A phosphene is a phenomenon characterized by the experience of seeing light without light actually entering the eye. The word phosphene comes from the Greek words phos (light) and phainein (to show). Phosphenes that are induced by movement or sound are often associated with optic neuritis. Phosphenes can be directly induced by mechanical, electrical, or magnetic stimulation of the retina or visual cortex as well as by random firing of cells in the visual system. Phosphenes have also been reported by meditators (commonly called nimitta), people who go for long periods without visual stimulation (also known as the prisoner's cinema), or those who are using psychedelic drugs.

 

The most common phosphenes are pressure phosphenes, caused by rubbing the closed eyes. They have been known since antiquity, and described by the Greeks.The pressure mechanically stimulates the cells of the retina. Experiences include a darkening of the visual field that moves against the rubbing, a diffuse colored patch that also moves against the rubbing, a scintillating and ever-changing and deforming light grid with occasional dark spots (like a crumpling fly-spotted flyscreen), and a sparse field of intense blue points of light. Pressure phosphenes can persist briefly after the rubbing stops and the eyes are opened, allowing the phosphenes to be seen on the visual scene. Hermann von Helmholtz and others have published drawings of their pressure phosphenes. One example of a pressure phosphene is demonstrated by gently pressing the side of one's eye and observing a colored ring of light on the opposite side, as detailed by Isaac Newton.
Another common phosphene is "seeing stars", from a sneeze, laughter, a heavy and deep cough, blowing of the nose, a blow on the head or low blood pressure (such as on standing up too quickly or prior to fainting). It is possible these involve some mechanical stimulation of the retina, but they may also involve mechanical and metabolic (such as from low oxygenation or lack of glucose) stimulation of neurons of the visual cortex or of other parts of the visual system.
Less commonly, phosphenes can also be caused by some diseases of the retina and nerves, such as multiple sclerosis. The British National Formulary lists phosphenes as an occasional side effect of at least one anti-anginal medication^{[citation needed]} (available only by medical prescription in the United Kingdom). In some cases, phosphenes can be produced by movement of the retina inside the eye, for example in cases of retinal detachment.