In: Binocular Vision |
ISBN: 978-1-60876-547-8 |
Editors: J. McCoun et al, pp. 139-153 |
© 2010 Nova Science Publishers, Inc. |
Chapter 6
BINOCULAR VISION AND DEPTH PERCEPTION:
DEVELOPMENT AND DISORDERS
Ken Asakawa* and Hitoshi Ishikawa
Department of Ophthalmology and Visual Science, Kitasato University Graduate School, Doctors Program of Medical Science.
Introduction
“Binocular vision” literally means vision with two eyes, and refers to the special attributes of vision with both eyes open, rather than one eye only. Our perception under binocular conditions represents a highly complex coordination of motor and sensory processes and is markedly different from and more sophisticated than vision with one eye alone. However, the use of a pair of eyes can be disrupted by a variety of visual disorders, e.g., incorrect coordination between the two eyes can produce strabismus with its associated sensory problems, amblyopia, suppression and diplopia. What, then, is the reason forand the advantage of-having two eyes? From our visual information input, we can perceive the world in three dimensions even though the images falling on our two retinas are only two-dimensional. How is this accomplished? This article is a review of our ability to use both eyes, while also providing basic
* E-mail address: dm07002u@st.kitasato-u.ac.jp. Correspondence to Ken Asakawa, CO (Orthoptist), Department of Ophthalmology and Visual Science, Kitasato University Graduate School, Doctors Program of Medical Science, 1-15-1, Kitasato, Sagamihara, Kanagawa, 228-8555, Japan
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information on the development of binocular vision and on the clinical disorders that interfere with our depth perception, such as strabismus and amblyopia.
1. Advantages of Binocular Vision
“Two eyes are better than one,” it is said; and, indeed, two eyes do offer a number of advantages over just one. It is reported that some 80% of the neurons in the visual cortex receive input from both eyes, which offers anatomical support for the view that binocular vision is an attribute of considerable value and importance. Clearly, binocular vision has a number of functional advantages, the main ones being: 1) Binocular summation, in which many visual thresholds are lower than with monocular vision[16]. Binocular visual acuity, for example, is typically better than monocular visual acuity; and two eyes offer better contrast detection thresholds than one does. 2) The binocular field of view is larger than either monocular field alone. We have a horizontal field of approximately 200 degrees, in which the two visual fields overlap by about 120 degrees when both eyes are used together[29]. We can see objects whose images are formed on both foveas as if their images fell on a single point midway between the two eyes, like an imaginary single eye in the middle of our forehead, named a “cyclopean eye” [7,45]. 3) If one looks at the fingertip in front of the eyes, noticing what can be seen behind it, and one first closes one eye, and then the other, the objects behind the fingertip should appear to move. This positional difference results from the fact that the two eyes are arranged laterally, and are a certain distance-the interocular distance (60 to 65 mm)-apart. They therefore see the world from two slightly different points. The subtle differences between the images entering each eye make possible the binocular form of depth perception, which is the true advantage of binocular vision, and is known as “stereopsis”,[15,55,56]. The large designed studies of binocular vision and stereopsis by Howard, Rogers[32], Saladin[51] and Watt[63] are investigated.
2. Foundations of Binocular Vision
Images of a single object that do not stimulate corresponding retinal points in both eyes are said to be disparate[22,37]; binocular disparity is defined as the difference in position of corresponding points between images in the two eyes [48,49,50] (figure 1). Binocular disparity can be classified as crossed or
Binocular Vision and Depth Perception: Development and Disorders 141
uncrossed in relation to the point at which the two eyes converge (the fixation point)[44]. Points perceived to be nearer than the fixation point (within the Vieth-Müller circle, a theoretical prediction of objects in space that stimulate corresponding points in the two eyes) generally have lines of sight that cross in front of the fixation point; these points are said to have crossed disparity. Points farther away than the fixation point have lines of sight that meet behind the fixation point; this is called uncrossed disparity. The Vieth-Müller circle intersects the fixation point and the entrance pupils of each eye. Diplopia is the result of a large binocular disparity; however, the visual system is able to combine two images into a single percept with smaller disparities. In binocular disparities associated with normal binocular vision, the relationship between motor and sensory fusion is more complex[25]. Panum’s area determines the upper limit of disparities that can produce single vision[41,54]. Small differences in the perception of the two eyes give rise to stereopsis—three- dimensional depth perception. When a distant object is fixated bifoveally, nearer objects in front of it will be imaged on the temporal retina of each eye on noncorresponding points, resulting in a double image; crossed diplopia. In contrast, when a near object is fixated and a distant object is seen double, this is called uncrossed diplopia. In this case, each image is formed on the nasal retina of the eye. These phenomenons called physiological diplopia. The double image arises from visual corresponding or non-corresponding retinal areas under binocular vision. Binocular retinal correspondence is defined by the set of retinal image locations that produces identical visual directions when viewing with both eyes at the same time. These object locations, imaged onto corresponding retinal points, can be imagined as a cylinder with an infinite radius of curvature. This surface of points, called the horopter, stimulates the perception of identical visual directions for the two eyes[57,58]. However, the horopter will not precisely intersect only the fixation target. Because single binocular vision only requires the retinal image to fall within Panum’s area, a small residual misalignment of the visual axis (vergence error) may occur, causing a constant retinal disparity of a fixated object without diplopia. Fixation disparity is used by the vergence eye movement system to maintain its innervational level and compensate for a heterophoria. In the United States, testing has primarily followed a motor approach, whereas a strong sensorybased analysis has been used in Germany[39].
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Figure 1. Crossed and uncrossed disparities result when objects produce images that are formed on closely separated retinal points. Any point within Panum’s area yields a percept of a single image, while points outside Panum’s area produce diplopia.
3. Stereopsis as the Highest Level of Binocular Vision
The first descriptions of binocular vision were described in detail by Worth (1921), who classified binocular vision as three grades. The first degree consists of the simultaneous perception of each eye’s image at once. The second degree consists of the combination of the two images into a single percept and fusion,