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The ABC’s of Vision and AAC

For Consumers: The ABC’s of vision in AAC

More people experience visual impairment (VI) than any other type of functional loss. If you haven't yet, just wait! Presbyopia lurks around that 40th corner!

VI is a term used to describe visual acuity between 20/70 and 20/200. Legally blind means visual acuity of 20/200 or less in the better eye with corrective lenses, or a visual field loss of 20 degrees or more.²

Legal blindness is an old-fashioned concept, rooted in thee premise that vision much below normal is useless.

Nearly 40 million people world-wide are classified as legally blind although most (80%) have some residual vision that may be useful. Approximately two-thirds of all people with VI are over 65 years of age. The prevalence of VI and blindness among minorities is high-the rate for African-Americans is double that of whites of comparable socio-economic status. Many individuals with multiple impairments have VI. Between 75 and 90% of school-aged children with severe/profound cognitive disabilities and approximately 40% of those with cerebral palsy (CP) have visual problems. Functional vision-vision that is reasonably useful-requires a fairly intact visual system and the motivation, experience, and understanding a person brings to the "seeing" task.

The visual system

The visual system is complex. The process of seeing involves a sequence of events-the reception of light and sensory stimuli through the eye, the transmission of electrical impulses along the optic nerve and the interpretation of these impulses as an image in the visual cortex of the brain. The anatomic structures involved are interrelated and very complex. Optometrists, ophthalmologists, VI specialists and manufacturers as well as some psychologists, educators, administrators and families, focus on solving the complex problems of individuals who are blind or visually impaired.

What can go wrong?

Low vision and blindness limit the quality and quantity of a person's experiences. Difficulties may originate in the cornea, lens, retina, optic nerve, brain stem and/or other parts of the visual pathway up to and including the visual cortex. Common visual impairments include:

• Nearsightedness. Can focus up close, but vision is blurred at a distance.
• Farsightedness. Can focus at a distance, but vision is blurred up close.
• Astigmatism. Visual image is distorted. Usually accompanied by nearsightedness or farsightedness.
• Cataracts. Lens becomes opaque, obstructing part or all of view.
• Glaucoma. Peripheral vision diminishes. Can cause total loss of vision.
• Detached retina. Retina comes loose causing blindness or blind spots.
• Macular degeneration. Failure of the small region in center of retina causing blind spots. Can interfere with fine discrimination needed for reading and using graphic symbols.
• Strabismus. Convergence and muscle imbalance resulting in poor focus or double vision makes focusing, fixing and tracking more difficult. Binocularity occurs in many children with CP.
• Amblyopia. Reduced vision from lack of use or lack of clarity of vision during early childhood. A consequence of strabismus.
• Hemianopia. Lack of peripheral vision on one side of the visual field of both eyes. Requires active scanning of visual information.
• Visual field defects. Blind spots which result in a lack of awareness (neglect) of objects. Requires active scanning of visual information.
• Nystagmus. Oscillations or tremors of the eyes occurring independently of normal eye movements.

Finding solutions

More than one type of visual problem can occur so it can take years to figure out the functional vision of multi-handicapped individuals who are unable to speak. In addition, a range of accommodations can be made; and more and more visual problems are now "fixable" using less invasive techniques. Be sure to check with a knowledgeable developmental or behavioral optometrist. Table I depicts components of vision that have an impact on the selection and use of AAC techniques.

Visual acuity. Impaired acuity, with a variety of etiologies, is the most common visual problem. Visual acuity allows us to discriminate details close up and far away. Acuity impairment classifications vary from partially sighted to totally blind and include visual field defects.

AAC accommodations. Just because you have a good map of a person's visual fields doesn't mean you know about functional vision. Check to see how "blind spots" affect function. Adjustments in positioning of symbols and displays and in mounting devices may be necessary.

Oculormotorfunctioning. Eye muscles allow people to scan, locate, fixate and track moving objects. People may need to shift their bodies to make accommodations. If motor problems or positioning constraints interfere, seeing can be difficult.

AAC accommodations. Pay attention to the design of displays. Adjust the positioning of a person and/or equipment. Sometimes the angle of a display is critical. How the person tilts his/her head also can make a difference.

Light and color sensitivity. Color and illumination factors can vary with the type of VI. For example, as we age and presbyopia occurs, more light is required to see. Sensitivity to certain colors may be depressed depending on the visual impairment, but color blindness is rare. AAC accommodations. Appropriate use of color and lighting enhances acuity and makes perception easier. Illumination on a display or device screen must be adequate and without glare. Sunny days can be a problem. Back lighting is important. Color provides contrast. Yellow backgrounds are often better than white.

Cortical vision. Cortical visual impairment (CVI) occurs with damage to visual pathways leading to and including the visual cortex. It is generally caused by a lack of oxygen to the brain (anoxia). Prematurity is a major etiology of CVI as more fragile babies are surviving. CVI also occurs following anoxic events associated with head trauma, hydrocephalus, meningitis and encephalitis. Studies suggest a gradual visual recovery extended over several months to years in people who acquire CVI. Those who have congenital CVI have more difficulty because learning is so dependent on vision and other problems associated with brain damage are often present. Clinical symptoms of CVI include visual inattentiveness and a lack of visual acuity. Because eye movements are not affected, individuals may not appear to have impaired vision.

AAC accommodations. Individuals who are unable to attach meaning to visual information remain severely compromised in learning, language development and communication. CVI will interfere with the use of AAC system components. Children with CVI often benefit from auditory scanning and motor experiences that allow them to interact with and learn the meaning of objects, events and people. Motor memory (i.e., the mental map we use to carry out our rote movements) may be critical to an individual with cortical blindness to establish meaning. A good source of information is the deaf-blind literature.

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