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VISUAL ROLE IN DYSLEXIA...
VISUAL SYSTEM = EYES + VISUAL PATHWAYS + BRAIN
When considering the visual system's role in Dyslexia, it is important to understand the Visual Perceptual Hierarchy in order to determine where the child is having difficulties....
VISUAL IMAGERY:
* easiest
* backbone of all visual memory skills
* ability to accurately create a mental picture of a visual stimulus
VISUALIZATION:
* ability to manipulate/rotate a visual image
* "seeing something in your mind" from symbols or cues
VISUAL MEMORY:
* ability to recall and accurately identify foreign visual stimuli that were rapidly presented
VISUAL SEQUENTIAL MEMORY:
* ability to remember a chain of foreign symbols in the order they were presented
VISUAL RECALL:
* most difficult: most cognitively demanding skill
* ability to recreate (FROM SCRATCH!!) a foreign visual image that was presented for only a few seconds
As mentioned earlier, there are different types of Dyslexia which have different symptoms and involve different areas of the brain. The two most common types are the Auditory-Liguistic type (Dysphonesia) and the Visual-Spatial type (Dyseidesia). Here is a more detailed summary of the two...
AUDITORY-LINGUISTIC (DYSPHONESIA)
* more prevelant
* inability to distinguish phonemic (smallest) units of speech
* inability to learn the relationships between appearances and sounds of letters and words
* poor auditory discrimination, auditory sequencing, auditory focus and auditory emotional perception
* maintains strong preference for leisure and academic activities that involve minimal listening skills
* can comprehend more efficiently when material is read silently rather than orally
* prefers learning from silent media (films) rather than audio media (tapes)
* has difficulty recalling everyday words in conversation
* takes wild guesses recalling everyday words in conversations
* has extreme difficulty blending letter sounds
* remembers better when shown what to do rather than when told what to do
* experiences difficulty in remembering information
* makes many spoonerisms (words that sound alike)
* does not follow oral directions well
* omits vowels in two syllable words
* substitutes vowels
VISUAL-SPATIAL (DYSEIDESIA)
* less common
* appear to have normal language development
* unable to learn spatial and visual requirements needed to acquire reading skills
* poor visualization, visual memory, visual discrimination, visual sequencing, visual figure-ground discrimination, visual depth perception and visual emotional perception
* limited sight word vocabulary and an over-reliance on phonetic decoding
* consistently uses finger or marker to maintain place while reading
* has difficulty copying information
* mixes up capital and small letters when writing
* often fails to notice changes in the environment
* has difficulty reading maps
* has a difficult time remembering directions when walking
* spells better aloud than in writing (often given correct letters just in the wrong sequence)
* has difficulty describing visual characteristics of familiar people and places
* prefers listening to audio tapes rather than watching films
* experiences difficulty when copying math signs and figures
* omits words (and lines of words) when reading
* excessive vocalization during silent reading
* has a better memory for what is said than for what was read
It is also important to note that there are some refractive and binocular visual conditions that are associated with development of a learning disability. Such visual conditions that create such a risk for developing a learning disability are:
1. HYPEROPIA &/or POOR NEAR VA
2. AMBLYOPIA/ANISOMETROPIA
3. OCULOMOTOR DYSFUNCTION
4. ACCOMODATIVE DYSFUNCTION
5. CONVERGENCE DYSFUNCTION
6. "AGAINST THE RULE" ASTIGMATISM
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VISION and READING...There are two sub-systems of the visual system that are involved while reading. The MAGNOCELLULAR (TRANSIENT) PATHWAY, also known as the "where" system, primarily detects motion (in the periphery) by detection of changes in an object's shape or position. The PARVOCELLULAR (SUSTAINED) PATHWAY, also known as the "what" system, primarily detects detailed patterns of stationary objects in the central vision.
MAGNOCELLULAR (TRANSIENT) PATHWAY
* bigger, faster and shorter system
* "where" system
* most sensitive to LOW spatial frequencies (for contrast sensitivity); more sensitive to LOW contrast
* most sensitive to HIGH temporal frequencies (more sensitive to movement or flickering stimuli)
* effectively "color blind" BUT may be enhanced by BLUE light and inhibited by RED light
* responds only at stimulus ONSET and OFFSET
* predominates in the periphery
* inhibits Parvocellular pathway
PARVOCELLULAR (SUSTAINED) PATHWAY
* smaller, slower and longer duration sytem
* "what" system
* most sensitive to HIGH spatial frequencies (for contrast sensitivity); more sensitive to HIGH contrast
* ability to distinguish between colors
* responds throughout stimulus presentation
* predominates in central vision
* inhibits Magnocellular pathway
It is important to note that BOTH high and low spatial frequencies are needed for good depth perception and normal vision.
