If you rub your closed eyes, you’ll “see” a virtual rainbow of colors, shapes, squiggles, and lines. Those are called phosphenes, and the eye and the brain work together to create these weird little visual blips.
Phosphenes occur when there is no external visual stimulus. That can happen when you close your eyes or when you’re focused on scenery with little to no input as to depth or changes, such as a dark highway at night.
People who spend long periods of time in sensory deprivation or meditation often report seeing visions, which can be chalked up to the appearance of phosphenes.
The presence of physical stimulus to the eye, like pushing on the eyeball, will create temporary phosphenes, and more traumatic events like head injuries can create permanent squiggles.
In these cases, phosphenes are present because the visual centers of the brain are active without the presence of external visual stimuli.
For example, when conscious patients undergoing brain surgery had different areas of their brains electrically stimulated, they reported seeing phosophenes.
In studies of blind people, it’s been found that the appearance of phosphenes happens in different areas along the sight pathway between the eye and the brain, depending on what part of the visual system has been damaged.
Humans aren’t the only ones who can see these dancing bits of light and color—the phenomenon has been observed in animals as well.