When in need of a quick psychedelic trip on a budget, everyone knows the fine art of squishing their eyes to see an in-head display of fuzzy colors, swirling visuals, and black and white checkerboards. Even the ancient Greeks wrote about it in some of the world’s earliest medical texts. But what’s the science behind this?

Scientists call the phenomenon phosphenes, essentially experiencing sensations light without light actually entering the eye. They come in a few different forms, but the most common experience is a pressure phosphene.

Within our eyeballs, there’s a type of neuron called the retinal ganglion cell whose job is to receive visual information from the light-sensitive photoreceptor cells in the retina, the lining inside the back of your eyeball. Usually, we see the world because the retinal ganglion cell receives information from photoreceptor cells that being stimulated by light entering the eye.

However, it’s also possible to activate the retinal cells through applying pressure. Gently pressing into your eye will apply pressure to the cells within the retina, “tricking” them into firing off in a similar way to activation by light. Totally unable to differentiate the stimulation, the central nervous system will perceive it in the same way it would seeing light. Even a sneeze or a cough is enough for some people to spark up a small phosphene.

Poking your eyeballs is not the only way to experience fuzzed-out fireworks – if you’ve ever had a migraine, you’ll know all too well. Scientists aren’t totally certain what causes the visual experiences that accompany a migraine, however, it could be due to a localized wave of electrical activity in the brain.

One study found that 47 out of 59 NASA and ESA astronauts also experience sudden phosphenes, mainly consisting of light flashes, when they are sent into low-Earth orbit. The researchers on this project came to believe that the phosphenes were actually being caused by radiation. Other studies have found it’s possible to induce phosphenes through direct electrical shocks of the brain’s visual cortex and through intense changes of magnetic fields.

There’s another related phenomenon known as prisoner's cinema well-documented among people confined to dark cells for prolonged periods. Faced with long periods of total sensory deprivation, people can see a "light show" of various colors that almost appear to be projected onto the pitch-dark walls around them.  

All of this is a great way to get you think about the senses, your perception, and how we interpret reality around us. Just don't poke your eyes too much or you'll do yourself a mischief.