious perceptual awareness.”

Here is Bob Myers’ contribution to the discussion:

“[I]f you chew on something hard, like a candy, you cause vibrations in your head which DO, in essence, move your eyes quickly over a relatively short distance. Remember, what’s really in the screen is a moving bright spot — and suddenly, that moving bright spot is in a different place, and it is different from where the eye/brain system expects it to be. That system can no longer integrate the information into a nice, steady whole, and we perceive the result as a flicker. This is somewhat of a case of a person temporarily getting around the whole persistence-of-vision effect, and seeing, sort of, what’s really going on in the time.”

The Critical Confusion Frequency?

Dr. Anshel provided a similar explanation, with some additional insight into why and when we perceive the flicker.

“Human vision has something called the critical fusion frequency, which is the point where we begin to perceive things that are flickering as if they are solid. Different factors influence that frequency, including the size of the object, its brightness, and which part of the retina it is seen by. The brighter the background, for example, the greater the flicker, and when you look straight ahead, you’ll see less flicker than if you are looking at something off to the side. My guess is that the action of chewing jars the visual axis and changes your line of sight relative to the particular point you are focused on, moving it far enough off the central retina to change your ability to perceive a flickering image as a stable one.”

So there you have it. To sum up: What you see when you eat while watching television or looking at a computer monitor is a result of the motion of chewing, which changes the way we perceive images on screen, disrupting our persistence of vision and allowing us to momentarily see the flicker that is actually there all the time, but beyond the range of our normal vision. And yes, I did see it too, but I had to slow down the refresh rate on my monitor before I was able to perceive it.

Of course this is mostly just speculation, albeit the speculation of the experts, for as Dr. McMenamin pointed out, “Truthfully, I really doubt whether this question will be addressed by the scientific community. It is certainly fascinating that so many people are interested in knowing about this though.”

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.