MadSci Network: Neuroscience

Re: What is the light on my eyelid when my eyes are closed?

Date: Wed Mar 3 14:28:01 1999
Posted By: Tom Stickel, Grad student, Optometry, Indiana University School of Optometry
Area of science: Neuroscience
ID: 920133799.Ns

Hey Samantha,
	Your question is a good question.  It's going to be one of those two-
part answers. The first part will deal with why you see the lights, and the 
second part has to do with why it looks like the lights are on the other 
side of your eye.  Here we go.
	Part 1 goes like this.  The reason you see them is because light
stimulates cells called photoreceptors in the back of your eye.  The 
photoreceptors send messages that go to nerve cells in the back of the eye 
called ganglion cells.  If a photoreceptor receives enough light, then it 
will send a signal to the ganglion cell that causes the ganglion cell to, in 
turn, send a signal to the brain.  And that's the very simplified version 
of how you see.
	Now, there are other things than light that can stimulate a ganglion 
cell to send a message to your brain.  For instance, if we stick a very 
small electrode in a ganglion cell in your eye (Ouch!) and fire little 
bursts of electricity, we can get your ganglion cell to fire. We could also 
get it to fire a message to the brain just by mechanically rubbing it.  
These other ways of getting ganglion cells to say "Hey!  I see something!" 
when they really don't are called phosphenes.  To get technical, a phosphene 
is defined as 

	"A subjective, luminous sensation due to stimulation of the retina 
	(back of the eye) by stimuli other than light."

And that is exactly what you do when you rub your eye.  You're making 
ganglion cells go off.  The trick is, your brain has no idea what's making 
the ganglion cell fire.  It just knows that the ganglion cell is firing.  
And your brain is wired so that when the ganglion cell goes off, 
you think you see light.
	OK, that was part 1.  Part 2 is a little harder to visualize.  You 
don't know it, but everything that you see is actually upside down.  Strange 
but true.  The way the optics of your eyes are set up, everything that you 
see above you is focused on the back of your eye at the bottom.  If we look 
at your right eye, things that are to your right are focused on the side of 
your eye by your nose.  So basically when the image of the world
hits the back of your eye, it's upside down and backwards.  How your brain 
straightens that out so you can make it around the world without falling 
over is a whole different story.
	Anyway, you can see that if you press on the top of your eye, you 
stimulate ganglion cells up there.  And those cells usually get stimulated 
when light from something down below hits them.  Sooo, your brain tells you 
that the light is coming from down below.  The same goes for when you press 
the right side of your eye and you see the circle on your left.  Funky, huh?
	Hope that helps answer your question!  Oh yeah, I'd like to thank Dr. 
Bob DeVoe for "showing me the light" on a few of the finer points of this 

	Tom Stickel

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