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Accomodation, Fusion and Binocularity

Accomodation, Fusion and Binocularity. Amy C. Nau, OD. Outline. Accommodation Binocular sensory function Vergence Pathophysiology of common accommodative and binocular anomolies. Ken Cuifredda, OD PhD, Glen McCormack, OD PhD in Borish, Clinical Refraction. History of Accomodation.

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Accomodation, Fusion and Binocularity

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  1. Accomodation, Fusion and Binocularity Amy C. Nau, OD

  2. Outline • Accommodation • Binocular sensory function • Vergence • Pathophysiology of common accommodative and binocular anomolies Ken Cuifredda, OD PhD, Glen McCormack, OD PhD in Borish, Clinical Refraction

  3. History of Accomodation • Descartes (1677)- proposes lenticular based focusing • Thomas Young- (1801)- demonstrates that the lens changes shape • Hermann von Helmholtz (1866)- first accurate description of accommodative processes

  4. Physiology of AccommodationThe Eye Ciliary m. relaxed Ciliary m. contracts http://hyperphysics.phy-astr.gsu.edu/hbase/vision/imgvis/accom2.gif

  5. Neural basis for accommodation ewn Magnocellular LGN Area 17 http://bicmra.usuhs.mil/MRA/Histology/Eye/Images/eye01.jpg http://static.howstuffworks.com/gif/brain-intro.gif

  6. Neural basis for accommodation EWN- CN3 (PS)-CilG- Short Cil N/Long Cil N. http://info.med.yale.edu/caim/cnerves/cn3/cn3_graphics/cn3_9.jpg

  7. Types of Accommodation • Reflex, +/- • Vergence, +/- • Proximal, +/- • Tonic, +/-

  8. Adapt. tonic - S S Reflex Accom Stimulus (D) Depth of focus gain Cm/lens retina + Systems Analysis of Accommodation

  9. Optical Cues H.O. Aberrations Microfluctuations in EOM and AR Non Optical Cues Size Proximity Apparent Distance Disparate retinal images Monocular depth cues Nonretinal Image Vestibular stimulation Mood Effort Cognitive Demand Instruction set Retinal Image Frequency Contrast Eccentricity Luminance Size Depth of focus Influences on Blur Accommodation Responses

  10. Fusion • How do the two eyes, separated in space, give us a single precept? • How are the eye movements coordinated with the retinal image to maintain singularity? • How does stereo vision enhance our perceptions of our surroundings?

  11. Stereograms http://www.lri.fr/~marche/images/cameleon-3d.gif

  12. Space Perception • Object Space “the real world” • Visual Space - the world as it is captured by our retina and processed by the visual cortex. • Visual space must reproduce object space

  13. The Cyclopean Eye • We percieve the world as though we have one eye which exists between our two eyes. • “egocenter” http://bearah718.tripod.com/sitebuildercontent/sitebuilderpictures/cyclops.jpg

  14. Visual Localization and Visual Direction • Egocentric localization of an object • Where is the object in 3D space in relation to my own position? • Can judge distance in a unit of measurement (i.e., feet or meters) • Imaginary arrow from the cyclopean eye to the object (i.e. straight ahead, etc.)

  15. Egocentric localization • The sense of direction from retinal locus is a property of oculo-cortical mapping • Each retinal point has a location relative to other retinal points in this map • Local sign is the term that refers to the unique direction for each retinal point. • Related to va and retinal location (fovea vs periph), amblyopia

  16. Principal visual direction • Elicits the sensation of “looking” at something • Associated with the fixation point /origin • Center of anatomic fovea • Center of oculocentric direction • Guides foveation • Registration…

  17. Example: Paretic strabismus • Past pointing test • Used to detect labyrinth disease but also can be used for paretic eom. • Egocentric direction judgement errors when monocularly fixating with the paretic eye

  18. Depth and Distance Perception • Distance perception = absolute depth How far any object is from the observer, from another object, able to judge in meters, etc. Depth perception (relative), perception of relative proximity of one object to another or the relative depth between two or more points in space.

  19. Depth: coffee cup is 50% farther than pencil Distance: 10 cm

  20. Monocular Cues to Depth • Object distance form observer is inversely proportional to retinal image size. • Linear/perspective • Texture density • Luminance variations, shadows, color • Atmospheric perspective • Overlay cue • Parallax • Head motion • Kinetic depth effect

  21. http://www1.cs.columbia.edu/~paley/spring03/assignments/HW5/bg2020/size.jpghttp://www1.cs.columbia.edu/~paley/spring03/assignments/HW5/bg2020/size.jpg

  22. Visual cliff experiment. Are cues learned or innate?

  23. Binocular cues to stereopsis • Separation of the eyes • Accommodation • Convergence

  24. Stimulus to Stereopsis • Lateral separation of eyes= different views of the world (binocular parallax) • Elicits convergence/divergence • The magnitude of horizontal disparity for any given point is a function of the lateral separation of the eyes divided by object distance

  25. Binocular Contribution to Depth and Distance Perception • Horizontal geometric disparity is insufficient to calculate percieved distance. • Registered convergence

  26. Spatial Stereopsis Limits • Retinal disparity is not compared with all retinal points • Actually any given retinal locus only interacts with a limited area of retinal points in the fellow eye

