Convergence micropsia
Encyclopedia
Convergence micropsia is a type of micropsia
characterized by the reduction in apparent size of objects viewed when the eye
s are more converged
than they need to be for the distance of the object from the eyes.
It occurs mainly during stereoscopy
and when viewing autostereogram
s (such as Magic Eye
pictures). In these cases, the object is depicted by the two half images of a stereogram
or by the contents of the autostereogram. Moving the stereogram or the autostereogram closer to the eyes increases convergence of the eyes and reduces the apparent size of the depicted object. If a correctly arranged stereogram or autostereogram is viewed with crossed eyes
, then the depicted objects will appear smaller than if it is viewed with eyes diverged or parallel.
Convergence micropsia could also occur if one looked at the world through base-out prisms.
, in which the apparent size of an afterimage
(e.g., the bright spot we see after looking at a camera flash) is influenced by where it is viewed. An afterimage has a fixed size on the retina, resulting from adaptation
of the rod cell
s and cone cell
s of the retina
. When an afterimage is viewed on the far wall of a room it looks large; when it is viewed on one's hand, it looks small. It is consistent with the geometry
of the world: an object with a particular visual angle
(given by the size of the retinal image) must be large if it is far away and small if it is near.
Convergence micropsia implies that the state of convergence of the eyes contributes to determining the distance of an object from the eyes, that it acts as a depth cue. At six meters, to view an object without double vision
the optic axes of the eyes are essentially parallel (no convergence). At closer distances, to view an object without double vision the optic axes need to approach each other at an angle (increasing convergence). Normally, the convergence angle gives accurate information about the distance of objects. Under the conditions that yield convergence micropsia, the overconverged vergence angle specifies a shorter distance than the actual distance of the object; this affects apparent size.
Micropsia
Micropsia is a condition affecting human visual perception in which objects are perceived to be smaller than they actually are. Micropsia can be caused by either optical distortion of images in the eye or by a neurological dysfunction. The condition of micropsia can be caused by more factors than...
characterized by the reduction in apparent size of objects viewed when the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
s are more converged
Convergence (eye)
In ophthalmology, convergence is the simultaneous inward movement of both eyes toward each other, usually in an effort to maintain single binocular vision when viewing an object. This action is mediated by the medial rectus muscle, which is innervated by Cranial nerve III...
than they need to be for the distance of the object from the eyes.
It occurs mainly during stereoscopy
Stereoscopy
Stereoscopy refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth...
and when viewing autostereogram
Autostereogram
An autostereogram is a single-image stereogram , designed to create the visual illusion of a three-dimensional scene from a two-dimensional image in the human brain...
s (such as Magic Eye
Magic Eye
Magic Eye is a series of books published by N.E. Thing Enterprises . The books feature autostereograms , which allow people to see 3D images by focusing on 2D patterns. The viewer must diverge his or her eyes in order to see a hidden three-dimensional image within the pattern...
pictures). In these cases, the object is depicted by the two half images of a stereogram
Stereogram
A stereogram is pair of two-dimensional panels depicting the view of a scene or an object from the vantage points of the right and left eyes. Observing the panels superimposed in a stereoscope results in the experience of three-dimensionality by virtue of the fact that object depth is encoded as...
or by the contents of the autostereogram. Moving the stereogram or the autostereogram closer to the eyes increases convergence of the eyes and reduces the apparent size of the depicted object. If a correctly arranged stereogram or autostereogram is viewed with crossed eyes
Autostereogram
An autostereogram is a single-image stereogram , designed to create the visual illusion of a three-dimensional scene from a two-dimensional image in the human brain...
, then the depicted objects will appear smaller than if it is viewed with eyes diverged or parallel.
Convergence micropsia could also occur if one looked at the world through base-out prisms.
Explanation
Apparent size is influenced by the size of the retinal image of an object and by its apparent distance from the eyes. This is shown clearly by Emmert's lawEmmert's law
Emmert's Law bears the name of Emil Emmert , since it was first described by him in 1881. Emmert noted that an afterimage appeared to increase in size when projected to a greater distance. It is unclear whether he intended this to mean physical distance or perceived distance, but most authors...
, in which the apparent size of an afterimage
Afterimage
An afterimage or ghost image or image burn-in is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased...
(e.g., the bright spot we see after looking at a camera flash) is influenced by where it is viewed. An afterimage has a fixed size on the retina, resulting from adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
of the rod cell
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
s and cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. When an afterimage is viewed on the far wall of a room it looks large; when it is viewed on one's hand, it looks small. It is consistent with the geometry
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
of the world: an object with a particular visual angle
Visual angle
The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc.It also is called the object's angular size....
(given by the size of the retinal image) must be large if it is far away and small if it is near.
Convergence micropsia implies that the state of convergence of the eyes contributes to determining the distance of an object from the eyes, that it acts as a depth cue. At six meters, to view an object without double vision
Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
the optic axes of the eyes are essentially parallel (no convergence). At closer distances, to view an object without double vision the optic axes need to approach each other at an angle (increasing convergence). Normally, the convergence angle gives accurate information about the distance of objects. Under the conditions that yield convergence micropsia, the overconverged vergence angle specifies a shorter distance than the actual distance of the object; this affects apparent size.
See also
- Binocular visionBinocular visionBinocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...
- Convergence insufficiencyConvergence insufficiencyConvergence insufficiency or Convergence Disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by an inability of the eyes to turn towards each other, or sustain convergence.-Symptoms:...
- Divergence (eye)Divergence (eye)In ophthalmology, divergence is the simultaneous outward movement of both eyes away from each other, usually in an effort to maintain single binocular vision when viewing an object. It is a type of vergence eye movement.-References:...
- Eye movement
- Extraocular musclesExtraocular musclesThe extraocular muscles are the six muscles that control the movements of the eye . The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.-List of muscles:-Importance:...
- MicropsiaMicropsiaMicropsia is a condition affecting human visual perception in which objects are perceived to be smaller than they actually are. Micropsia can be caused by either optical distortion of images in the eye or by a neurological dysfunction. The condition of micropsia can be caused by more factors than...
- Moon illusionMoon illusionThe Moon illusion is an optical illusion in which the Moon appears larger near the horizon than it does while higher up in the sky. This optical illusion also occurs with the sun and star constellations. It has been known since ancient times, and recorded by numerous different cultures...
- VergenceVergenceA vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision..When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both...