Monocular vision: Difference between revisions

The eyes of an animal with monocular vision are positioned on opposite sides of the animal's head, giving it the ability to see two objects at once. This is usually most commonly seen with prey animals, as the reason why their eyes are placed on either side of their head is to make it easier for them to look out for predators, which usually have forward-facing eyes to make it easier to find prey. However, there are some exceptions to this rule, usually if the predator is an animal that is often preyed upon by a greater predator (because of this, [[apex predatorspredator]]s usually tend to have forward-facing eyes) or sport an anatomy that makes it very difficult for it to see straight, such as a short, stiff neck that would limit its head movement, and therefore would require its eyes to be on either side (this is often seen with marine predators such as sharks and killer whales).

'''Monocular vision impairment''' refers to having no vision in one eye with adequate vision in the other.<ref>{{cite web |url=http://www.guidedogsqld.com.au/cgi-bin/index.cgi/monocular/mvi |title= Monocular Vision Impairment (MVI)|website=www.guidedogsqld.com.au |archive-url=https://web.archive.org/web/20061208095213/http://www.guidedogsqld.com.au/cgi-bin/index.cgi/monocular/mvi |archive-date=December 8, 2006}}</ref>

'''Monopsia''' is a medical condition in humans who cannot perceive [[Depth perception|perceive depth]] even though their two eyes are medically normal, healthy, and spaced apart in a normal way. Vision that perceives three-dimensional depth requires more than [[parallax]]. In addition, the resolution of the two disparate images, though highly similar, must be simultaneous, subconscious, and complete. (After-images and "phantom" images are symptoms of incomplete visual resolution, even though the eyes themselves exhibit remarkable acuity.) A feature article in ''[[The New Yorker]]'' magazine published in early 2006 dealt with one individual in particular, who, learning to cope with her disability, eventually learned how to see three-dimensional depth in her daily life. Medical tests are available for determining monoptic conditions in humans.<ref>Monocular individuals face increased challenges with driving. These specifically relate to depth perception and peripheral vision.

* [[Accommodation (eye)|Accommodation]] – This is an oculomotor cue for depth perception. When we try to focus on distant objects, the [[ciliary muscles]] relax allowing the eye lens to flatten, making it thinner. The [[Proprioception|kinesthetic sensations]] of the contracting and relaxing ciliary muscles (intraocular muscles) isare sent to the visual cortex where it is used for interpreting distance/depth.

* Relative size – If two objects are known to be the same size (e.g. two trees) but their absolute size is unknown, relative size cues can provide information about the relative depth of the two objects. If one subtends aappears larger visual angle on the retina than the other, the object which subtends theappears larger visualappears angleto appearsbe closer.

* [[Peripheral vision]] – At the outer extremes of the [[visual field]], parallel lines become curved, as in a photo taken through a [[fish-eye lens]]. This effect, although usually eliminated from both art and photos by the cropping or framing of a picture, greatly enhances the viewer's sense of being positioned ''within'' a real, three-dimensional space. (Classical perspective has no use for this "distortion", although in fact the "distortions" strictly obey optical laws and provide perfectly valid visual information, just as classical perspective does for the part of the field of vision that falls within its frame.)

* Texture gradient – Suppose you are standing on a gravel road. The gravel near you can be clearly seen in terms of shape, size and colourcolor. As your vision shifts towards the more distant part of the road it becomes progressively less easy to distinguish the texture.

Vision has been known to play an important role in balance and postural control in humans, along with proprioception and vestibular function. Monocular vision affects how the brain perceives its surroundings by decreasing the available visual field, impairing peripheral vision on one side of the body, and compromising depth perception, all three of which are major contributors to the role of vision in balance.<ref name="Berela">Berela, J. et al. (2011) [https://iovs.arvojournals.org/article.aspx?articleid=2120979 Use of monocular and binocular visual cues for postural control in children]. Journal of Vision. 11(12):10, 1-8</ref><ref>Wade, M. and Jones, G. (1997) [https://academic.oup.com/ptj/article-pdf/77/6/619/10761662/ptj0619.pdf The role of vision and spatial orientation in the maintenance of posture]. Physical Therapy. 77, 619-628</ref> Studies comparing monocular vision to binocular (two eyes) vision in cataract patients (pre and post surgery),<ref>Schwartz, S. et al. (2005) [https://arvojournals.org/article.aspx?articleID=2124095 The effect of cataract surgery on postural control]. Investigative Ophthalmology and Visual Science. 46(3), 920-924</ref> glaucoma patients (compared with healthy age matched controls),<ref>Shabana, N, et al. (2005) [https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1442-9071.2005.01003.x Postural Stability in primary open angle glaucoma]. Clinical and Experimental Ophthalmology. 33, 264-273</ref> and in healthy adults and children (in both binocular and monocular conditions)<ref>Berela, J. et al. (2011) [https://iovs.arvojournals.org/article.aspx?articleidname=2120979 Use of monocular and binocular visual cues for postural control in children]. Journal of Vision. 11(12):10, 1-8<"Berela"/ref> have all shown to negatively impact balance and postural control than when both eyes are available. Each of the studied populations still displayed better balance when having only one eye compared to having both eyes closed.