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Doug in Vermont asked:
If I am in a dark room and I rub my closed eyes, why does it appear that I see bright lights or vivid color patterns?
I have been enjoying your podcasts for a couple of years.† My commute to work is always educational (and fun) with the Naked Scientists on board.† Thanks for all of the time you put into this and keep up the good work.
Chris - What you're doing when you're squeezing on your eye ball, itís triggering whatís called Ďentopticí phenomenon. In other words, itís a visual hallucination originating from inside your own eye ball. When you apply pressure to the eye ball, what you're doing is pressing on the retina and the retina is the extremely complicated, cell-rich, very highly metabolically active structure that turns light waves into brain waves, to put it simply.
When you apply pressure to the retina, two things happen. One, you deform the retina a little bit and this makes the photo receptors, which are the specialised cells that pick up photons of light and change their pattern of firing activity in response to them which is how we see, it makes those cells change their activity. The other thing that pressing on the retina would do is it may affect its ability to pick up oxygen from the blood because the photoreceptors are right at the back of the eye, close to something called the choroid plexus. The choroid plexus is a very dense network of blood vessels. In fact, the retina has one of the highest metabolic rates of any tissue in the body and if you affect the way at which oxygen moves out of the choroid plexus and into the photoreceptors for even a fraction of a second, they start to deliver abnormal firing activity, and you start to see these funny lights.
You might have seen this if you stand up quickly out of the bath, when you've had a hot bath. You may have noticed a similar strange wooziness, but also, you'll have noticed some perhaps funny lights in front of your eyes. Thatís because your blood pressure temporarily dips when you stand up and you deprive the photoreceptors of their oxygen supply momentarily, and they respond by firing off all these funny blazes of colours. So, itís an entoptic phenomenon secondary to physical deformation of the retina, but also probably because you're affecting the ability of the retina to grab its oxygen supply.
Doug in Vermont asked the Naked Scientists:
You are mechanically stimulating your retina RD, Sat, 2nd Oct 2010
But why do you see those specific patterns, and why do you see patterns as opposed to something random? Yair, Mon, 30th Dec 2013
No, I know what he means. I often see a red and black checker board pattern, sometimes it moves in a spiral. I don't think it's related to the pattern of blood vessels, but patterns that result when you mechanically stimulate nerve cells in the retina. I couldn't find an exact explanation for the pattern, but maybe neurons in the brain that detect edges, parallel and perpendicular lines, are easier to stimulate than neurons that detect other shapes. And since there is no actual sensory content when you close your eyes and press on them, that's the default image that the brain comes up with.