Phosphene

A phosphene is the phenomenon of seeing light without light 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 may be associated with optic neuritis.^[1][2]

Phosphenes can be induced by mechanical, electrical, or magnetic stimulation of the

The most common phosphenes are pressure phosphenes, caused by rubbing or applying pressure on or near the closed eyes. They have been known since antiquity, and described by the Greeks.^[5] 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, well defined shapes such as bright circles that exist near or opposite to where pressure is being applied,^[6] 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.^[7][8][9]

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.^{[citation needed]}

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.

The name "phosphene" was coined by J. B. H. Savigny, better known as the ship's surgeon of the wrecked French frigate Méduse.^[10] It was first employed by Serre d'Uzes to test retinal function prior to cataract surgery.^[11]

Electrical stimulation

Phosphenes have been created by electrical stimulation of the brain, reported by

In recent years, researchers have successfully developed experimental brain–computer interfaces or neuroprostheses that stimulate phosphenes to restore vision to people blinded through accidents. Notable successes include the human experiments by William H. Dobelle^[13] and Mark Humayun and animal research by Dick Normann.

A

Phosphenes can be caused by some medications, such as Ivabradine.^[22]

Mechanism

Most vision researchers believe that phosphenes result from the normal activity of the visual system after stimulation of one of its parts from some stimulus other than light. For example, Grüsser et al. showed that pressure on the eye results in activation of retinal ganglion cells in a similar way to activation by light.^[23] An ancient, discredited theory is that light is generated in the eye.^[5] A version of this theory has been revived, except, according to its author, that "phosphene lights are [supposed to be] due to the intrinsic perception of induced or spontaneous increased biophoton emission of cells in various parts of the visual system (from retina to cortex)"^[24]

Anthropological research

In 1988,

• Research has looked into visual prosthesis for the blind, which involves use of arrays of electrodes implanted in the skull over the occipital lobe to produce phosphenes. There have been long term implants of this type. Risks, such as infections and seizures, have been an impediment to their development.^[26]
• A possible use of phosphenes as part of a brain to brain communication system has been reported. The system called BrainNet, produces phosphenes using
  EEG produced a unique signal, which was transmitted to the TMS helmet of the receiver, who perceived phosphenes which differed for the 15 Hz and 17 Hz signal, and rotated the block in the relevant direction. The experiment achieved 81% success.^[27]

References