|MadSci Network: Neuroscience|
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 question. Sincerely, Tom Stickel
Try the links in the MadSci Library for more information on Neuroscience.