phosphene

   Also known as unstructured photopsia. The term phosphene comes from the Greek words phos (light) and phainein (to shine). It is used to denote a transient flash or spark of light, commonly referred to as 'seeing stars'. Phosphenes are * visual hallucinations of a * simple or * unformed nature. They are classified either as a type of *photopsia or as a type of *positive spontaneous visual phenomenon (PSVP). Phosphenes have been described in the medical literature since at least the fifth century BC. Ancient texts on phosphenes tend to focus on the alleged capacity of the eye to generate light for the purpose of vision. During the 19th century, the Bohemian physiologist Johannes Evangelista Purkyne (1787-1869) was among the first to make a significant contribution to the modern conception of phosphenes. Today the terms * deformation phosphene and * pressure phosphene are used interchangeably for a type of phosphene that can be provoked under physiological conditions through pressure onto the eyeball. Such types of phosphene may take the shape of a darkening of the visual field, of diffuse colour patches, of changing, scintillating, and deforming light-grids with occasional dark spots, and of fields with sparse, but intense blue points of light. Brief, recurrent, vertical phosphenes occurring in the temporal field of one eye are referred to as * Moore's lightning streaks. When phosphenes can be provoked by horizontal movement of the eyes, they are called * movement phosphenes. When they follow rapid eye movements (* REMs), especially in a dark environment, they are called * flick phosphenes. When they occur in reaction to a sudden sound, they are called * sound phosphenes. Phosphenes occurring in reaction to convergence of the eyes are designated as * convergence phosphenes. Phosphenes presenting as * formed hallucinations are sometimes referred to as * imagistic phosphenes. Pathophysiologically, phosphenes are associated with the random firing of neurons within any part of the *visual system, i.e. ranging from the retina to the lateral geniculate and stri-ate cortex. They can be provoked artificially by means of mechanical, electrical, and magnetic stimulation. Pathological causes of phosphenes include retinal detachment, optic neuropathy, and occipital haemorrhage. Movement phosphenes are associated primarily with optic neuritis. When phosphenes are attributed to intraocular mechanisms, they are classified as *entoptic phenomena.

   References

   Grüsser, O.-J., Hagner, M. (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, 57-85.

   Purkinje, J. (1819). Beiträge zur Kenntniss des Sehens in Subjectiver Hinsicht.Prague: Calve.

   Purkinje, J.E. (1819). Beobachtungen und Versuche zur Physiolgie der Sinne: Beiträge zur Kennt-niss des Sehens in subjectiver Hinsicht. Prague: Calve.