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*Correction: This episode was written by Jon Parnell*

What's the deal with all those little flashes of light you see when you close your eyes? And why do truck drivers and people in jail see glowing circles and spirals?

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So, you know when cartoon characters bump their head and then there’s, like, stars start whirling around them?

Well, there’s actually some truth to that: if you bump your head hard enough, you might actually see little flickering lights in your eyes. These flickers are an example of what’s known as an entoptic phenomenon, where your brain misinterprets signals so you see things that aren’t really there.

It’s not quite the same as a full-blown hallucination, but it is messing with your perception of reality. Back in the 1660s, Isaac Newton experimented with this by STICKING A GIANT METAL PIN INTO HIS EYE SOCKET TO PRESS ON THE BACK OF HIS EYE. Why, Isaac Newton?

You’re supposed to be a smart guy! The good news is, you don’t need to stick things in your eye to experience entoptic phenomena. And also you shouldn’t.

You can see similar flickers by rubbing your eyes, or even staring at something boring for too long. And all because of some quirks in your eyes and brain. For example, no matter how dark it is in a room, you’ll never see it as 100% completely, totally black.

That’s because the rod cells in your retina, which detect light, are really sensitive. In fact, the rods have been known to respond to just a single photon of visible light. They work using a protein called rhodopsin.

There’s lots of rhodopsin in each rod cell, and when light hits rhodopsin, it changes form, in a process called isomerization. The change causes a special cell called a ganglion cell to send a signal to your brain, which processes the signal into an image. But other types of energy can cause rhodopsin to change form, too — like thermal energy.

So your eyes can respond to heat instead of light! It’s pretty rare for an individual rhodopsin protein to react this way, but there’s so much of the stuff in a single rod cell that the average cell will mistakenly send a electrical signal to the brain once every couple of minutes. And because there are over a hundred million rods in your retina, that translates to a lot of random signals coming in pretty much all the time.

So even if it is totally dark, without a single photon of visible light in the room, thermal energy will activate your rod cells anyway, and you will not see total darkness. Instead, you’ll see a very dark shade of grey, called eigengrau. You can try this for yourself by going into a totally pitch-black room and closing your eyes.

If there are no photons entering your eye, you’ll see eigengrau. But you can’t blame your rods for all entoptic phenomena. Sometimes, the ganglion cells fire a signal even when they’re not excited by the rod cells.

It turns out that pressure can also excite ganglion cells. So if you carefully rub your eyes and apply a little bit of pressure, you’ll cause ganglion cells to fire signals as if there’s light entering your eyes. Your brain interprets those signals as little flashes of lights called phosphenes.

Specifically, they’re pressure phosphenes, because they’re caused by a change in pressure in your eye. But there are other ways to see phosphenes — like if you’ve ever had your vision go wonky when you stood up too fast. The retina has an especially high metabolic rate, which means that it needs tons of glucose and oxygen for its size.

When you stand up too fast, there’s a sudden drop in blood pressure, and for a few seconds your retinas might not get enough blood to work properly. That causes your ganglion cells to misfire random signals, and you see those little dots and flashes. And if you hit your head really hard and see stars?

That’s because you bumped it hard enough to mess with your occipital lobe: the part of your brain that does visual processing. Some people experience entoptic phenomena more than others. For prisoners and truck drivers, for example, phosphenes can get pretty severe because of the lack of visual stimulation.

In some cases, they’ll see phosphenes as more than just a flicker: they’ll see glowing circles or spinning spirals, in what’s known as a prisoner’s cinema, because it’s kind of like you’re seeing a full cinema show. Prisoner’s cinema is probably caused by a combination of phosphenes and sensory deprivation. Even though a prisoner’s or truck driver’s eyes are open, they’re staring at what’s basically an unchanging scene, so their brain isn’t getting much new information.

And when you’re visually deprived, your brain amplifies the signals coming from your ganglion cells to make up for the lack of stimulation, and you end up seeing those circles or spirals. So they might not be full-on hallucinations, but there are lots of ways that your eyes and brain play tricks on you. And you do not need to stick horrifically large things into your eye socket to experience them!

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