A phosphene is the phenomenon 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 may be associated with optic neuritis.
Phosphenes can be directly induced by mechanical, electrical, or magnetic stimulation of the retina or visual cortex, or by random firing of cells in the visual system. Phosphenes have also been reported by meditators (called nimitta), persons who endure long periods without visual stimulation (the prisoner's cinema), or those who ingest psychedelic drugs.
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. 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, 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.
The name "phosphene" was coined by J. B. H. Savigny, better known as the ship's surgeon of the wrecked French frigate Méduse. It was first employed by Serre d'Uzes to test retinal function prior to cataract surgery.
Phosphenes have been created by electrical stimulation of the brain, reported by neurologist Otfrid Foerster as early as 1929. Brindley and Lewin (1968) inserted a matrix of stimulating electrodes directly into the visual cortex of a 52-year-old blind female, using small pulses of electricity to create phosphenes. These phosphenes were points, spots, and bars of colorless or colored light. Brindley and Rushton (1974) 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 neuroprostheses 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.
Phosphenes have been produced by intense, changing magnetic fields, such as with transcranial magnetic stimulation (TMS). These fields can be positioned on different parts of the head to stimulate cells in different parts of the visual system. They also can be induced by alternating currents that entrain neural oscillation as with transcranial alternating current stimulation. In this case they appear in the peripheral visual field. This claim has been disputed. The alternative hypothesis is that current spread from the occipital electrode evokes phosphenes in the retina. Phosphenes created by magnetic fields are known as magnetophosphenes.
Astronauts exposed to radiation in space have reported seeing phosphenes. Patients undergoing radiotherapy have reported seeing blue flashes of light during treatment; the underlying phenomenon has been shown to resemble Cherenkov radiation. 
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. An ancient, discredited theory is that light is generated in the eye. 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)"
In 1988, David Lewis-Williams and T. A. Dowson published an article about phosphenes and other entoptic phenomena. They argued that non-figurative art of the Upper Paleolithic depicts visions of phosphenes and neurological "form constants", probably enhanced by hallucinogenic drugs.
- 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.
- A possible use of phosphenes as part of a brain to brain communication system has been reported. The system called BrainNet, produces phosphenes using transcranial magnetic stimulation (TMS). The goal of the research is to connect thoughts brain to brain using a system where signals are sent using electroencephalography (EEG) and received using transcranial magnetic stimulation (TMS). An experiment was conducted with 5 different groups, each contained three people. The subjects were split into two groups. Two subjects functioned as the senders, and were connected to EEG electrodes, and a third person functioned as the receiver, who wore the TMS helmet. Each person was stationed in front of a television screen with a Tetris-style game. The senders had to determine if there was a need to rotate the falling blocks, but without the ability to rotate them – only the receiver was able to perform this operation. At the edges of each screen, were two icons with two flashing lights in two different frequencies, (one at 15 Hz and the other at 17 Hz). The sender focused on one icon, or the other to signal that the block should be rotated to the right or the left. The 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. These results could eventually lead to mind-to-mind communication technology.
- "Phosphenes: The Evidence". Suzanne Carr. 1995. Retrieved 2008-03-25.
- Davis F. A., Bergen D., Schauf C., McDonald I., Deutsch W.; Bergen; Schauf; McDonald; Deutsch (November 1976). "Movement phosphenes in optic neuritis: a new clinical sign". Neurology. 26 (11): 1100–4. doi:10.1212/wnl.26.11.1100. PMID 988518.CS1 maint: multiple names: authors list (link)
- Page N. G., Bolger J. P., Sanders M. D.; Bolger; Sanders (January 1982). "Auditory evoked phosphenes in optic nerve disease". J. Neurol. Neurosurg. Psychiatry. 45 (1): 7–12. doi:10.1136/jnnp.45.1.7. PMC 491258. PMID 7062073.CS1 maint: multiple names: authors list (link)
- Philip T. Nicholson (March 27, 2002). "The Soma Code, Part III: Visions, Myths, and Drugs". Electronic Journal of Vedic Studies. 8 (3c). ISSN 1084-7561. Retrieved March 1, 2013.
- Kluver, H. 1966 Mescal and mechanisms and hallucinations University of Chicago Press. p. 70
- Grüsser, Otto-Joachim; Hagner, Michael (February 1990). "On the history of deformation phosphenes and the idea of internal light generated in the eye for the purpose of vision". Documenta Ophthalmologica. 74 (1–2): 57–85. doi:10.1007/bf00165665. PMID 2209368.
- March 11, TK Sellman ·; 2018 (2018-03-11). "Phosphenes: Your Own Personal Aurora Borealis | MS". MultipleSclerosis.net. Retrieved 2020-02-02.CS1 maint: numeric names: authors list (link)
- Newton, Isaac. "Laboratory Notebook". cudl.lib.cam.ac.uk. Cambridge Digital Library. Retrieved 9 October 2014.
- McGuire J. E., Tammy M. Certain philosophical Questions. Cambridge Univ. Press, 2002. 386.
- "From the library". British Journal of Ophthalmology. 87 (10): 1308. 2003. doi:10.1136/bjo.87.10.1308.
- Savigny, J. B. H. (1838). "Phosphenes ou sensations loumineuses". Arch. Gen. Med. Third Series. 2: 495–97.
