binocular single vision in humans in detail

Binocular single vision

1. Binocular single vision By, Lakshmi.K.S Moderator: Dr. Asha Achar

2. Binocular Single Vision Binocular Single Vision  may be defined as the state of simultaneous vision, which is achieved by the coordinated use of both eyes, so that separate and slightly dissimilar images arising in each eye are appreciated as a single image by the process of fusion

3. Evolution of Binocular Vision The animals that are preyed upon have eyes pointing in different directions, take advantage of almost 360 degree field of vision The animals that are predatory tend to have eyes facing in the same direction to take advantage of steriopsis

4. Advantages of a Binocular vision The first and the foremost advantage of a binocular vision is  single vision. In addition to single vision it results in  stereopsis  – the most precise kind of depth perception i.e appreciation of 3 dimensions of an object Enlargement of the field of vision Compensation for blind spot Optical defects present in 1 eye are made less obvious by normal image in opposite eye

5. Normal binocular single vision Normal binocular single vision (BSV) involves the simultaneous use of both eyes with bifoveal fixation, so that each eye contributes to a common single perception of the object of regard

6. Conditions necessary for normal BSV are Normal routing of visual pathways with overlapping visual fields . Clear visual axis Binocularly driven neurons in the visual cortex. Normal retinal (retinocortical) correspondence (NRC) resulting in sensory fusion Accurate neuromuscular development and coordination, so that the visual axes are directed at, and maintain fixation on, the object of regard resulting in motor fusion Approximately equal image clarity and size for both eyes.

7. Terminologies Related to Binocular Vision Visual axis (line of vision) passes from the fovea, through the nodal point of the eye to the point of fixation (object of regard).

8. Visual direction Visual direction is the projection of a given retinal element in a specific direction in subjective space. a Principal visual direction- line of sight, usually foveal direction b Secondary visual directions are the projecting directions of extrafoveal points

9. Projection is the subjective interpretation of the position of an object in space on the basis of stimulated retinal elements .

10. Retinomotor values Retinomotor value, zero at the fovea, increases progressively towards the retinal periphery The image of an object in the peripheral visual field falls on an extrafoveal element. To establish fixation on this object a saccadic version of accurate amplitude is required

11. Retinal Correspondence Retinal elements of the two eyes that share a common subjective visual direction are called corresponding retinal points 1. Normal retinal correspondence 2. Abnormal retinal correspondence *cover test basis

12. The horopter Imaginary plane in external space, relative to both the observer's eyes for a given fixation target, all points on which stimulate corresponding retinal elements and are therefore seen singly and in the same plane. Theoretical horopter circle- vieth muller circle Empirical horopter curve- flatter with increased radius of curvature

13. Panum fusional space Panum fusional space (‘volume’) is a zone in front of and behind the horopter in which objects stimulate slightly non-corresponding retinal points (retinal disparity ). Objects within the limits of the fusional space are seen singly and the disparity information is used to produce a perception of binocular depth (stereopsis ). Objects in Panum fusional areas are seen singly and stereoscopically.

14. Fusion Fusion is defined as the unification of visual excitations from the corresponding retinal images into a single visual percept. Takes place in visual cortex. Stimulus is retinal disparity outside the pannums fusional area. Sensory Fusion: ability to appreciate two similar images, one with each eye and interpret them as one . 2. Motor Fusion: ability to align the eyes in such a manner that sensory fusion can be maintained .

15. Retinal Rivalry / Binocular Rivalry When dissimilar contours are presented to corresponding retinal areas fusion becomes impossible and retinal rivalry may be observed Confusion: Simultaneous appreciation of two superimposed but dissimilar images caused by stimulation of corresponding retinal points (usually the foveae) by images of different objects . L eads to complete sensory dominance of the other eye . Return of retinal rivalry is a must for binocular single vision

16. Suppression I nhibitory mechanism in which when corresponding retinal areas are stimulated by dissimilar stimuli or when non-corresponding retinal areas are stimulated by similar stimuli, one or the other is temporarily inhibited or suppressed to prevent confusion or diplopia respectively Facultative:occurs only under binocular conditions. visual acuity is not reduced under monocular conditions Obligatory: monocular conditions resulting in diminution of visual acuity (amblyopia)

17. Steriopsis (Disparity Sensitivity) Ability to fuse images that stimulate horizontally disparate retinal elements within Panum’s fusional area resulting in binocular appreciation of visual object in depth i.e. in the third dimension O bject confined to the horopter is seen as flat because it projects to corresponding retinal regions, causing zero horizontal disparity

18. Non-zero disparities giving rise to stereoscopic depth are divided into crossed and uncrossed . 1. Crossed disparities are created by objects in front of the horopter (near objects). 2. Uncrossed disparities are created by objects located behind the horopter (far objects).

