Entoptic phenomenon

"This is a medical definition of entoptic phenomena. For an alternative use within archaeology please see Entoptic phenomena (archaeology)"Entoptic phenomena are visual effects whose source is within the eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake; see entopic.)In Helmholtz's words::"Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called "entoptical."

Entoptic images have a physical basis in the image cast upon the retina. Hence, they are different from optical illusions, which are perceptual effects that arise from interpretations of the image by the brain. Because entoptic images are caused by phenomena within the observer's own eye, they share one feature with optical illusions and hallucinations: the observer cannot share a direct and specific view of the phenomenon with others.

Helmholtzref|helmholtz comments on phenomena which could be seen easily by some observers, but could not be seen at all by others. This variance is not surprising because the specific aspects of the eye that produce these images are unique to each individual. Because of the variation between individuals, and the inability for two observers to share a nearly identical stimulus, these phenomena are unlike most visual sensations. They are also unlike most optical illusions which are produced by viewing a common stimulus. Yet, there is enough commonality between the main entoptic phenomena (see below) that their physical origin is now well-understood.

During the 1920s, some theosophists, unaware of the physical explanation, maintained that the moving spots seen in the blue field entoptic phenomenon were "vitality globules" related to the concept of prana in yoga.ref|zusne

Some examples of entoptical effects include:

* Floaters or "muscae volitantes" are "slowly" drifting transparent blobs of varying size and shape, which are particularly noticeable when lying on the ground looking up at the sky. They are caused by imperfections in the fluid of the eye.
* The blue field entoptic phenomenon has the appearance of "tiny" bright dots moving "rapidly" along squiggly lines in the visual field. It is much more noticeable when viewed against a field of pure blue light and is caused by white blood cells moving in the capillaries in front of the retina.
* Haidinger's brush is a very subtle yellow-and-blue pattern that is seen when viewing a field of light that is polarized.
* The Purkinje tree is an image of the retinal blood vessels in one's own eye. It can be seen by shining a bright, moving light like a penlight onto the sclera (the white of the eye) in a darkened room. Normally the image of the retinal blood vessels is invisible because of adaptation. The unusual angle casts the image onto unadapted portions of the retina. Unless the light moves, the image disappears within a second or so. If the light is moved at about 1 Hz, adaptation is defeated, and a clear image can be seen indefinitely. The vascular figure is often seen by patients during an ophthalmic examination when the doctor is using an ophthalmoscope. In the process of aligning the instrument so that the doctor can view the blood vessels through the pupil, the light from the instrument often falls briefly on the sclera, so that the patient gets a quick glimpse of the vascular figure.
* A phosphene is the perception of light without light actually entering the eye, for instance caused by pressure applied to the closed eyes.

A phenomenon that could be entoptical if the eyelashes are considered to be part of the eye is seeing light diffracted through the eyelashes. The phenomenon appears as one or more light disks crossed by dark blurry lines (the shadows of the lashes) each having fringes of spectral colour. The disk shape is given by the circular aperture of the pupil.

Notes

* H. von Helmholtz, "Handbuch der Physiologischen Optik," published as "Helmholtz's Treatise on Physiological Optics, Translated from the Third German Edition," ed. James P. C. Southall; 1925; The Optical Society of America.
* Leonard Zusne, 1990: "Anomalistic Psychology: A Study of Magical Thinking"; Lea; ISBN 0-8058-0508-7 [http://www.amazon.com/gp/reader/0805805087/]

External links

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* [http://www.eye-floaters.info/floaters/entoptic_phenomenon-entoptische_erscheinungen.pdf Picture of the entoptic phenomenon: Vitreous Floaters] (PDF file, requires an Acrobat Reader or plugin)