Sometimes, when you shut your eyes tightly or are in a pitch black room, your eyes decide it’s time for a light show. Random, psychedelic-like patterns and flashes of color will dance around your field of vision like it’s 1969. While you can appreciate the entertainment, in the back of your mind you wonder what is causing the phenomena.
Are these flashes and patterns of light and colors indicative of a health problem? Are they something that can be treated or cured? Or are they completely normal? Today we’re going to crash the party in your eyes and find out what’s going on.
Light Need Not Apply
One common misconception about a human’s vision is that our eyes require light (photons) to be stimulated. Even in the absence of photons, the neurons in our visual system are always active. The thalamus (part of the brain responsible for relaying sensory signals), your visual cortex, and your retina are in an “always on” state – even while you sleep. The neurons in these visual systems are continuously relaying signals and information.
This continuous activity is sometimes referred to as background activity. It’s this background activity that is responsible for your trippy visuals and interesting patterns in the absence of light. The technical term for this phenomena is a “Phosphene“.
When you are in a completely dark room or just closing your eyes to go to sleep, the neurons in your visual system will spontaneously fire, and in doing so, will activate other visual neurons and so on. These neurons acting together collectively are what is causing those interesting and strange visual effects. The colors, duration, frequency, duration and type of effects seen will vary depending on which part of the visual system the neurons are acting up in.
“Seeing stars” is another common phosphene or visual effect that can occur, and is most common immediately after blowing your nose, intense laughter, a sneeze, a heavy cough, a blow to the head, or standing up too quickly. However, these phosphenes are usually the result of mechanical or metabolic stimulation, as opposed to spontaneous activity.
Low blood pressure, low oxygenation or a lack of glucose are most common causes of metabolic stimulation. Certain drugs can also produce metabolic stimulation of the visual system to varying degrees, the most potent and well known among them being LSD and psilocybin.
Artificially Induced Phosphenes
A common demonstration by professors in college is to have their students gently press on their eyeball in the dark. When done correctly, a football shaped white patch will be seen just off center of your field of vision. What you’re seeing is a bundle of axons attached to your fovea becoming active.
Those particular axons are a little thicker than your retina so the pressure from your finger pushes them onto the rods and cones at the back of your eye. This creates a phosphene. While it’s an interesting demonstration, we do not recommend attempting this without the assistance of a medical professional.
“Phosphenes: The Evidence“. Suzanne Carr. 1995
Bokkon, I. (2008). Phosphene phenomenon: A new concept. BioSystems, 92, 168–174.
“The sensations produced by electrical stimulation of the visual cortex“. The Journal of Physiology 196.