Entoptic Phenomenon
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Entoptic phenomena () are visual effects whose source is within the
human eye The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. ...
itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.) In
Helmholtz Hermann Ludwig Ferdinand von Helmholtz (31 August 1821 – 8 September 1894) was a German physicist and physician who made significant contributions in several scientific fields, particularly hydrodynamic stability. The Helmholtz Association, ...
's words: "Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called ''entoptical''."


Overview

Entoptic images have a physical basis in the image cast upon the retina. Hence, they are different from
optical illusions Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; thei ...
, which are caused by the
visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
and characterized by a visual
percept Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
that (loosely said) appears to differ from
reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that which is only imaginary. The term is also used to refer to the ontological status of things, indicating their existence. In physical terms, r ...
. 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. Helmholtz commented on entoptic 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 that their physical origin is now well understood.


Examples

Some examples of entoptical effects include: *
Floaters Floaters or eye floaters are sometimes visible deposits (e.g., the shadows of tiny structures of protein or other cell debris projected onto the retina) within the eye's vitreous humour ("the vitreous"), which is normally transparent, or between ...
or ''muscae volitantes'' are slowly drifting blobs of varying size, shape, and transparency, which are particularly noticeable when viewing a bright, featureless background (such as the sky) or a point source of diffuse light very close to the eye. They are shadow images of objects floating in liquid between the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
and the jelly inside the eye (the vitreous) or in the vitreous humor itself. They are visible because they move; if they were pinned to retina by the vitreous or fixed within the vitreous they would be as invisible as ordinary viewing of any stationary object, such as the retinal blood vessels (see Purkinje tree below). Some may be individual red blood cells swollen due to osmotic pressure. Others may be chains of red blood cells stuck together; diffraction patterns can be seen around these. Others may be "coagula of the proteins of the vitreous gel, to embryonic remnants, or the condensation round the walls of
Cloquet's canal Hyaloid canal (Cloquet's canal and Stilling's canal) is a small transparent canal running through the vitreous body from the optic nerve disc to the lens. It is formed by an invagination of the hyaloid membrane, which encloses the vitreous body. ...
" that exist in pockets of liquid within the vitreous. The first two sort of floaters may collect over the fovea (the center of vision), and therefore be more visible, when a person is lying on his or her back looking upwards. *
Blue field entoptic phenomenon The blue field entoptic phenomenon is an entoptic phenomenon characterized by the appearance of tiny bright dots (nicknamed blue-sky sprites) moving quickly along squiggly lines in the visual field, especially when looking into bright blue light ...
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 cell White blood cells, also called leukocytes or leucocytes, are the cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and de ...
s moving in the
capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
in front of the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
. White cells are larger than red blood cells and can be larger than the diameter of a capillary, so must deform to fit. As a large, deformed white blood cell goes through a capillary, a space opens up in front of it and red blood cells pile up behind. This makes the dots of light appear slightly elongated with dark tails. *
Haidinger's brush Haidinger's brush, more commonly known as Haidinger's brushes is an image produced by the eye, an entoptic phenomenon, first described by Austrian physicist Wilhelm Karl von Haidinger in 1844. Haidinger saw it when he looked through various miner ...
is a very subtle bowtie or hourglass shaped pattern that is seen when viewing a field with a component of blue light that is
plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * ''Planes' ...
or circularly polarized. It's easier to see if the polarisation is rotating with respect to the observer's eye, although some observers can see it in the natural polarisation of sky light.Minnaert, M. G. J. (1940). Light and colour in the open air (H. M. Kremer-Priest, Trans.). London: G. Bell and Sons. If the light is all blue, it will appear as a dark shadow; if the light is full spectrum, it will appear yellow. It is due to the preferential absorption of blue polarized light by pigment molecules in the fovea. *
Purkinje images Purkinje images are reflections of objects from the structure of the eye. They are also known as Purkinje reflexes and as Purkinje–Sanson images. At least four Purkinje images are usually visible. The first Purkinje image (P1) is the reflectio ...
are the reflections from the anterior and posterior surfaces of the cornea and the anterior and posterior surfaces of the lens. Although these first four reflections are not entoptic—they are seen by others who are looking at someone’s eye— Becker described how light can reflect from the posterior surface of the lens and then again from the anterior surface of the cornea to focus a second image on the retina, this one much fainter and inverted. Tscherning referred to this as the sixth image (the fifth image is formed by reflections from the anterior surfaces of the lens and cornea to form an image too far in front of the retina to be visible) and noted it was much fainter and best seen with a relaxed
emmetropic Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the ciliary muscle in a relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the axi ...
eye. To see it, one must be in a dark room, with one eye closed; one must look straight ahead while moving a light back and forth in the field of the open eye. Then one should see the sixth Purkinje as a dimmer image moving in the opposite direction. * The Purkinje tree is an image of the retinal blood vessels in one's own eye, first described by Purkyně in 1823. It can be seen by shining the beam of a small bright light through the pupil from the periphery of a subject's vision. This results in an image of the light being focused on the periphery of the retina. Light from this spot then casts shadows of the blood vessels (which lie on top of the retina) onto unadapted portions of the retina. Normally the image of the retinal blood vessels is invisible because of
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
. 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 examiner is using an
ophthalmoscope Ophthalmoscopy, also called funduscopy, is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination and may be done as part ...
. Another way in which the shadows of blood vessels may be seen is by holding a bright light against the eyelid at the corner of the eye. The light penetrates the eye and casts a shadow on the blood vessels as described previously. The light must be jiggled to defeat adaptation. Viewing in both cases is improved in a dark room while looking at a featureless background. This topic is discussed in more detail by Helmholtz. * Purkinje's blue arcs are associated with the activity of the nerves sending signals from where a spot of light is focussed on the retina near the fovea to the optic disk. To see it, one needs to look at the right edge of a small red light in a dark room with the right eye (left eye closed) after dark-adapting for about 30 seconds; one should see two faint blue arcs starting at the light and heading towards the blind spot. When one looks at the left edge, one will see a faint blue spike going from the light to the right. * A
phosphene A phosphene is the phenomenon of seeing light without light 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 wit ...
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
eyelash An eyelash (also called lash) (Latin: ''Cilia'') is one of the hairs that grows at the edge of the eyelids. It grows in one layer on the edge of the upper and lower eyelids. Eyelashes protect the eye from debris, dust, and small particles and p ...
es are considered to be part of the eye is seeing light
diffracted Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
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 ''Spectral'' is a 2016 3D military science fiction, supernatural horror fantasy and action-adventure thriller war film directed by Nic Mathieu. Written by himself, Ian Fried, and George Nolfi from a story by Fried and Mathieu. The film stars ...
colour. The disk shape is given by the circular
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...
of the
pupil The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...
.


