Binocular Single Vision and Stereopsis

(1)
University of Sydney, Sydney, Australia
 

Overview: The Physiology of Binocular Vision

  • Binocular vision is obtained from two retinal images that are fused through motor and sensory processes culminating in the perception of a single image and stereoscopic depth.
  • Visual information from each eye remains segregated until it passes to binocular neurons in the primary visual cortex (see Chap. 14).
  • Stereopsis is the sense of 3-dimensional depth perception based on slight binocular image disparity detected by these cortical neurons.
  • The paramount consequence of binocular vision is fine stereopsis.
  • This is because the following are required to achieve fine stereopsis:
    (a)
    Central fixation with normal visual acuity in each eye
     
    (b)
    Precise oculomotor control that achieves bifoveal fixation
     
    (c)
    Normal retinal correspondence regarding visual direction in space
     
    (d)
    The ability to perceive slight discrepancies in image location from each eye to generate a sense of depth
     

Binocular Single Vision

Binocular single vision (BSV) is the ability to see one image with both eyes simultaneously.
1.
Levels of binocular single vision [1–5]
(i)
Simultaneous perception
  • The subject simultaneously perceives an object with each eye.
 
(ii)
Fusion
  • Cortical fusion of the two retinal images leads to sensation of a single image.
 
(iii)
Stereopsis
  • Images are fused; however, slight horizontal disparity gives the perception of depth.
  • This is the highest level of BSV.
 
 
2.
Retinal correspondence
  • Corresponding retinal areas share a common subjective visual direction. When co-stimulated, these result in a sensation of single vision [6, 7].
  • Non-corresponding retinal areas, when co-stimulated, result in a sensation of diplopia.
  • Normal retinal correspondence, a cortical phenomenon, implies that corresponding areas of each retina have the same position relative to each fovea [6].
  • Normal ocular alignment and good image clarity for both eyes during early childhood is necessary for the development of normal retinal correspondence [7–10].
 
3.
The horopter and Panum’s fusion area
  • The points in space that project to corresponding retinal areas lie on an imaginary curved arc, the horopter, which is centered on the point of fixation (Fig. 25.1) [7, 11, 12].
    A347009_1_En_25_Fig1_HTML.gif
    Fig. 25.1
    The horopter and Panum’s fusion area
  • An object located on the horopter does not induce binocular image disparity.
  • Objects located outside the horopter (anterior or posterior) induce image disparity [10, 13].
  • Small amounts of disparity can be overcome by the visual system’s ability to fuse disparate images.
  • Hence, objects located within Panum’s fusion area (PFA), a narrow region anterior and posterior to the horopter, result in single vision [7, 10, 14].
  • The slight image disparity induced by objects in PFA results in the sensation of stereopsis [10, 13].
  • Close to fixation very little disparity is tolerated; more is tolerated farther toward the periphery [15].
  • Correspondingly PFA is narrow centrally and broad peripherally.
  • Objects outside PFA cause image disparity beyond the limits of fusion: these cause diplopia [16].
 
4.
Fusion
  • Fusion can be divided into sensory and motor fusion [6, 17]:
(i)
Sensory fusion
  • Sensory fusion is based on normal retinal correspondence.
  • There is an orderly topographical relationship between each retina and the visual cortex, whereby corresponding retinal points project to the same cortical locus resulting in a single image [18, 19].
 
(ii)
Motor fusion
  • This is a corrective vergence movement in response to image disparity [20, 21].
  • Motor fusion adjusts eye position to maintain sensory fusion.
  • As a fixation target approaches the observer, the retinal images move temporally from each fovea if the eye remains in an unchanged position.
  • To prevent diplopia, the crossed image disparity induces both eyes to converge (turn inwards) and maintain the image focused on the foveae [22].
  • A similar divergent movement occurs as objects move from near to far.
  • Fusional reserve indicates the level at which motor fusion breaks down, usually causing diplopia.
  • It can be measured by adding prism bars (base in or out) until fusion is lost (Table 25.1) [23].
    Table 25.1
    Normal fusional amplitudes [1, 24, 25]
    Testing distance (m)
    Convergence (prism diopters, Δ)
    Divergence
    (prism diopters, Δ)
    Vertical (prism diopters, Δ)
    6
    16
    6
    5–6
    0.25
    32
    16
    3–4
    Torsional fusional vergences also exist for up to 6–10° of torsional image disparity
 
 

