Seeing stars when you're bonked on the head isn't just for cartoon characters. That phantom astrological event happens in real life, too. It's pretty weird.
One moment you Manny Pacquiao's handsome mug in front of you; the next his face is dappled with what appears to be the Milky Way. What is it about getting smacked on the dome gives us such strange reactions?
Naturally, it starts with your brain.
A little set up: The part of the brain that handles what you see is called the occipital lobe,; it's located at the back of your head. Its job is to take the information sent from the retina and turn it into something that makes sense to us. So before I know that the thing in front of me is actually my cat chewing on a power cord, my retina has to take the observable light, convert it first into a chemical signal and then into an electrical impulse, before sending back to my brain for processing. The occipital lobe will then say, yup, based on this information, that's your cat trying to off himself. You get the idea; your eyes and your brain work together to understand what's in front of you.
Your brain reads other types of stimulation, too. Robert Wade Crow, an assistant clinical professor of neuro-ophthalmology at UC Irvine explains, "If you irritate the brain, it may create a response like it's normally used to creating, which is, in this case, a visual response." Poking the occipital lobe can make it cry vision. The thing is, the response is not anchored to anything, so instead of seeing floating suicidal cats or baseballs speckling your vision when the occipital lobe is bumped, you just see light.
Now this doesn't happen all that often. When you hit your head on a counter while bending down to tie your shoes, for instance, your head stops suddenly, but your brain keeps moving. But because your brain is surrounded by cerebrospinal fluid, it's cushioned from running into your skull. This fluid is what makes head banging and football possible without immediate brain injury. But when you or the thing hitting you moves at a high velocity, the situation changes.
It takes a pretty serious bump for the fluid cushioning system to fail to protect our brains. That baseball that zoomed through your hands and slammed into your forehead forces your skull to snap back, which can cause it to smack against the front part of the brain. Then, when you fall and hit the back of your head against the ground, your occipital lobe continues downward and can hit your skull. Voila: stars.
But no worries, there are less dramatic ways to get this effect. When you rub your eyes in the morning, you can see stars, too. "You can activate the retina just like you would when it sees light," Crow explains. This is because the retina only really knows how to do one thing; it either sees light or it doesn't. So when you apply pressure, you can basically make it think that that switch has been flipped. The duped retina will then send your brain something like an email with the subject line "light!" with no text in the body. So that's what you get: flecks of uninformed light on top of the landscape that your brain can make sense of.
In any case, next time you see stars, let's hope they're the ones in the sky.
Rachel Swaby is a freelance writer living in San Francisco. Check her out on Twitter.