Why does using an electric toothbrush alter my vision?

04 July 2010

Question

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?

Answer

We posed this question to Mikael Karlberg from Lund University Hospital in Sweden... 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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