What causes the bright patterns when you rub your closed eyes?

03 October 2010

Question

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.

Answer

Chris - What you're doing when you're squeezing on your eye ball, it's triggering what's called an 'entoptic' phenomenon. In other words, it's a visual hallucination originating from inside your own eyeball.

When you apply pressure to the eyeball, 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 photoreceptors, which are the specialised rod and cone cells that pick up photons of light, change their pattern of firing activity in response, which is how we see.

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, which you see as funny "lights".

You might have also seen this if you stand up quickly out of 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. That's 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 these blazes of colours.

So, in summary, it's 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.

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