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- Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called entoptical.
(Occasionally, these are called entopic phenomena, which is probably a typographical mistake; see entopic.)
They are different from optical illusions, which are perceptual effects occurring within the brain. Most entoptical phenomena have a direct, known physical cause. However, like optical illusions or hallucinations, the observer of an entoptical effect cannot give others a direct view of what he or she observes. Helmholtz comments on phenomena which could be seen easily by some observers, but could not be seen at all by others. Thus, entopical phenomena are experienced subjectively. 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.
Some examples of entoptical effects include the following:
- 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 Vascular figure 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.
- ^ 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 0805805087 
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