Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 06/07/2010 15:28:36
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When I brush my teeth with an electric toothbrush my vision remains normal unless I look at a computer screen or digital LED clock in which case the image is shimmering. Why the difference?
Asked by John Woolf
Hear the answer to this question on our podcast (http://www.thenakedscientists.com/HTML/podcasts/show/2010.07.04/)
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We posed this question to Mikael Karlberg from Lund University Hospital in Sweden...
[img float=right]http://www.thenakedscientists.com/forum/copies/RTEmagicC_Electrical_toothbrush_20050717_001.jpg.jpg[/img]Mikael - It’s an interesting question, I noticed that phenomenon 10 years ago also while brushing my teeth, and watching television at the same time. I thought - this is very funny and then we did some experiments in front of computer screens, making a farting sound with your lips and watching the screen on the same time, and seeing exactly the same visual phenomenon. And then I specialized in inner ear disorders, on disorders of balance. We started to study the inner ear receptors, the vestibular receptors, that is the perception of vertigo, and the sensory cells are really small sensors that are sensitive to vibration, just like the auditory hair cells that make us hear.
It turned out, as I found in experiments from the 70s, that if you vibrate the skull and measure what the vestibular receptors do, they phase lock to the vibrations, so they start to signal with the same frequency as the vibration that is applied to the skull. So if you vibrate at 100 hertz, the vestibular cells start to fire at 100 hertz.
So, what I believe happens is that we watch a screen and the screen is updated around 50 – 60 hertz, 50 to 60 times every second, and then we apply a vibratory stimulus to the vestibular hair cells, so they start to fire at a different frequency. And what we really notice here is some kind of interference phenomenon between the updating frequency of the screen and the vibration applied to the sensory hair cells.
Chris - And what those vestibular system hair cells are actually doing is controlling your eyes so that the eyes move in the opposite direction to the movements your head makes. Now this is normally very important to help you to see objects clearly even when your head is moving around, but the vibrations from the electric toothbrush can disturb this control which makes the eyes move unnecessarily and consequently, the LED clock or the television, or whatever you're looking at, which is turning on and off very quickly, gets strobescopically caught by your moving eyes, the visual system, and the numbers appear to flicker.
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http://www.thenakedscientists.com/forum/index.php?topic=27094.msg286786#msg286786
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Presumably the sources you are looking at are flicking on and off very rapidly and the vibrations from your toothbrush are moving your eyes sufficiently fast to "catch the clock or screen in the act" of updating itself. In other words, you are seeing two snapshots of the clock - one with the eye in the first position, the second, with the eye in a new position; if the clock or screen flicks on whilst this is happening the image will appear to jump, which is what you are seeing. Your own reflection, on the other hand, remains static because you have remained constantly visible during the eye movement.
Chris
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It is because, your eyes have got used to the movement, and when you get still to watch the image, your eyes can no more focus.
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Have you tried using a Mouth harp/Jews harp and doing this?
You get the same effect but the speed of the vibrations changes with the tone of the instrument. It is also possible using the same instrument to get the vibrations to seem to stop and hover on the screen and even change direction.
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This is the first article I have found that mentions any connection between the inner ear receptors and use of an electric toothbrush. I have been using an electric toothbrush for about 6 years now, and I also have been experiencing a ringing in my ears - sometimes intermittent, sometimes constant - for almost that long. It only just occurred to me that perhaps these two things are related; when I first started using the brush, the way it was vibrating my entire skull gave me a moment of pause - I felt uneasy about how it was shaking my entire skull. But, of course, I got used to it eventually. However, if possible can you please elaborate a little more on how the vibrations from using an electric toothbrush affect the ear? I would like to know if it is at all possible that constant exposure to the vibration is what's causing my ear to constantly ring. Thanks!