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Dysfunction of the Visual System SPE 516 What Can Go Wrong Acuity Field Color Perception Binocularity Cortical Visual Impairment Visual Acuity What is visual acuity? How is it measured? What conditions typically impact visual acuity? How is acuity measured? Visual acuity is measured by determined by the smallest object or line which can be seen clearly and distinguished at a set distance. Typically expressed as 20/???. Snellen Acuities. The higher the bottom number, the worse the vision Can be converted from 10/200 equivalents Acuity and Refraction What is refraction? The bending of light. How is it measured? Diopter = unit of refraction How Much Refraction Takes Place in the Normal Eye? Normal eye generates approximately 60 diopters. Each of the clear surfaces of the eye contribute at least some refraction. Amount of Refraction Contributed by Each Structure Tears 1-2 diopters Conjunctiva 3-4 diopters Cornea Acqueous humor Lens Viterous humor 40 diopters 4-5 diopters 5-8 diopters 5 diopters Conditions Which Cause Refractive Problems Myopia (nearsightedness)- too much bending power or the eye is too long Hyperopia (farsightedness)- too little bending power of the eye is too short Astigmatism (everything’s blurry) A problem with almost any structure in the eye can limit visual acuity The Lens The plasticity of the lens (manipulated by the ciliary muscle and the zonules) allows for a change in bending power (diopters) by changing its shape – This allows us to see items closer that are closer than optical infinity. This ability is lost with lens removal or replacement and later in life due to age Presbyopia Optical Infinity = 20 feet or 6 meters Less than Optical Infinity Other Issues Impacting Acuity Medication Fatigue Illness Motor Issues Environment Light Color/Contrast Space Correction of Refractive Errors The myopic eye has too much refraction (bending power): Ex: Normal = 60 and Myopic = 70 The hyperopic eye has too little refraction: Ex: Normal = 60 and Hyperopic = 50 The presbyopic eye cannot accommodate Requires addition of near lens or bifocal Visual Field 170 to 180 degrees Central and peripheral vision Central losses are the most serious Field Losses Various conditions often result in a certain type of field loss Most change is the stasis of the eye can impact retina health Anything that impacts the health of the retina will typically result in field losses. Surgical repairs of torn or fallen retinas will result in field losses Color Perception Color perception losses vary Can be total or partial The condition is typically congenital so the individual may not know what they are missing. Typically considered a minor disability – it can have a serious impact on functioning in school Color Vision Common- red-green deficits Rare-blue yellow deficits Extremely rare- Achromatopsia- loss of cone receptors resulting in complete loss of color vision Most common in boys Binocularity image. Stereopsis – using two eyes to work together to form one three dimensional Depth Perception – the ability to see the world in three dimensions and to perceive distance using both binocular and monocular clues. Balanced Eyes Esotropia – turning in Exotropia – turning out Cortical Visual Impairment CVI is disturbed or reduced vision due to various brain abnormalities CVI is a neurological disorder, which results in unique visual responses to people, educational materials, and to the environment It is a brain rather than an eye problem CVI Individuals with CVI typically have: Limited functional vision Fluctuation of visual abilities Attracted to lights and movement “look away touch” Good color vision and preference for certain colors Difficulty with clutter and complexity of visual targets Field losses CVI The individual may also have ocular impairments According to APH, CVI is the fastest growing visual impairment diagnosis today Four major causes -Asphyxia; Brain mal-development; Head injury; Infection