19. Stereoscopic acuity it is the minimum disparity beyond which no stereoscopic effect is produced it follows that stereopsis cannot work beyond a certain critical distance i.e between 125-200 meters steroacuity threshold for static targets is in the range of 2-10 arc sec. For targets in motion threshold increases to about 40 arc sec Stereacuity is maximal about 0.25 degrees off dead center in the foveola , and diminishes exponentially with increasing eccentricity along the x-axis. Stereopsis is nil beyond 15 degrees eccentricity

20. Development of Binocular Vision Factors necessary: Anatomical factors Shape of the orbit Presence of adjacent ligaments, muscles and connective tissues . These bring about motor correspondence responsible for Enlarge the field of view by transforming the field of vision into the field of fixation. Bring back the object of attention on to the fovea and to maintain it. Position the two eyes in such a way that at all the times they are properly aligned

21. Physiological Factors Fixation reflex Compensatory fixation reflex- gravitational, based on head and body posture Orientation fixation reflex  - in slow movement of head Accommodation convergence reflex- correctly alligning eyes and keeping them focused on an object 2 . Re fixation reflex 3. Pupillary reflex

22. Fusional reflexes ( psychoptical reflexes ) C onditioned reflexes, acquired and maintained by cerebral activity W ith continued reinforcement it becomes an unconditioned reflex I t consists of all the activities mediated from the retina through the brain to maintain the images received on the two foveae with the ultimate aim of attaining a single binocular vision

23. Elements of fusion mechanism Fixation reflex Refixation reflex Conjugate fusional reflexes – maintains the parallelism of the two eyes in all positions of gaze. Disjunctive reflexes convergence/divergence reflexes.

24. Fixation reflex Poorly developed at birth 2- 3 weeks- follows light uniocularly 6 weeks to 6 months - follows light binocularly Convergence- starts developing at 1 month of age and is well established by 6 months. The development of accommodation lags behind the development of convergence due to the delay in the development of ciliary muscles, parallels with the convergence by 6 months of age.

25. Binocular vision B egins at about 4 months of age , peaks at 2 years, is well developed by 4 years of age slowly declines to cease by 9 years of age . * sensitive period – upto 2 years. Therefore any obstacles in this period hamper the development of binocular vision.

26. O bstacles Sensory obstacles Dioptric obstacles – e.g. media opacities, uncorrected errors of refraction. Prolonged uniocular activity- e.g. severe ptosis, anisometropia Retinoneural obstacles – lesions of retina, optic nerve Proprioceptive obstacle

27. 2. Motor obstacles Congenital craniofacial malformations Conditions effecting extra-ocular muscles 3.  Central obstacles CNS lesions- involving the nerve trunks, root of nuclei

28. The presence of these obstacles gives rise to various sensory adaptations to binocular dysfunction especially if the disruptive factor is present in the sensitive period. This can be in the form of : Abnormal Retinal Correspondence Suppression Amblyopia

29. Theories of Binocular Vision Correspondence and disparity theory Alternation theory of binocular vision Projection Theory Motor theory Theory of isomorphism

30. Correspondence and disparity theory: It assumes the presence of one to one retinocortical relationship between the two eyes When stimulated simultaneously by two object points that differ in character – binocular rivalry occurs I f horizontal disparity remains within limits of Panum’s area, a single visual impression is elicited with depth or stereopsis . Perceived depth increases with increasing disparity. With increasing disparity quality of stereopsis decreases which may eventually lead to diplopia

31. * neurophysiologic Basis: from animal studies by Hubel and Wiesel. 80% of the neurons in the striate cortex can be driven from either eye in response to a visual stimulus from the retina, assuming that there exists a precise and orderly arrangement of connections along the entire retino -geniculate striate pathway. 25 % of these binocularly driven cells were stimulated equally from each eye, while 75% represented graded response from either left or right eye.

