# Convergence micropsia

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{{Short description|Medical condition}}
'''Convergence micropsia''' is a type of [micropsia](/source/micropsia) characterized by the [reduction in apparent size](/source/demagnification) of objects viewed when the [eyes](/source/human_eye) are more [converged](/source/Convergence_(eye)) than they need to be for the distance of the object from the eyes.<ref>{{cite book |author=Enright JT |chapter=The eye, the brain and the size of the moon : Toward a unified oculomotor hypothesis for the moon illusion |editor=Hershenson, Maurice |title=The Moon illusion |publisher=L. Erlbaum Associates |location=Hillsdale, N.J |year=1989 |pages=59–121 |isbn=978-0-8058-0121-7 }}</ref><ref>{{cite journal |author=Enright JT |title=Manipulating stereopsis and vergence in an outdoor setting: moon, sky and horizon |journal=Vision Res. |volume=29 |issue=12 |pages=1815–24 |year=1989 |pmid=2631401 |doi=10.1016/0042-6989(89)90162-4 |s2cid=26009881 }}</ref><ref>{{cite journal |author=Hollins M |title=Does accommodative micropsia exist? |journal=Am J Psychol |volume=89 |issue=3 |pages=443–54 |date=September 1976 |pmid=998809 |doi=10.2307/1421617 |publisher=The American Journal of Psychology, Vol. 89, No. 3 |jstor=1421617}}</ref>

It occurs mainly during [stereoscopy](/source/stereoscopy) and when viewing [autostereogram](/source/autostereogram)s (such as [Magic Eye](/source/Magic_Eye) pictures). In these cases, the object is depicted by the two [half image](/source/half_image)s of a [stereogram](/source/stereoscopy) 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](/source/autostereogram), 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](/source/base-out) [prism](/source/Triangular_prism_(optics))s.

==Explanation==
Apparent size is influenced by the size of the [retinal image](/source/retinal_image) of an object and by its apparent distance from the eyes. This is shown clearly by [Emmert's law](/source/Emmert's_law), in which the apparent size of an [afterimage](/source/afterimage) (e.g., the bright spot we see after looking at a [camera flash](/source/camera_flash)) is influenced by where it is viewed. An afterimage has a fixed size on the retina, resulting from [adaptation](/source/adaptation) of the [rod cell](/source/rod_cell)s and [cone cell](/source/cone_cell)s of the [retina](/source/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](/source/geometry) of the world: an object with a particular [visual angle](/source/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](/source/depth_cue). At six meters, to view an object without [double vision](/source/diplopia) the [optic axes](/source/optic_axis) 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==
{{col div|colwidth=30em}}
*[Binocular vision](/source/Binocular_vision)
*[Convergence insufficiency](/source/Convergence_insufficiency)
*[Divergence (eye)](/source/Divergence_(eye))
*[Eye movement](/source/Eye_movement_(sensory))
*[Extraocular muscles](/source/Extraocular_muscles)
*[Micropsia](/source/Micropsia)
*[Moon illusion](/source/Moon_illusion)
*[Vergence](/source/Vergence)
{{colend}}

==Notes==
{{reflist}}

Category:Optical illusions
Category:Eye
Category:3D imaging

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Adapted from the Wikipedia article [Convergence micropsia](https://en.wikipedia.org/wiki/Convergence_micropsia) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Convergence_micropsia?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.