The Visual System is the part of the nervous system which allows organisms to see. It interprets the information from visible light to build a representation of the world surrounding the body. The visual system has the complex task of (re)constructing a three dimensional world from a two dimensional projection of that world. The psychological manifestation of visual information is known as visual perception.
The visual cortex is the most massive system in the human brain and is responsible for higher-level processing of the visual image. It lies at the rear of the brain (highlighted in the image), above the cerebellum. The interconnections between layers of the cortex, the thalamus, the cerebellum, the hippocampus and the remainder of the areas of the brain are under active investigation.
Visual processing disorder refers to a reduced ability to make sense of information taken in through the eyes. This is different from problems involving sight or sharpness of vision. Difficulties with visual processing affect how visual information is interpreted or processed. A person with visual processing problems may have 20/20 vision but may have difficulties discriminating foreground from background, forms, size, and position in space. The person may be unable to synthesise and analyse visually presented information accurately or fast enough. The eyes look and the brain sees.
Visual problems related to sensory processing can take many forms. Often children cannot find what they are looking for. Their writing may appear untidy, they have difficulty staying on the line and spacing out their work. They may also be visually distracted when there is too much on the walls in the classroom or by movements.
Difficulties in visual closure can be seen in such school activities as when the young child is asked to identify, or complete a drawing of a human face. This difficulty can be so extreme that even a single missing facial feature (a nose, eye, mouth) could render the face unrecognisable by the child.
The ability to perceive the location of objects in relationship to other objects is a critical skill in reading, math and handwriting, where a child must be able to recognise the different symbols, perceive their direction, tell the difference between similar shapes and determine where these are located in relationship to each other. Individuals who have difficulty with spatial relationships may seem unusually clumsy or accident prone may have difficulty reading or may refuse to read, or may have poor handwriting (dysgraphia).
Whether it is the differentiation of the shape of a circle from a square or the letter b from d, the ability to perceive the shapes of objects and pictures is an important skill for the developing child to acquire. There is hardly an academic activity that does not require the child to engage in form discrimination. The most obvious classroom activity requiring the child to discriminate forms is that of reading. The learning of the letters of the alphabet, syllables, and words will undoubtedly be impeded if there is difficulty in perceiving the form of the letters, syllables, and words.
Visual discrimination allows us to tell the difference between similar objects, tell where one object ends and another begins, and to recognise objects and symbols when only part of it can be seen (or when it is fuzzy). Individuals who have visual discrimination disorders often mix up letters or numbers and have difficulty reading or scanning pictures for information.
Visual memory is a critical part of academic skills. It allows us to recognise objects and to remember letters, numbers, symbols, words, and pictures. In cases of partial visual agnosia (visual access problems), what is learned on day one, "forgotten" on day two, may be remembered again without difficulty, on day three.
Some individuals have difficulty perceiving or integrating the relationship between an object or symbol in its entirety and the component parts which make it up. Some children may only perceive the pieces, while others are only able to see the whole. Children with a visual integration disorder will have difficulty learning to read (dyslexia) and recognising symbols.
Visual Pursuit and Tracking:
The ability to track moving objects while seated or standing, and the ability to keep a stable visual image when the head or eyes are in motion is part of the ocular-vestibular system. Individuals who have visual pursuit disorders have difficulty watching moving objects (e.g. on videos or computer programs), difficulty copying from the board, and difficulty reading.
SIGNS OF VISUAL INPUT DYSFUNCTION (no diagnosed visual deficit):
1. HYPERSENSITIVITY TO VISUAL INPUT (over-responsiveness)
- sensitive to bright lights; will squint, cover eyes, cry and/or get headaches from the light
- has difficulty keeping eyes focused on task/activity he/she is working on for an appropriate amount of time
- easily distracted by other visual stimuli in the room; i.e., movement, decorations, toys, windows, doorways etc.
- has difficulty in bright colorful rooms or a dimly lit room
- rubs his/her eyes, has watery eyes or gets headaches after reading or watching TV
- avoids eye contact
- enjoys playing in the dark
2. HYPOSENSITIVITY TO VISUAL INPUT (under-responsive or difficulty with tracking, discrimination, or perception):
- has difficulty telling the difference between similar printed letters or figures; i.e., p & q, b & d, + and x, or square and rectangle
- has a hard time seeing the “big picture”; i.e., focuses on the details or patterns within the picture
- has difficulty locating items among other items; i.e., papers on a desk, clothes in a drawer, items on a grocery shelf, or toys in a bin/toy box
- often loses place when copying from a book or the chalkboard
- difficulty controlling eye movement to track and follow moving objects
- has difficulty telling the difference between different colors, shapes, and sizes
- often loses his/her place while reading or doing math problems
- makes reversals in words or letters when copying, or reads words backwards; i.e., “was” for “saw” and “no” for “on” after first grade
- complains about “seeing double”
- difficulty finding differences in pictures, words, symbols, or objects
- difficulty with consistent spacing and size of letters during writing and/or lining up numbers in math problems
- difficulty with jigsaw puzzles, copying shapes, and/or cutting/tracing along a line
- tends to write at a slant (up or down hill) on a page
- confuses left and right
- fatigues easily with schoolwork
- difficulty judging spatial relationships in the environment; i.e., bumps into objects/people or missteps on curbs and stairs