Dyslexia

We read with our ears

Children acquire language by listening to those around them talking. In the first year of life they are building an ever-increasing store of speech sounds. This store is phonological memory — the units of sounds that make up words. If these sounds are stored in phonological memory in a faulty manner, the child’s perception of speech will be compromised, as will reading and spelling. Research by Paula Tallal shows that Dyslexic (and language impaired) children are unable to perceive fast sounds. These are the stop consonants that change to the vowel frequency before 40 milliseconds. Consonants such as b, t, k, d not perceived by the slow sensory processing system of the average Dyslexic and consequently auditory nerves are not stimulated into action in the same way. Many speech sound distinctions are lost.

Tallal, P. (1980) "Brain & Language" (9)
Tallal P & Piercy, M (1973) Nature, (241)
Nagarajan S et al "Cortical auditory processing in poor readers"
Proceedings National Academy of Sciences, vol. 96, issue 11, (1999)
 

This abnormal auditory processing is due to smaller neuronal fields in the left medial geniculate nuclei (MGN) according a post-mortem study of Dyslexics (Galaburda A et al, 1994, "Evidence for Aberrant Auditory Anatomy in Developmental Dyslexia" in Proceedings of the National Academy of Sciences, Vol. 91), and backed up by brain imaging studies showing the "knock-on" effect of auditory inefficiencies are weak phonological processing in Broca’s area (left frontal gyrus) which is often the target of stroke damage, which suggests this area is responsible for speech articulation. When speech is lost due to damage here it is called aphasia.

Other research suggests a timing circuit throughout the brain that simultaneously identifies letters (in the visual cortex) while the phonological analysis is progressing. After this meaning is mediated by the superior temporal gyrus and parts of the middle temporal and supramarginal gyri.

(Shaywitz S. "Dyslexia" Scientific American, Nov 1996).
Tallal P. The Science of Literacy Proceeding National Academy of Sciences 97: 2402
 

Some Dyslexics show a pattern of under activation in the visual regions with a corresponding over activation in the phonological regions, when measured by Qeeg (quantitative eeg, a measure of brain activation). There seem therefore to be both sub-types and developmental stages of Dyslexia. The consistency of the brain based under and over activation in certain areas suggests a genetic causation and this is backed up by studies of families where Dyslexia affects generation after generation.

But early middle ear infections too, can cause a child to perceive speech sounds unevenly so that some are heard before others and the whole auditory system can be mistimed, and sounds misequenced. Thus there is a delay (in milliseconds) in the perception of speech sounds, which others notice as a time lag between their speaking and the child’s response. This affects the rate at which phonemes are matched to the syllable to recognize a word, the "inner voice" can’t keep up with the eyes and reading is inefficient.

Another environmental cause is premature birth This also is highly correlated with difficulties in the acquisition of literacy since the auditory nerve active in the last trimester, is not activated and primed as much as it would be if the baby was full term, especially if the baby is put into an incubator which cuts off sound. This has effects on the perception of speech sounds. Another environmental cause can be early exposure to the sounds of a second language before the child is secure about the sounds of the first language. While most children manage this quite successfully and go on to become bi-lingual, if there is any inefficiency in auditory perception it can inhibit the acquisition of literacy.

 

And our Eyes must synchronize

Slowness in processing affects all the senses in Specific Learning Difficulties. Perception refers to the interpretation the brain makes of information from the senses. If the senses can’t convey a rapid feedback due to lack of neurons devoted to the function then information is mistimed and misequenced.

In fact most Dyslexic children have been taken to have their hearing and eyes checked early on, only to be told by the optician and audiometric Ian that there is nothing wrong with their eyes or ears. This is sometimes bad news for them — their parents or teachers may make a negative judgment — laziness or stupidity or any number of reasons for their failure in learning to read. The accounts children give of blurring print, losing lines in reading text or music, headaches on reading, not being able to see the blackboard, copy fast enough or listen to the teacher are all hard for a parent to interpret after being told there is nothing wrong at the physical level. But these complaints are common and are due to inadequate processing of visual, auditory and kinesthetic information.

