Bright halos in your head
January 2, 2006 12:42 AM   Subscribe

At nighttime, close your eyes, then flick them around in your head. Why do you see flashes of bright white halos?

It's easier to see at night but you can do it during the day too. It's easier with your eyes shut, but I found you get a similar, though diminished, effect by keeping your eyes open in the pitch black and flicking them around. What are these bright halos? I'm assuming they're an effect of the brain, due to the lack of light. Is it some sort of brain reset of the visual system or something? Are they connected to how we have dreams during REM?
posted by wackybrit to Health & Fitness (10 answers total)
You mean eye floaters?
posted by helios at 1:04 AM on January 2, 2006

People have a tendency to call a whole range of visual phenomena "floaters." I don't have this, but it's clearly not those.
posted by abcde at 1:29 AM on January 2, 2006

Response by poster: I can only see floaters during the day when in bright light, but the effect is not similar.
posted by wackybrit at 1:37 AM on January 2, 2006

It's your optic nerve. Seriously. Or more precisely, the rods and cones around them.

Any kind of stimulation will set off your eyeball's sensors, including pressure. This is why you see bizarre patterns when you rub your eyes, for example. This is merely another manifestation. When you move your eyes quickly, the optic nerve tugs on your retina and sets off some fireworks in the adjacent cells. (Remember the nerve comes through your eyeball and connects to the front of the retina. This is what causes your blind spot.)

I have noticed this happens more when I'm very tired than otherwise.

If it is extremely bright or you see other kinds of flashes, you may be developing a detached retina. Extremely nearsighted people are particularly at risk of this, I believe. You should be seeing an opthamologist on a regular basis to detect this possible condition as early as possible.
posted by kindall at 3:00 AM on January 2, 2006

I remember a great exhibit at the Exploratorium in San Francisco that explained this (or something similar). You might check out their website, particularly the online exhibitions section. The museum has tons of cool exhibts, but I'm not sure if they're all online.

(If you have the means, I highly recomend you visit the museum sometime also)
posted by bargex at 3:02 AM on January 2, 2006

With eyelids closed at night, I've noticed a waxing and waning effect. A blurry elliptical (or somesuch) whiteish pattern develops radially, only to then be replaced by a developing black pattern. And so on. I assumed that this is the visual conscious output of pattern generators* in the cortex.

*there's always some activity going on in the brain. Even in the absence of external input, there's oscillatory baseline activity occuring, hence the term pattern generators.
posted by Gyan at 3:47 AM on January 2, 2006

From my Psychology: 7th Edition (Myers, p.204) text:
In human eyes, the information from the retina's nearly 130 million receptor rods and cones is received and transmitted by the million or so ganglion cells, whose fibers make up the optic nerve. ... The same sensitivity that enables retinal cells to fire messages can lead them to misfire as well. Turn your eyes to the left, close them, and then gently rub the right side of your right eyelid with your fingertip. Note the patch of light to the left, moving as your finger moves. Why do you see light? Why at the left?

Your retinal cells are so responsive that even pressure triggers them. But your brain interprets their firing as light. Moreover, it interprets the lighta s coming from the left—the direction light normall comes from when it activates the right side of the retina.
Cool, huh?
posted by disillusioned at 4:04 AM on January 2, 2006

It looks like people have already taken care of the "why" of the matter, but the "luminous impression that occurs when the retina undergoes nonluminous stimulation (as by pressure on the eyeball when the lid is closed)" is a phosphene.
posted by catdog at 6:32 AM on January 2, 2006

An easier way to do this, is to close your eyes at any time of day, very gently and barely press with your index fingers onto the outer corner of your eyeball (on the lid's surface), and look to the far left or right for a bright-like circle in either direction.
posted by vanoakenfold at 9:41 AM on January 2, 2006

Trying to 'control' these can be a good way to get to sleep. I concentrate on trying to see them as an organized spiral, moving inwards ... it actually takes the kind of concentrated relaxation which is a good precursor to sleep.

In general, I believe visual effects generated inside the optic nerve or even brain are known as "entoptic" phenomena and have been discussed before on mefi.
posted by Rumple at 11:56 AM on January 2, 2006

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