11. The Space Sense III 2223.pptx

The Space Sense III: Fusion, suppression and stereopsis (clinical tests) Introduction. Objectives and Readings. Sensory fusion and suppression. Assessment of stereopsis. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Describe how to assess • Elliott. Clinical...

The Space Sense III: Fusion, suppression and stereopsis (clinical tests) Introduction. Objectives and Readings. Sensory fusion and suppression. Assessment of stereopsis. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Describe how to assess • Elliott. Clinical Procedures in Primary Eye care (ebook). – Pgs. 429-436 (fusion). – Pgs. 437-450 (stereopsis) sensory fusion. • Explain under what circumstances sensory suppression may occur. • Describe differences and similarities between commonly used stereotests. SFO1004 • Fricke T. & Siderov J. Stereopsis, stereotests, and their relation to vision screening and clinical practice. Clinical and Experimental Optometry, 1997; 80(5): 165-172. • #https://doi-org.libaccess.hud.ac.uk/ 10.1111/j.1444-0938.1997.tb04876.x Dr Sarah J Waugh© Recall – Fusion • Is the process that invokes both a neurophysiological and perceptual (psychological) response by which the brain combines the two images (one from each eye) into a single percept. • Requires that the images fall on (near) corresponding points in the two eyes (within the limit of Panum’s area). SFO1004 • Panum’s area determines the size of the region in space where there is binocular single vision. Dr Sarah J Waugh© Suppression – what causes it? • A binocular imbalance – Differential blur (one eye is more blurred than the other). – Differential visual acuity (one is poorer visual acuity). – Differential eye alignment (the two SFO1004 eyes are not – Difference in refractive error (we also use this property in binocular refraction). – Including conditions such as amblyopia and others that reduce VA. – Strabismus (heterotropia) or large heterophoria that is outside of the fusional vergence ability. Dr Sarah J Waugh© Suppression – how is it measured? • Tests for suppression must present both monocular and binocular information to the two eyes in the presence of binocular viewing. • Use a test that separates elements to create different images to each eye. SFO1004 • Clinical tests typically use filters. – Polarising. – Anaglyphs (e.g., red/green). • With the filters on, part of the target object is seen by the right eye and part by the left. Other parts are seen by both eyes. Dr Sarah J Waugh© Suppression – Worth (4-dot) and variants • Four different coloured targets (‘dots’). – Red. – Green. – White. • With red and green filters. – Eye with the red filter sees 2 dots. – Eye with the green SFO1004 filter sees 3 dots. Dr Sarah J Waugh© Worth (4-dot) – possible outcomes 4 dots seen fusion 2 dots seen suppressio n 3 dots seen suppressio n 5 dots seen -? RE SFO1004 LE Suppression – Worth (4-dot) and variants • Thomson chart variant – Although 4-dots and red/green filters are used there is a variation in the colours and hence the appearance of the dots. – Nevertheless, the principles are identical. SFO1004 Dr Sarah J Waugh© Suppression – red/green methods • Utilise tests for eye alignment in this case the fixation disparity. • Under normal fusion, with red/green glasses worn, both top and bottom lines are seen. • With suppression, only one line is seen depending on the suppressing eye. SFO1004 Distance Thomson target Dr Sarah J Waugh© Suppression – polarization methods • Utilise tests for eye alignment in this case the fixation disparity. • Under normal fusion, with polarising glasses worn, both sets of horizontal and vertical lines are seen. • With suppression, only one half is seen depending on the suppressing eye. SFO1004 Distance projector target Dr Sarah J Waugh© Suppression – polarisation methods • Utilise tests for eye alignment in this case the fixation disparity at near. • With polarising glasses worn, both the top and bottom line is seen. • With suppression, only one (top or bottom) is seen depending on the suppressing eye. SFO1004 Near Mallet type unit Enlarged Dr Sarah J Waugh© Suppression – polarisation methods • Mirror – polarisation or mirror-pola test • Patient wears polarising glasses and views reflection in a mirror. • With suppression, only one eye is seen depending on the suppressing eye. The other eye appears ‘black’ (or darkened). SFO1004 Dr Sarah J Waugh© Suppression – other methods • Bagolini striated lens • Lenses with small parallel striations engraved on them. • When viewing a spot of light, the striations create an a line image (similar in a way to the Maddox Rod). • Fusion is seen when the lines cross (X) with the spot in the middle. SFO1004 fusion Dr Sarah J Waugh© Interval • This is a good time to pause and reflect on the first section of the presentation. • When you are ready, continue to the next series of slides. SFO1004 Dr Sarah J Waugh© Recall: Perception of depth – stereopsis • The binocular perception of depth in the absence of monocular cues. • Stereopsis is the binocular perception of relative distances between objects that results from the relative horizontal retinal disparities between the monocular images. SFO1004 • The difference in depth of two objects produces ‘retinal disparity’. This is the cue that allows stereoscopic depth Dr Sarah J Waugh© perception to occur. Stereopsis – how is it measured? • Tests for stereopsis must present the target in such a way that the image seen in each eye creates a relative retinal disparity. • Use a test that separates elements to create different images to each eye. SFO1004 • Clinical tests typically use filters. – Polarising. – Anaglyphs (e.g., red/green). • Other methods include – Real depth objects Dr Sarah J Waugh© Stereopsis – contour type tests • Titmus Fly and Wirt circles. • A commonly used test. • Requires polarising filters to observe the depth. • Range of disparities tested from 3000” to 40”. • Animal images and Fly for children. • Also has suppression check. SFO1004 Dr Sarah J Waugh© Stereopsis – contour and random-dot type tests • Randot test. • Requires polarising filters to observe the depth. • Range of disparities tested from 500” to 20”. • Animal images and shapes for children. • Uses random-dots. • Also has suppression check. SFO1004 Dr Sarah J Waugh© Stereopsis – random-dot type tests • TNO test. • Red/green anaglyphic. • Demonstration and suppression plates • Find the missing ‘cake’ portion • Lowest stereoacuity to 30” (in latest version). • May be too challenging for young children SFO1004 Dr Sarah J Waugh© Stereopsis – random-dot type tests • Frisby test – real depth so filters not required. • A series of random elements painted on one surface in squares. • On one square some of the elements (in the shape of a circle) are painted in the back surface. • Different thickness plates create different disparities SFO1004 Dr Sarah J Waugh© Stereopsis – random-dot type tests • Lang test (I and II) – filters also not required. • A random-dot test using small cylindrical lenses to separate the images to each eye. • Patient identifies the shapes (star, car and cat in Lang I). • Only measures gross levels of stereopsis. SFO1004 Dr Sarah J Waugh© Questions? • Remember you can ask questions via Brightspace discussion board or the chat function on Teams. SFO1004 Dr Sarah J Waugh©