See also

* * * * * * * *
Theory of Colours ''Theory of Colours'' (german: Zur Farbenlehre, links=no) is a book by Johann Wolfgang von Goethe about the poet's views on the nature of colours and how these are perceived by humans. It was published in German in 1810 and in English in 1840. ...


References


Sources

* Jan E. Purkyně, 1823: ''Beiträge zur Kenntniss des Sehens in subjectiver Hinsicht'' in ''Beobachtungen und Versuche zur Physiologie der Sinne'', In Commission der J.G. Calve'schen Buchhandlung, Prag. * 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;

* Becker, O., 1860, “Über Wahrnehmung eines Reflexbildes im eigenen Auge
bout perception of a reflected image in your own eye Bout can mean: People *Viktor Bout, suspected arms dealer * Jan Everts Bout, early settler to New Netherland * Marcel Bout Musical instruments * The outward-facing round parts of the body shape of violins, guitars, and other stringed instrum ...
” Wiener Medizinische Wochenschrift, pp. 670 672 & 684 688. * M. Tscherning, 1920, ''Physiologic Optics''; Third Edition, (English translation by C. Weiland). Philadelphia: Keystone Publishing Co. pp. 55–56. * White, Harvey E., and Levatin, Paul, 1962, "'Floaters' in the eye," Scientific American, Vol. 206, No. 6, June, 1962, pp. 199 127. * Duke Elder, W. S. (ed.), 1962, System of Ophthalmology, Volume 7, The Foundations of Ophthalmology: heredity pathology diagnosis and therapeutics, St. Louis, The C.V. Mosby Company. p450. * Snodderly, D.M., Weinhaus, R.S., & Choi, J.C. (1992). Neural-vascular relationships in central retina of Macaque monkeys (Macaca fascicularis). Journal of Neuroscience, 12(4), 1169-1193. Available online at: http://www.jneurosci.org/cgi/reprint/12/4/1169.pdf. * Sinclair, S.H., Azar-Cavanagh, M., Soper, K.A., Tuma, R.F., & Mayrovitz, H.N. (1989). Investigation of the source of the blue field entoptic phenomenon. Investigative Ophthalmology & Visual Science, 30(4), 668-673. Available online at: http://www.iovs.org/. * Giles Skey Brindley, Physiology of the Retina and Visual Pathway, 2nd ed. (Edward Arnold Ltd., London, 1970), pp. 140–141. * Bill Reid, “Haidinger's brush,” Physics Teacher, Vol. 28, p. 598 (Dec. 1990). * Walker, J., 1984, “How to stop a spinning object by humming and perceive curious blue arcs around the light,” Scientific American, February, Vol. 250, No. 2, pp. 136 138, 140, 141, 143, 144, 148.


External links

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Picture of the entoptic phenomenon: Vitreous Floaters
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Diagram of entoptic subjective visual phenomenaVideo describing history and science of first entoptic viewing techniqueVideo describing history and science of second entoptic viewing techniqueThe Relation Between Migraine, Typical Migraine Aura and “Visual Snow”
{{Visual phenomena Ophthalmology Vision *