Phosphene

A phosphene is an entoptic phenomenon characterized by the sensation of seeing light. Phosphenes are caused by mechanical, electrical, or magnetic stimulation of the retina or visual cortex as well as random firing of cells in the visual system.

Phosphenes in daily life
The most common phosphenes are pressure phosphenes, caused by rubbing the closed eyes. 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. Christopher Tyler (1978) has published drawings of pressure phosphenes. One example of a pressure phosphene is demonstrated by gently pressing the right side of your right eye or the left side of your left eye and observing a colored ring of light on the opposite side.

Another common phosphene is “seeing stars,” from a sneeze, 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 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.

The British National Formulary lists phosphenes as an occasional side effect of at least one anti-anginal medication (available only by medical prescription in the United Kingdom).

Phosphenes from radiation
Astronauts exposed to radiation in space report seeing phosphenes, and neutrons generated in particle accelerators also trigger phosphenes in the human visual system.

Phosphenes from magnetic stimulation
Phosphenes have also been created by intense, changing magnetic fields, such as with transcranial magnetic stimulation. These fields can be positioned on different parts of the head to stimulate cells in different parts of the visual system.

Restoring vision
Phosphenes have also been created by electricity. For example, Brindley and Rushton (1974) inserted a matrix of stimulating electrodes directly into the visual cortex of a 64-year-old blind man, using small pulses of electricity to create phosphenes. These phosphenes were points, spots, and bars of colorless or colored light. Brindley and Rushton used the phosphenes to create a visual prosthesis, in this case by using the phosphenes to depict Braille spots.

In recent years, researchers have successfully developed experimental brain-computer interfaces or neuroprosthesis that stimulate phosphenes to restore vision to people blinded through accidents. Notable successes include the human experiments by William H. Dobelle and Mark Humayun and animal research by Dick Normann.

Anthropological research
In 1988, J. D. Lewis-Williams and T. A. Dowson published an article about phosphenes and other entoptic phenomena. They argued, among other things, that non-figurative art of the Upper Paleolithic depicts actual visions of phosphenes and neurological “form constants,” probably enhanced by hallucinogenic drugs.

Phosphenes were used as a learning technique by French scientist and scholar, Francis Lefebure.