Stereopsis

  • Stereopsis is the ability to perceive depth due to relative binocular image disparity [4, 10].
  • Stereopsis occurs when retinal disparity is too great to permit simple superimposition of the two retinal images, but not great enough to elicit diplopia [13].
  • Stereopsis is produced predominantly by horizontal image disparity; vertical disparity contributes to slant perception and helps interpret the scale of horizontal disparities [10, 26, 27].
1.
Parvocellular and magnocellular stream-mediated stereopsis
  • Parvocellular and magnocellular stream-mediated stereopsis coexist [28–30].
  • Parvocellular mediated stereopsis is most sensitive for centrally located, static stimuli.
  • It is capable of fine stereoacuity and is color sensitive [28, 29, 31].
  • It is best suited to random dot stereogram testing [29, 32].
  • Magnocellular mediated (motion) stereopsis is most sensitive for peripherally located, moving stimuli and only capable of gross stereoacuity. It is color insensitive [33].
 
2.
Stereoacuity
  • Stereoacuity is measured as the smallest relative binocular disparity that can produce stereopsis.
  • Stereoacuity is greatest at central fixation and declines with eccentricity [34].
  • Optical defocus (especially asymmetric refractive error) [35], reduced contrast [36], aniseikonia [37], and high and low spatial frequencies [36] all reduce stereoacuity [10].
 
3.
Tests of stereoacuity
  • Several clinical tests can evaluate stereoacuity; these are designed as screening tools for distinguishing normal from abnormal binocular vision [5].
  • They can be divided into two broad types: contour and random dot tests [38, 39].
  • Contour stereopsis tests involve horizontal separation of the image to each eye with polarized or red-green dissociative glasses (e.g., Titmus fly test) [40].
  • Random dot stereopsis tests embed the stereo figures in a background of random dots e.g., TNO, Lang Stereotest (Switzerland) [41].
 
4.
Other mechanisms of depth perception
  • Stereopsis is not synonymous with depth perception: there are other clues of depth perception that are helpful for monocular individuals [42–44].
  • Monocular clues include object overlap, apparent size, highlights and shadows, motion parallax, and perspective.
  • For far (>6 m) distances, depth perception is based almost entirely on monocular clues.
 

Abnormalities of Binocular Single Vision

  • Misaligned eyes (known as strabismus) can result in visual confusion and/or diplopia [45].
  • Visual confusion is the stimulation of corresponding points by dissimilar images (typically both foveas), resulting in the images appearing to be on top of one another.
  • Diplopia is the stimulation of non-corresponding retinal areas by the same image, resulting in double vision [46, 47].
  • Abnormal BSV can result in subnormal stereopsis or in some cases amblyopia [48–51].

Sensory Adaptations to Strabismus

These include:
1.
Suppression
  • Suppression is a cortical mechanism to ignore one of the images, to prevent confusion (central image suppression) or diplopia (peripheral suppression) [52, 53].
  • The size and density of the suppression scotoma is variable [54–56].
  • Non–alternating (monocular) suppression can lead to amblyopia [57].
  • Alternating suppression does not lead to amblyopia but if present during childhood can result in subnormal development of depth perception [48–51].
 
2.
Abnormal retinal correspondence
  • This is a cortical mechanism to permit non–corresponding retinal points to stimulate the same area of occipital cortex to produce one image [18, 58].
  • Abnormal retinal correspondence (ARC) permits a small amount of BSV despite misaligned eyes [9].
 
3.
Abnormal head posture
  • This is a behavioral mechanism used by children to maintain BSV, by bringing the object into a field of visual space in which single vision is possible [47, 59, 60].
  • An abnormal head posture suggests that the patient is capable of binocular vision.
 

Subjective Testing for Suppression and Abnormal Retinal Correspondence

  • These are dissociative tests to assess for suppression under binocular conditions [1, 5].
  • They can only be interpreted in conjunction with a cover test to assess for strabismus.
  • BSV in the context of a manifest ocular deviation implies ARC.
  • These tests include:
    1.
    Base-out prism test (Fig. 25.2)
    A347009_1_En_25_Fig2_HTML.gif
    Fig. 25.2
    The 20 Δ base-out prism test
    • This is a test for fusion in children, used as an indirect marker of binocular vision [61].
    • A 20 Δ base-out prism is held in front of one eye.
    • If fusion is present, there will be a corrective contralateral saccade of both eyes followed by a slow refixation of the eye without the prism.
    • A 4 Δ prism can be used in a similar manner to detect a small scotoma in monofixation syndrome [62].
     
    2.
    Bagolini striated glasses (Fig. 25.3) [63, 64]
    A347009_1_En_25_Fig3_HTML.gif
    Fig. 25.3
    Possible results of the Bagolini striated glasses. (a) Normal BSV. (b) Diplopia. (c) Right suppression
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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Binocular Single Vision and Stereopsis

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