lecture 9

Category: Education

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Stereopsis is measured in seconds of arc ( 1° = 60 minutes of arc ; 1minute = 60 seconds of arc ). It is useful to remember that normal spatial VA is 1 minute and normal stereo-acuity is 60 seconds ( which equals 1 minute ). The lower the value the better the acuity . Various tests are employed using different test principles. Random dot tests ( TNO, Frisby ) provide the most definitive evidence of high grade BSV . TESTS FOR STEREOPSIS

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Where this is weak and / or based on ARC , less dissociation contour based tests ( e.g. Titmus ) may give more reliable evidence of stereopsis . TNO The TNO random dot test consists of 7 plates of randomly distributed paired red and green dots which are viewed with complementary red-green spectacles . Within each plate the dots of one colour forming the target shape ( squares , crosses etc. ) are displaced horizontally in relation to their paired dots of the other colour so that they have a different retinal disparity from those outside the target .

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Some control shapes are visible even without red-green spectacles , while the test targets are only visible to an individual with stereopsis , while wearing red-green spectacles . The first 3 plates are used to establish the presence of stereopsis vision and subsequent plates to identify it . Because there are no monocular clue , the TNO test provides a truer positive measurement of stereopsis than Titmus test , but can give false negative errors when fusion is poor .

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The disparities measured range from 480 to 15 seconds of arc tested at 40 cm . Most children are able to do this ( and the Frisby test ) from the age of about 4 years.

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The Frisby stereotest consists of 3 transparent plastic plates of varying thickness . On the surface of each plate are printed four squares of small randomly distributed shapes . One of the squares contains a hidden circle , in which the random shapes are printed on the reverse of the plate . The patient is required to identify this hidden circle . The test does not require special spectacles because the disparity is created by the thickness of the plate and can be varied by increasing or decreasing the working distance , which need to be accurately measured . FRISBY

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The disparities measured range from 600 – 15 seconds of arc . It is important not to allow the subject to tilt the plate or move their head during testing as this gives monocular clues .

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The Lang stereotest does not require special spectacles ; the targets are seen alternately by each eye through the built-in cylindrical lens elements. Displacement of the dots creates disparity and the patient is asked to name or point to a simple shape , such as a star , on the card . The Lang test can be used to assess stereopsis in very young children and babies who may reach out to touch the pictures . The examiner can also observe the child’s eye movements from picture to picture on the card . However , the cards must be held exactly parallel to the plane of the face for the effect to be seen and the Frisby screening test may be superior for demonstrating stereopsis . LANG

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( e.g. to confirm BSV in infants with suspected squint ). The degree of disparity is quite gross , ranging 1200- 600 seconds of arc at 40 cm.

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The Titmus test cosists of a three-dimensional Polaroid vectograph consisting of two plates in the form of booklet viewed through Polaroid spectacles . On the right is a large fly , and on the left is a series of circles and animals . The test is performed at a distance of 40cm . TITMUS

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1- FLY is a test of gross stereopsis ( 3000 seconds of arc ) , and is especially useful for young children . The fly should appear to stand out from the page and the child is encouraged to pick up the tip of one of its wings between finger and thumb . In the absence of gross stereopsis the fly will appear as an ordinary flat photograph . If the book is inverted , the targets will appear to be behind the plane of the page. If the patient states that the fly’s wings are still popping out , then they are not appreciating true stereoscopic vision .

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2- CIRCLES comprise a graded series which tests fine depth perception . Each of the nine squares contains four circles . One of the circles in each square has a degree of disparity and will appear forward of the plane of reference in the presence of normal stereopsis. The disparities measured range from 800-40 seconds of arc . If a patient perceives the circles to be shifted to the side , then they are not appreciating stereoscopic vision , but are using monocular clues instead .

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3- THE ANIMALS Are similar to the circle test but consist of three rows of five animals , one of which will appear forward of the plane of reference . The degree of disparity ranges from 400-100 seconds of arc . 4- FRISBY-DAVIS DISTANCE STEREOTEST This consists of a large cube with an open front through which four small objects are visible . Testing is usually performed at 6 meters . The patient has to decide which of four objects within the box is closest to them .

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