32. Grades of Binocular Vision- three grades ( Worth's classification) Grade I:  Simultaneous macular perception   O ccurs when the visual cortex perceives separate stimuli to the two eyes at the same time and concerns itself essentially with the absence of suppression. True sensory fusion

33. Grade II:  It represents  true fusion  with some amplitude Some effort is made to maintain this fusion in spite of difficulties i.e motor response added to simple sensory fusion . Grade III:  In the highest type of binocularity, a  stereoscopic effect Not only are the images of the two eyes fused, but they are blended to produce a  stereoscopic effect

34. Tests for Binocular Vision Before any test is undertaken it is essential to assess the: visual acuity fixation in the squinting eye direction and size of deviation Tests for assessing the presence or absence of; Normal or abnormal retinal correspondence Suppression Simultaneous perception Fusion with some amplitude Stereopsis

35. 1. Normal or abnormal retinal correspondence Clinically the tests used can be based on either of the two principles: A) Assessment of relationship between the fovea of the fixing eye and the retinal area stimulated in the squinting eye. This includes: Bagolini's striated glasses test R ed filter test Synaptophore using SMP slides for measuring the objective and subjective angles Worth's 4 dot test

36. B) Assessment of the visual directions of the two foveae. Included in this are: After image test (Hering Bielschowsky) Cuppers binocular visuoscopy test (foveo-foveal test of Cuppers)

37. Bagolini's striated glasses test For this the patient fixates a small light, after being provided with plano lenses with narrow fine striations across one meridian The lenses are usually placed at 45 degree OS and 135 degree OD

38. Interpretation: •Crossing of the lines at right angles to each other • Foveal suppression scotoma (fixation point scotoma ) with peripheral fusion, if no shift occurs with cover test, NRC exists, if shift occurs, ARC exists •Single line represents suppression

39. Red filter test Used in heterophoria Red filter in front of fixating eye Asked to look at a light source Responses: Esotropia with NRC Exotropia with ARC Harmonious ARC Unharmonious ARC Suppression

40. Synaptophore using SMP slides for measuring the objective and subjective angles (angle of anomoly) Angle of Anomaly = Objective Angle – Subjective Angle( degree of shift in visual direction) If Subjective Angle = Objective Angle → NRC If Subjective Angle < Objective Angle → ARC If Angle of Anomaly = Objective Angle →Harmonious ARC (full sensory adaptation) If Angle of Anomaly < Objective Angle →Unharmonious ARC

41. Worth's 4 dot test B. NRC with no heterotropia Harmonious ARC with manifest squint . C. The patient sees two red dots, suppression of left eye . D. The patient sees three green dots, suppression of right eye . E. five dots . -Alternately- alternating suppression -uncrossed diplopia with esotropia, red dots appear to the right -crossed diplopia with exotropia, red dots appear to the left of the green dots

42. After image test (Hering Bielschowsky) BATTERY POWERED CAMERA FLASH, HORIZONTAL (BETTER EYE) AND VERTICAL (POORER EYE) GLOWING FILAMENTS WITH BLACK CENTER A ) Cross response: A symmetrical cross with the central gaps superimposed indicates a normal bifoveal correspondence B) ARC- in a case of esotropia (R), horizontal to right C ) ARC- in case of exotropia (R), horizontal to left Single line indicates suppression

43. Cuppers binocular visuoscopy test The patient fixates with the normal eye on the central light of a Maddox scale via a plano mirror, which for the convenience of the examiner is turned in such a manner that the amblyopic eye looks straight ahead. The visuoscope asterisk is projected by the examiner onto the fovea of the amblyopic eye. The figure of the Maddox scale on which the patient sees the asterisk indicated the angle of anomaly. Determines whether the two foveae have common or different visual directions . It permits quantitative analysis of the angle of anomaly when eccentric fixation is present.

44. Tests For Suppression To diagnose: Worth's four dot test Synaptophore Friend test Amsler Grid 4 ∆ prism base out test Red filter test Bagolini’s striated glasses

45. FRIEND test T he letters F, I, N are in green and the letters R, E, D are in red Patient sees FRIEND at once—normal binocular vision Patient sees onlyRED–left eye suppression Patient sees only FIN –right eye suppression Patient sees FIN and RED alternatively –alternate suppression

46. Amslers Grid

47. 4 ∆ prism base out test Image displacement with a weak base out prism, while one observes the resulting binocular (version) and monocular (fusional) eye movements, is a quick, sensitive screening test to assess whether bifoveal fusion or suppression of one fovea is present Sudden displacement of an image with a base out prism from one fovea onto the parafoveal temporal retina will elicit a refixation movement if the image has been shifted within a normally functioning retina, but no movement will occur if the image has been shifted within a nonfunctioning (that is, scotomatous ) area.

48. To test the extent of suppression Charted under binocular conditions (fixating with one eye while the field of other eye is charted). Prisms Synaptophore Lee’s screen or Hess screen Polaroid Scotometer Phase difference haploscopy of Aulhorn

49. With more dissociating tests like prisms, Lee’s etc. single large coarse scotomas are seen; these extend from fovea to the diplopia point. With less dissociating tests like Aulhorn phase difference haploscope and Polaroid scotometer, two discrete scotoma are seen. These are foveal scotoma about 2-3 degrees in size and diplopia point scotoma .

