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Binocular vision is vision in which creatures having two eyes use them together. The word binocular comes from two Latin roots, ''bini'' for double, and ''oculus'' for eye.〔Harper, D. (2001). Online etymological dictionary. Retrieved April 2, 2008, from http://www.etymonline.com/index.php?term=binocular〕 According to Fahle (1987), having two eyes confers six advantages over having one. #It gives a creature a spare eye in case one is damaged. #It gives a wider field of view. For example, humans have a maximum horizontal field of view of approximately 190 degrees with two eyes, approximately 120 degrees of which makes up the binocular field of view (seen by both eyes) flanked by two uniocular fields (seen by only one eye) of approximately 40 degrees.〔 Henson, D.B. (1993). Visual Fields. Oxford: Oxford University Press.〕 #It can give stereopsis in which binocular disparity (or parallax) provided by the two eyes' different positions on the head gives precise depth perception. This also allows a creature to break the camouflage of another creature. #It allows the angles of the eyes' lines of sight, relative to each other (vergence), and those lines relative to a particular object (gaze angle) to be determined from the images in the two eyes. These properties are necessary for the third advantage. #It allows a creature to see more of, or all of, an object behind an obstacle. This advantage was pointed out by Leonardo da Vinci, who noted that a vertical column closer to the eyes than an object at which a creature is looking might block some of the object from the left eye but that that part of the object might be visible to the right eye. #It gives binocular summation in which the ability to detect faint objects is enhanced.〔 〕 Other phenomena of binocular vision include utrocular discrimination (the ability to tell which of two eyes has been stimulated by light), eye dominance (the habit of using one eye when aiming something, even if both eyes are open), allelotropia (the averaging of the visual direction of objects viewed by each eye when both eyes are open), binocular fusion or singleness of vision (seeing one object with both eyes despite each eye's having its own image of the object),〔 Panum, P. L. (1858). Über die einheitliche Verschmelzung verschiedenartiger Netzhauteindrucke beim Sehen mit zwei Augen. Kiel.〕 and binocular rivalry (seeing one eye's image alternating randomly with the other when each eye views images that are so different they cannot be fused). Binocular vision helps with performance skills such as catching, grasping, and locomotion.〔Heinen, T., & Vinken, P. M. (2011). Monocular and binocular vision in the performance of a complex skill. ''Journal of Sports Science & Medicine'' 10(3), 520-527. Retrieved from: http://www.jssm.org/〕 It also allows humans to walk over and around obstacles at greater speed and with more assurance. Orthoptists are eyecare professionals who fix binocular vision problems. ==Field of view and eye movements== Some animals, usually, but not always, prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view. Examples include rabbits, buffaloes, and antelopes. In such animals, the eyes often move independently to increase the field of view. Even without moving their eyes, some birds have a 360-degree field of view. Other animals, usually, but not always, predatory animals, have their two eyes positioned on the front of their heads, thereby allowing for binocular vision and reducing their field of view in favor of stereopsis. Examples include humans, eagles, wolves, and snakes. Some predator animals, particularly large ones such as sperm whales and killer whales, have their two eyes positioned on opposite sides of their heads, although it is possible they have some binocular visual field. Other animals that are not necessarily predators, such as fruit bats and a number of primates also have forward-facing eyes. These are usually animals that need fine depth discrimination/perception; for instance, binocular vision improves the ability to pick a chosen fruit or to find and grasp a particular branch. In animals with forward-facing eyes, the eyes usually move together. When the eyes move laterally, in the same direction, this is called a version. When the eyes move in opposite directions, to an object closer than where the eyes are pointing or farther than where the eyes are pointing, this is called a vergence. When the eyes move in, it is a convergence eye movement; when the eyes move out, it is a divergence eye movement. Some animals (including some humans, notably exotropes) use both of the above strategies. A starling, for example, has laterally placed eyes to cover a wide field of view, but can also move them together to point to the front so their fields overlap giving stereopsis. A remarkable example is the chameleon, whose eyes appear as if mounted on turrets, each moving independently of the other, up or down, left or right. Nevertheless, the chameleon can bring both of its eyes to bear on a single object when it is hunting, showing vergence and stereopsis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Binocular vision」の詳細全文を読む スポンサード リンク
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