Stereoacuity PDF

Stereoacuity Ariette Acevedo, O.D. PPO1 Stereopsis • The ability to perceive depth or relative distance on the basis of retinal disparity clues. • Depth perception, 3D viewing • Retinal Disparity: the small difference between images projected on the 2 retinas when looking at a single object. Co...

Stereoacuity Ariette Acevedo, O.D. PPO1 Stereopsis • The ability to perceive depth or relative distance on the basis of retinal disparity clues. • Depth perception, 3D viewing • Retinal Disparity: the small difference between images projected on the 2 retinas when looking at a single object. Corresponding Retinal Point (CRP) • For a given fixation point, each point within the binocular visual field is considered to be imaged on a point in each of the two retinas. • These points have the same visual direction and are known as corresponding retinal points (CRP). • Corresponding points are a pair of points, one in each retina, having the same direction and sending impulses to the same point in the visual cortex. • The nerve fibers from the corresponding retinal points project on a single point or area in the visual cortex, causing fusion or unification. • Only one image is perceived Retinal Disparity • If an object fails to stimulate corresponding retinal points for both eyes, it stimulates disparate points, resulting in retinal disparity. • Large retinal disparity causes diplopia, but small amounts of retinal disparity result in stereopsis. • Points closer to or farther away from a fixation point are perceived as being in 3D or in depth. • Stereoscopic perception of depth exists because both eyes are separate in space. • It varies with interpupillary distance (IPD) and can be enhanced by the increase of the IPD. Retinal Disparity • Our eyes are naturally separated by ~7cm. • Due to this separation each eye has a slightly different view, even when looking at the same object. • These images are united (fused) in the brain and are seen as one single image. • Image is seen as 3D due to the added depth dimension. Stereopsis • This is the awareness of the relative distances of the objects from the observer, by means of binocular vision (BV) and based on retinal disparity. • Requirements for stereopsis: • The person must be binocular, both eyes must be looking at the same target and be healthy. • No unilateral strabismus or amblyopia as this will prevent binocularity. • Stereopsis contributes to the judgment of depth, distance and recognition of solid objects. Binocular Vision and Stereopsis • Helps perceive objects more quickly and more accurately • Better motor control, quicker and more accurate cognitive information (i.e. estimate time of collision) • Motor control (i.e. for reaching objects) • Help in fine motor tasks BV Requirements • Clear bifoveal images: for efficient and functioning BV. • At near appropriate convergence and accommodation • Accommodation and convergence: must be coordinated for clear vision and acute stereopsis. • BV impairment will result in a partial or total loss of stereopsis. • Most common cause for BV loss is strabismus, but other conditions may also cause poor BV. • Such as: anisometropia, uncorrected refractive error, nystagmus, cerebral palsy, congenital cataracts or accommodative dysfunctions. Poor Binocular Vision • Will produce: • • • • • • Fatigue Blur Headache Eyestrain (asthenopia) Decreased visual comfort Decreased overall task performance • Patients with poor BV will notice difficulties with stereopsis and other critical tasks. • In some cases may report diplopia. • Patients with strabismus may have reduced or no binocular capabilities. Depth and Distance Perception • The perception of 3D space can be subdivided in distance and depth perception. • Distance Perception: Absolute Depth Perception • How far is a given object from the observer or from another object? • Depth Perception: Relative Depth Perception • The perception of the relative nearness of one object to another, as perceived by the observer. Depth and Distance Perception • Depth perception includes many factors: • BV factors: • Retinal disparity: for precise stereopsis • Vergence alignment: for gross stereopsis • Pictorial depth cues such as: • Occlusion, linear perspective, texture, gradients, relative size, height in visual field, shadow, luminance and aerial perspective. • Monocular factors such as: • Accommodation, looming (expanding), motion parallax and kinetic depth effect. Stereopsis • Ogle: “The ability to perceive depth through stereopsis is considered innate, but the ability to perceive depth by monocular cues must be learned.” • Monocular cues: • Geometrical Perspective: roads or tracks converge in the horizon. • Overlay: an object blocking another is perceived as being the closer of the two objects. • Aerial Perspective: objects high above in the horizon are interpreted a being closer than objects near the horizon. • Light and Shadow • Size: a large object is interpreted as being closer than a small object. Monocular Cues • Looming, motion parallax and kinetic depth monocular cues stimulate depth and distance perception when motion is present. • Looming: sense of