50. The depth or intensity of scotoma can be seen by using differential stimulation of the two eyes. The graded density filter bar of Bagilini is useful. As the denser filters are brought over the dominant eye, the relative scotoma of the amblyopic eye start disappearing or shrinking in size.

51. Tests for simultaneous macular perception Synaptophore: using SMP slides Eg: bird and cage, lion and cage, butterfly and net

52. If superimposition occurs, target slides of different sizes used Simultaneous foveal perception slide – subtend an angle of 1 degree at the nodal point Simultaneous parafoveal perception slides – subtend angle of 1-3 degree Simultaneous paramacular perception slides – subtend angle of 3-5 degree Simultaneous peripheral perception slides – subtend angle greater than 5 degrees

53. Tests for Fusion 1. Worth’s 4-dot test 2. Bagolini’s striated glasses 3. Synaptophore: using slides in which similar pictures with different controls are presented to the eyes simultaneously Eg: Letter L and F fused into E Rabbit with a tail and rabbit with flower in hand, fused into one rabbit having tail and flower.

54. Tests for Stereopsis- Static steriopsis Stereopsis tests may be qualitative or quantitative. 1. Qualitative tests for Stereopsis : Lang’s 2 pencil test Synaptophore 2. Quantitative tests for Stereopsis : Stereopsis is measured in seconds of arc. Random dot test Titmus fly test TNO Test Lang’s stereo test Howard dolman peg test

55. Lang’s two pencil test Lang’s two pencil test: 3000-5000 sec of arc For detecting the presence or absence of gross steriopsis. Patient asked to touch the tip of one pencil with another that examiner is holding Normally- can be done with both eyes open and cannot be done with 1 eye closed Cannot be done with both eyes open if steriopsis absent

56. Synaptophore

57. Synaptophore The Diagnostic Uses Of The Synoptophore : 1. Measurement of the objective and subjective angle of deviation. 2. Measurement of angle kappa. 3. Measurement of primary and secondary deviation. 4. Measurement of deviation in cardinal directions of gaze. 5. Estimation of status of binocular vision a. state of retinal correspondence: normal, abnormal. b . presence and type of suppression. c . presence of fusion and measurement of fusional amplitudes . d . presence of stereopsis.

58. Titmus stereo test Fly test- test gross steriopsis. Threshold of 3000 sec of arc. Only for near steriopsis Animal test- 3 rows of animals, one animal imaged disparately (100, 200, 400 sec of arc) with a misleading dark animal Circle test- 9 sets of 4 circles in 9 diamonds. Only 1 circle of the 4 in each diamond disparately imaged (800-40 sec arc) All 3 viewed wit polaroid spectacles

59. Random dot E test 3 cards to be viewed with polaroid spectacles. Held at a distance of 50 cms and increased to quantitate the stereoacuity 1 st card- bas relief model of stereotype figure 2 nd card- E stereo with random dot background 3 rd card- stereoblank with an identical random dot background 504 sec of arc for 50 cm to 50 sec of arc for 500 cms

60. TNO random dot test Booklet of 7 plates with 1 set for gross stereopsis without glasses and the 2 nd set with red green glasses that allow to quantiate the stereopsis 15 to 480 sec of arc

61. Lang test Random dot stereogram with panographic presentation with star, car, cat embedded are seen disparately through the cylindrical lenses imprinted on the surface lamination Glasses not required Held at 40 cms Car and star- 600 sec of arc Cat- 1200 sec of arc

62. Frisby test 3 plastic cards with 4 squares of small random shapes. One of the squares in each card has a circle that is viewed disparately by the thickness of the plate Glasses not required If fails , implies not 540 sec of arc

63. Howard-Dolman Peg Test A real-space test of local stereopsis two vertical pegs seen against a high-contrast background and through an aperture that conceals their ends One peg is moveable toward and away from the observer observation distance to the stationary peg is usually 4 to 6 m Two psychophysical thresholds- null threshold, the just-noticeable difference (JND) threshold A null threshold test score of 14 seconds of arc or better (smaller numerical value ) is expected for an adult with a normal oculomotor system

64. Motion steriopsis tests To be tested on a phone or personal computer or any monitor

65. References Anatomy and physiology of the eye, A K Khurana , Indu Khurana Binocular vision, Rahul Bhola , eyerounds.org Orthoptics and Ocular Examination Techniques , William E.Scott Duanes ophthalmology, PAUL R. MITCHELL and MARSHALL M. PARKS Adlers physiology of the eye