Phosphenes: the stars you see when you see stars

Have you ever seen “stars” after knocking your head, or even if you stood up too quickly? Those tiny points of light or other apparently visual sensations are known as “phosphenes”. Scientists are seeking to understand as much as they can about them, not least because such knowledge could help give a form of vision to blind people.

 Here are a few facts about phosphenes:

·  A dictionary definition of a phosphene is: “A sensation of seeing light caused by pressure or electrical stimulation of the eye”

·  The word comes from the Greek for “light” and “to show”. It can therefore be expressed as “light that shines forth”

·  Even people who have been blindfrom birth can see them

·  Phosphenes are generated by the visual cortex (at the rear of the brain) independently of stimulation from the optic nerve

·  Phosphenes can be experienced by rubbing your closed eyes, but can also be seen with the eyes open in a darkened room

· Other causes of phosphenes include electrical stimulation, strobe lights, substances such as LSD, and migraine headaches

·  Phosphenes can be generated most readily when electrical pulses are applied at the same rate as brain waves (between 5 and 40 cycles per second)

·  Many different shapes and colours of phosphenes have been recorded, including impressions of spiders’ webs, bolts of lightning and geometric shapes

·  Visions and hallucinations, as recorded from earliest times, may have phosphenes as likely explanations

·  If phosphenes are regularly seen during eye movements, this can be a symptom of a detached retina

·  Phosphenes may well have influenced the work of artists such as Miro, Dali and Kandinsky

·  Children are able to see phosphenes more readily than adults; this ability tends to decline after adolescence

·  It is believed that phosphenes are experienced by animals other than humans; experiments with primates shows this almost certainly to be the case

·  The first detailed scientific account of phosphenes was published in 1819 by Johannes Purkinje, a physiologist from Bohemia

·  Research into phosphenes is looking at the links between visual stimulation and brain waves; there is a chance that linking a camera to the visual cortex could provide a means of allowing blind people to see

(An artistic depiction of phosphenes, by “A12”; licensed for general use)

The experience of phosphenes is widespread, can have many causes, and can take many forms. By studying the phenomenon in depth scientists hope to learn more about how the brain works and to apply this knowledge to produce practical benefits.