movement in depth, stimulated by changes in retinal image. • Kinetic depth: provides a sense of depth based on the differential motion of portions of the retinal images. • https://www.youtube.com/watch?v=mkhY5lANs-k • Wiggle or kinetic depth of stereopsis: moving parts of the image or picture gives the depth perception effect. • https://www.youtube.com/watch?v=gq70rca1P1I • Motion parallax: sense of depth or distance stimulated by the differential motion of retinal image of objects that are farther or nearer than the point of fixation. • As we move, objects that are closer to us move farther across our field of view than objects that are in the distance. Motion Parallax • Close on eye and hold two index fingers up, one slightly higher and behind the other. • Fixate on the far finger and move your head side to side. • Notice that the far finger appears stationary while the near finger appears to move opposite to the direction of your head. • Switch fixation to the near finger and you will notice how the far finger appears to be moving in the same direction as your head. Stereopsis • Binocular clues for perception of distance • Lateral geniculate nuclear has parvocellular and magnocellular neurons for fine and coarse stereopsis • Retinal disparity • Convergence • Accommodation • Damage to any of these areas can cause specific stereopsis problems. Fine and Coarse Stereopsis • Coarse stereopsis: responds to lower spatial frequency targets (large objects), larger retinal disparities (30-600 min of arc) and moving targets. • It is active in both foveal and peripheral vision and may also be related to coarse disparity vergence control. • Appear to be supported by magnocellular system • Fine stereopsis: responds to higher spatial frequencies (fine details), retinal disparity of less than 30 min of arc and to stationary or slowly moving objects. • Is mainly found in foveal vision and supported by parvocellular system. Titmus Stereotest • Polarized test at 40cm • Performed with correction in normal room illumination • Patient wears polarized lenses or R/G lenses • Other tests: • • • • Stereofly test Bernell Reindeer Randot Random Dot E Stereofly • 3 tests performed at 40cm • Fly test for stereopsis • Gross stereopsis of 3,000 sec of arc • Circle patterns for depth perception • 800-40 sec of arc • Animal Series (children) • 400-100 sec of arc Bernell Stereo Reindeer • The examiner can make the reindeer’s nose wiggle for gross stereoacuity measurement. Randot • Targets are generated with computer patterns to eliminate monocular cues. • 3 tests performed at 16 in. • Randot Forms Gross Test: • 600 sec of arc • Randot Animal Forms Graded: • 400-100 sec of arc • Randot Circles: • 8 grades from 400-20 sec of arc Random Dot E • One demonstration plate and 2 test plates. • Demonstration plate has an elevated (raised) E that can be seen by everyone. • Test plates require polarized glasses. • If the patient has stereopsis when showed a test plate they will recognize an “E” while the other plates will appear blank. • Present at 50cm while shuffling the testing plates before each presentation, asking them to point to the one that has the letter “E”. • This has to be repeated at least 4 times, shuffling the cards each time. • If answering correctly at 50cm then perform at 1m. • Disparity at 50cm: 500 sec of arc • Disparity at 1m: 250 sec of arc AO Vectographic Project-o-Chart Slide • Comes for adults and pediatrics • Uses the projector and phoropter polarized lenses • Retinal disparity stimulated ranges from 240-30 sec of arc Stereoacuity Test Procedure • Patient wears polarized using habitual correction. • If polarized glasses are too big use tape to keep them up. • • • • Performed at 40cm (16 in.) Overhead lamp illuminating the target but not causing reflection or glare. Both eyes open Ask the patient “which of these circles/animals is closer to you or seems to be floating?” • Keep going until the patient has 2 consecutive incorrect answers • You cannot move the book closer toward the patient, you can flip the book but not tilt. Stereoacuity Test Procedure • If the patient cannot see the circles, try the animals. • If they cannot see the animals go to the gross stereoacuity targets. • Large shapes • If patient cannot distinguish any of the gross targets it is considered the patient has “no gross stereopsis” • Once this happens suppression needs to be evaluated: • Ask the patient to report what they see in the vertical box (R/+/L) • Do not ask the patient is they can see the R/L, just ask what they see. Recording • Recording must include: • • • • Near Sc or cc Amount of stereopsis reported in sec of arc Which test was used • Example: • SA at N sc 50 sec of arc, Titmus • Stereo at N cc 3,000 sec of arc, Stereofly • If not perceived: record no stereo or no gross stereopsis. If this happens you must check for suppression. Expected • SA at near 20 sec of arc • Some tests only measure up to 40 sec of arc. • Below or worse than 40 sec of arc is considered decreased and a cause has to be determined. • Most common causes: • Uncorrected RE • Accommodative or Binocular Dysfunction • Strabismus (most common cause of poor BV)