There are four basic components to eye movements while reading...
1. SACCADES: the eye movement itself
2. FIXATIONS: intersaccadic interval
3. REGRESSIONS: right-to-left eye movements
4. RETURN-SWEEPS: going from the end of one line to the beginning of another
The two most basic eye movements while reading are saccades and fixations which are both driven by cognitive processing. When reading, the sustained system is responsible for the fixational component which is when the central detailed information is extracted from the reading material. Then, a saccade is made (by the transient system) in order to bring new information into sight for fixation and for extraction of this new information. It does appear that these two systems inhibit the actions of one another. In other words, when one system is activated the other system is turned "off" and is shut down.
Some people believe that it is an oculomotor dysfunction that is the key to Dyslexia. Such symptoms of an oculomotr dysfunction would include:
1. Loss of place while reading
2. Re-reading words or lines of text
3. Word omissions, additions or word transpositions
4. Uses finger or marker to keep place while reading
5. Return sweep confusion (trouble finding beginning of new line after finishing reading previous line)
6. Illusory text movement (text appears to move)
7. Reduced reading rate and poor comprehension
8. Asthenopia
9. Generalized fatigue
There are five major types of reading problems...
1. FAULTY WORD IDENTIFICATION
* poor visual analysis of words due to lack of visual perceptual information processing skills to visually identify a word
* limited phonic knowledge
* poor sight word vocabulary
* excessive locational errors
2. INAPPROPRIATE DIRECTIONAL ATTACK
* orientational word confusion
* word transpositions (reading words in the wrong order)
* faulty eye movements (regressions: right-to-left eye movements)
3. DEFICIENCIES IN BASIC COMPREHENSIVE ABILITIES
* poor comprehension of words
* lack of paragraph or section organization
4. DEFICIENCIES IN BASIC STUDY SKILLS
* poor map, graph & table reading skills
* poor organizational skills (of materials to read)
5. DEFICIENCIES IN RATE OF READING WITH COMPREHENSIVE
* poor sight word vocabulary
* poor vocabulary knowledge
* over-analytical reading (reads word-by-word while trying to decode EVERY word phonetically instead of using visual processing which is faster)
* poor use of context cues
* subvocalizations (reads the words out loud in a very soft voice so that they can HEAR the words which can assist in word recognition)
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NORMAL vs DISABLED READERS...When testing the performance patterns of Dyslexic readers, many differences were found from that of normal readers.
"Normal" (adult) readers have a normal fixation length of approximately 1.1 to 1.3 words (~14 letters or 1.4 degrees). They also tend to fixate toward the beginning of the word (in order to get a cognitive idea of what the word is) and then they move on.
Dyslexic readers appeared to have more regressions (right to left eye movements), longer fixation durations (due to slower cognitive processing), an increase in the number of fixations made and greater difficulty with return-sweeps. It was also found that their saccadic function was dramatically different than normal readers in that they made shorter saccades and more frequently. They also have more "express saccades" (very short saccades). It would seem that the Dyslexic individual tends to make a saccade to the next information before they actually get a fixation of the current information.
A few interesting findings between the two groups was that Dyslexic individuals performed better at "one word at a time" tasks than normal readers and that normals had a dramatic decrease in performance when they had to read with blue filters or slightly blurred text (which either improved the performance or had no effect on the Dyslexic readers).
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MAGNOCELLULAR PATHWAY DEFICIT... Much research has been done that connects reading disabilities to a deficit in the MAGNOCELLULAR PATHWAY of the visual system. Since the magnocellular pathway plays an important role in the neurological control processing for smooth pursuit eye movements, such a deficit could produce the illusion of text movement or overlapping text. It could also disrupt the proper timing and accuracy of saccadic eye movements, the proper spatial and temporal disposition of visual attention and temporal order of visual processing of words.
Many dyslexic readers experience a phenomena called VISUAL PERSISTENCE. This is the term given to describe what the reader experiences when the current image remains even after a saccadic movement has been made to the next image, which causes an overlap of the images (before and after the saccade) and creates much confusion for the reader! It is caused by a defective transient cut-off saccadic mechanism which therefore causes the words to "smear" into one another. It can be corrected by slowing down the sustained system (ex: with colored filters) so that the transient system can "catch up".
There is much evidence that supports this idea that disabled readers have a magnocellular pathway deficit, with the most conclusive coming from testing of contrast sensitivity, flicker frequency, VEP and smooth pursuit testing. For instance, Dyslexic readers have lower contrast sensitivity for LOW spatial frequencies, poor temporal resolution and integration, lower flicker/movement detection and have abnormal visual attention reflexes.
  
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