- Serre d'Uzes P. M. Essai sur les phosphenes ou anneaux de la retine. Paris, Mason, 1853.
- G. S. Brindley; W. S. Lewin (May 1, 1968). "The sensations produced by electrical stimulation of the visual cortex". The Journal of Physiology. 196 (2): 479–493. doi:10.1113/jphysiol.1968.sp008519. PMC 1351724. PMID 4871047.
- Cowey, Alan; Walsh, Vincent (2001). "Chapter 26: Tickling the brain: studying visual sensation, perception and cognition by transcranial magnetic stimulation". In Casanova, Christian; Ptito, Maurice (eds.). Vision: From Neurons to Cognition, Volume 1. Gulf Professional Publishing. pp. 411–25. ISBN 9780444505866.
- E. J. Tehovnik; W. M. Slocum; C. E. Carvey; P. H. Schiller (2005). "Phosphene Induction and the Generation of Saccadic Eye Movements by Striate Cortex". Journal of Neurophysiology. 93 (1): 1–19. CiteSeerX 10.1.1.326.9609. doi:10.1152/jn.00736.2004. PMID 15371496.
- Kanai R.; Chaieb L.; Antal A.; Walsh V.; Paulus W. (2008). "Frequency-dependent electrical stimulation of the visual cortex". Curr. Biol. 18 (23): 1839–43. doi:10.1016/j.cub.2008.10.027. PMID 19026538.
- Kar K, Krekelberg B (2012). "Transcranial electrical stimulation over visual cortex evokes phosphenes with a retinal origin". J Neurophysiol. 108 (8): 2173–8. doi:10.1152/jn.00505.2012. PMC 3545027. PMID 22855777.
- Schwiedrzik C. M. (2009). "Retina or visual cortex? The site of phosphene induction by transcranial alternating current stimulation". Front Integr Neurosci. 3: 6. doi:10.3389/neuro.07.006.2009. PMC 2691656. PMID 19506706.
- Schutter D. J.; Hortensius R. (2010). "Retinal origin of phosphenes to transcranial alternating current stimulation". Clin Neurophysiol. 121 (7): 1080–1084. doi:10.1016/j.clinph.2009.10.038. PMID 20188625.
- Fuglesang, Christer; Narici, Livio; Picozza, Piergiorgio; Sannita, Walter G. (April 2006). "Phosphenes in Low Earth Orbit: Survey Responses from 59 Astronauts". Aviation, Space, and Environmental Medicine. 77 (4): 449–452. ISSN 0095-6562. PMID 16676658.
- Tendler, Irwin I.; Hartford, Alan; Jermyn, Michael; LaRochelle, Ethan; Cao, Xu; Borza, Victor; Alexander, Daniel; Bruza, Petr; Hoopes, Jack; Moodie, Karen; Marr, Brian P.; Williams, Benjamin B.; Pogue, Brian W.; Gladstone, David J.; Jarvis, Lesley A. (2020). "Experimentally Observed Cherenkov Light Generation in the Eye During Radiation Therapy". International Journal of Radiation Oncology*Biology*Physics. 106 (2): 422–429. doi:10.1016/j.ijrobp.2019.10.031. ISSN 0360-3016. PMC 7161418. PMID 31669563.
- Tardif JC, Ford I, Tendera M, Bourassa MG, Fox K (2005). "Efficacy of ivabradine, a new selective I(f) inhibitor, compared with atenolol in patients with chronic stable angina". Eur. Heart J. 26 (23): 2529–36. doi:10.1093/eurheartj/ehi586. PMID 16214830.
- Grüsser O. J.; Grüsser-Cornehls U.; Hagner M.; Przybyszewski A. W. (1989). "Purkyne's description of pressure phosphenes and modern neurophysiological studies on the generation of phosphenes by eyeball deformation". Physiologia Bohemoslovaca. 38 (289–309): 1059–1068.
- Bókkon I (2008). "Phosphene phenomenon: A new concept". BioSystems. 92 (2): 168–174 . CiteSeerX 10.1.1.377.2281. doi:10.1016/j.biosystems.2008.02.002. PMID 18358594.
- Lewis-Williams JD (1988). "The signs of all times: entoptic phenomena in Upper Palaeolithic art". Current Anthropology. 29 (2): 201–45. doi:10.1086/203629. JSTOR 2743395.
- Lewis, P. & Rosenfield, J. (2016). "Electrical stimulation of the brain and the development of cortical visual prostheses: An historical perspective". Brain Research. 1630: 208–224. doi:10.1016/j.brainres.2015.08.038. PMID 26348986.CS1 maint: multiple names: authors list (link)
- Rao, Rajesh P. N.; Prat, Chantel S.; Abernethy, Justin A.; Losey, Darby M.; Stocco, Andrea; Jiang, Linxing (2019-03-21). "BrainNet: A Multi-Person Brain-to-Brain Interface for Direct Collaboration Between Brains". bioRxiv: 425066. arXiv:1809.08632. doi:10.1101/425066.
- Entoptic Phenomena: Review of J. D. Lewis-Williams and T. A. Dowson's research
- Altered States: Review of J. D. Lewis-Williams and T. A. Dowson's research
- Biophysical pictures within the brain