Visual Processing Disorder and Dyslexia
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 Motion Detection in Dyslexia
On this page of the web site we concentrate on the visual difficulties associated with dyslexia and Learning Difficulties. There are also associated Phonetic or Auditory Processing Difficulties which are discussed on the Central Auditory Processing Disorder page of this web site.
The nerve endings (rods and cones) at the back of the retina relay visual infornation to the lateral geniculate body of the Thalamus. These visual signals are processed by two specialised types of cells:
- Parvocellular pathways (small cell bodies) concerned mostly with colour hues and contrast and
- Magnocellular pathways (large cell bodies) concerned mostly with movement detection.
Considerable research indicates that in Dyslexia these cell bodies may not differentiate their functions adequately. Consequently Dyslexics may have a range of visual dysfunctions and overlapping of functions between these two pathways. There is growing evidence that dysfunction in parvo and Magnocellular pathways are responsible for Visual Motion Detection Difficulties in Dyslexics and some forms of Learning Difficulties.
- The white page of a book (background contains all colours and hues) can overwhelm Parvocellular (P) cells and also activate Magnocellular (M) cells by mistake.
- Words can seem to move on the page
- The page of text can appear to be mostly blocks of blurred black (text) with rivers of white running in between.
- The orientation of letters and text may keep changing "d and b" becoming difficult to differentiate. Hence the child may reverse letters.
- Using a coloured visual filter (Irlen lenses) can help some people with dyslexia by reducing the colour that may be overwhelming or scrambling the P and/or M pathways.
- Children with Dyslexia can find it difficult to visually track text on a page as this requires fine movement and very tight eye muscle control.
- The cerebellum, a part of the brain that plans our movements a fraction of a second before we execute them, orchestrates this extremely fine motor control. Children with dyslexia often also suffer from dyspraxia (difficulties with fine and/or gross motor control (e.g. awkward gait).
Two tests, used at the Behavioural Neurotherapy Clinic, can test whether Visual Motion Detection dysfunction may be responsible for reading difficulties or suspected Dyslexia. Follow the link to see Scheuerpflug's 2003 study.
In the first test, a number of dots move randomly on the screen. The test requires the subject to press a key to indicate that he/she has detected that a percentage of the dots move towards the left or the right at the same time (coherent motion). The percentage of coherent dots is increased until the child can detect coherent motion. Children with Magnocellular dysfunction detect coherent motion at a higher percentage of coherent dots than those without visual difficulties.
In the second test, during the motion-onset task, children look at a large monitor on which a vertical sine wave grating pattern runs. The pattern is faded in and moved at 3 different velocities (2, 8,
16 degrees per second) to the right or to the left. The child has to press one of two buttons to indicate detection of motion (left or right).
Electrophysiological recordings detect the brain signals associated with movement detection and these are compared to those seen in Scheuerpflug's 2003 study.
This methodology is used to establish whether Magnocellular pathways may be involved in learning difficulties and to evaluate the effects of nutritional treatment and/or behavioural optometry exercises prescribed.
Visual Processing Disorders and Learning Difficulties
Visual processing disorders, which are NOT related to the ability to see clearly, involve difficulties understanding visual information such as movement, spatial relationships, form, or direction. Visual processing disorders, together with Central Auditory Processing Disorders, frequently result in dyslexia or poor academic performance.
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.
Visual Processing Assessment
A Behavioural Optometrist working in conjunction with the Clinic carries out assessment for Visual Processing Disorder. It is important that the assessment be thorough and identifies the specific dysfunction that can then be addressed by a combination of the most effective Interventions.
Visual Processing Interventions
A successful intervention must be based on identifying the specific types of visual processing problems that exist. Since each assessment activity is designed to address a specific processing problem, interventions must be designed for a specific individual based on standardised test results. Typically, interventions involve daily eye exercises, work with specific computer programs, Neurotherapy, physical games and activities, and academic adjustments. The behavioural Optometrist will make recommendations for optometric exercises and the Behavioural Neurotherapy Clinic may recommend Nutrient Supplements and/or Neurotherapy protocols to retrain relevant brain areas associated with visual processing that are identified through Brainmapping as having less than optimum function.