  27. Horopter • Center of the region of stereopsis • Defined by all those points in object space that stimulate corresponding retinal points (points in each eye that retain the same sense of visual direction). • Any object not on the horopter may appear to be in different directions ot the two eyes. • It is the center of range of binocularity and region of highest stereoscopic acuity

  28. Horoptor • Crossed disparity- closer to observer than the horopter,image is observed on opposite side , (images fall temporal) • Uncrossed disparity - object is farther from observer than the horopter (images are nasal from corresponding points ) observed on same side

  29. Measuring retinal disparity • Maddox rod test • Exophoria- streak falls on temporal retina, stimulates crossed disparity • Esophoria-streak falls nasally, simulates uncrossed disparity. • Anomalous retinal correspondence_ identical sense of visual direction with very dissimilar retinal points

  30. Spatial Limits of Fusion • Occurs only when corresponding points are stimulated • Small to moderate retinal disparities are stimulated. • Pannum’s space the space around the current fixation point which can be fused, extents around 15 minarc. • Outside this region, some qualitative depth can still be perceived, but stimuli can not be fused. • Humans and many other animals sample the surrounding space by constantly changing the fixation point of their eyes.

  31. Nonspatial Limits of Fusion • As images become more dissimilar fusion is interrupted (color/shape etc). • Diplopia • Binocular rivalry • Sustained suppression • Luminance Luster • You can get partial fusion • Red green glasses…..

  32. Anomalous retinal correspondence

  33. Fine and Coarse Stereopsis • Fine: parvocellular, higher spatial frequency, smaller retinal disparities, stationary targets. Foveal vision, similar size and shape images necessary. • Coarse: magnocellular, lower spatial frequency, larger retinal disparities, moving targets, periphery, similarity of images not such an issue. (motion in depth mechanism)

  34. Local and Global Stereopsis • Stereo contributes to pattern recognition • Randot: visual system performs interocular image disparity computations across binocular visual field in a process known as global stereopsis • Stereograms: When color and contrast reveal the form, dispartiy processing limited to the immediate vicinty of the form is sufficient to reveal the depth = local stereopsis. • Fine and coarse = local • Fine= global

  35. LOCAL STEREOPSIS http://archive.museophile.org/3d/pub/tunnel.gif

  36. Stereo Acuity • Ability to discriminate very fine differences in depth from geometric disparity • Minimum geometric disparity that elicits a sensation of depth. • Fine- 2 sec arc • hyperacuity

  37. Binocular Sensory Fusion Summary • Single precept from two ocular images • Ensures visual space represents object space. (no diplopia)

  38. Binocular Motor FunctionEye Movements • support foveal vision • Saccades/smooth pursuits/vergence • support stable retinal imagery • Vestibular/optokinetic • Vertical eye movements • Supranuclear level- different from horizontal • Torsional eye movements • Compensate for head tilts • Hering’s law- motor embodiment of cyclopean vision

  39. Horizontal Vergence Movements • Tonic, +/- • Accommodative, +/- • Proximal, +/- • Fusional, +/- Maddox, 1893

  40. Disparity Vergence (allows stereo) • Vergence innervation is stimulated by retinal disparity • Corresponding retinal points are not stimulated • Purpose is to place targets of interest on the horopter to maximize steropsis • Coarse –v- Fine innervation

  41. + S S Verg. Resp. Disparity (MA) Pannum’s Area gain EOMs retina Adapt. tonic - Feedback Control of Disparity Vergence and Fixation Disparity

  42. Adapt. tonic - + S S S S Verg. Resp. Reflex Accom Disparity (MA) Stimulus (D) Depth of focus Pannum’s Area gain gain EOMs Cm/lens retina + retina Adapt. tonic - Convergence Accommodation CA/C

  43. Adapt. tonic - + S S S S Verg. Resp. Reflex Accom Disparity (MA) Stimulus (D) Depth of focus Pannum’s Area gain gain Cm/lens EOMs retina + retina Adapt. tonic - Accommodative Convergence AC/A CA/C

  44. Adapt. tonic - + AC/A S S Prox Accom gain S S Verg. Resp. Reflex Accom Pannum’s Area Depth of focus gain gain EOMs Cm/lens Stimulus (D) Disparity (MA) Percieved Nearness retina retina + Prox Vergence gain Adapt. tonic - CA/C Proximal Vergence

  45. Accommodative insufficiency Accommodative excess Accommodative infacility Convergence Insufficiency Convergence Excess Pathophysiology of common binocular anomolies

  46. Accommodative Insufficiency • Accomodation is about 2D lower than age expected norms • Ill sustained/fatigue • Paralysis or paresis • Unequal

  47. Accommodative Excess • Accommodative response is greater than expected • Accommodative spasm • Usually presents with convergence insufficiency

  48. Accommodative Infacility • Dynamics of latency/ velocity/ time constant are slower than normal • Accommodative amplitudes are normal • Difficulty changing focus from far to near and back. • flippers

  49. Convergence Insufficiency • High near exophoria, low AC/A ratio, low positive relative convergence, excess accommodation at near • Poor or nonexistant convergent adaptation • Reduced accommodative vergence=low ac/a • Abnormallly high convergence accommodation

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