In fact up to 25/30% of children may have light sensitivity and colour based visual perceptual problems. The current opinion is that at least some of the observed problems (glare off the page, moving and blurring of text, sore, watery eyes on reading, losing lines, needing to reread constantly to get the sense,) are due to an analogous deficit (to the auditory problems) in the visual pathways. The lateral Geniculate nucleus has been identified as smaller in cellular content in Dyslexics than in normal readers by Margaret Livingstone at Harvard University "Physiological and anatomical evidence for a magnocellular defect in developmental Dyslexia" Proc. Natl.Acad. Sci. 1991:88.

Most visual information moving from the retina via the lateral geniculate nucleus of the thalamus travels through one of 3 visual pathways. One of these, the magnocellular is thought to carry visual information about space — such as movement, depth and the relationships between them. The magnocellular is thickly myelinated (for rapid transmission) and ends up in the parietal cortex. The other important visual pathway, the parvocellular, the "what" pathway, which ends in the temporal cortex, must synchronize for efficient reading, so the theory is that when the magnocellular is not able to keep up with the parvocellular, visual tracking and fixation is unstable when the eyes sweep across a page. Visual information carried through the magnocellular in Dyslexic brains, has been shown by brain imaging studies (Eden, G. in Nature, 1995) to be poor in identifying movement in comparison to normal readers. By the time the information gets to the visual cortex the signal is quite faint in Dyslexics compared to normal readers. Thus the magno can’t control eye movements or guide them to the object to be looked at. Some researchers believe this means the magno or "What" pathway acts as an attentional spotlight. (Vidyasagar T. (1999) "Impaired Visual Search in Dyslexia related to the role of the Magnocellular pathway in attention" Neuroreport; 10) Studies showed that Dyslexic children are poorer at a visual search task than normal readers and the more distracters there were in the background the worse they did because reading places great demands on the attentional spotlight, far more than a complex visual scene. In other words a slow visual processing analogous to the slow auditory processing mentioned above. This has led some researchers to propose an auditory magnocellular system, analogous to the visual

For some children the interaction of these problems causes a light sensitivity with headaches, pattern glare off white pages (copying their work on pastel colours helps) — this is known as scotopic sensitivity, first identified by Helen Irlen — see her help for parents book "Reading by the colours". Her solution is to screen the child or adult using coloured lenses until the right portion of the spectrum is inhibited or enhanced. This detected by reading rate, clarity of depth perception and subjective feeling of comfort.

Behavioral optometrists give vision exercises to help strengthen the convergence and accommodation.

Other techniques are monocular occlusion (covering the left eye with opaque lenses while reading)

 

Sub-types of Dyslexia

Reading requires both;

Phonological (sound based) analysis

Levels of phonological awareness

1

Syllables

c/a/t

2

onset and rime

tr / ip

3

analogy

zip/nip — beak/bean

(need good auditory sequencing and sound discrimination)

 

Orthographic (visual code) analysis

1

requires orientation

 

2

visual sequencing

 

3

visual tracking

 

details of shape — horizontals, verticals, dots etc.

Letter to sound mapping is all locked into a precise, hierachchial sequence of neural events. This must be carried out in milliseconds or comprehension collapses.

A grammatical analysis is also going on. The subject and object of a sentence must be identified before the brain starts to analyze the meaning of the sentence, so if there is any slowness in the phonological or orthographic analysis then the brain never gets to the meaning level, or only imperfectly so.

 

Phonological sensitivity

In western writing we break each syllable down into individual phonemic segments represented by alphabetic symbols. (Cat = c/a/t/) Phonemes are a human invention, unlike syllables they are not generated by neurologically distinct programmes, i.e. they are physiologically arbitrary

Phonological skill correlates with the ability to switch attention from a word's meaning to an analysis of its acoustic properties.

Normal readers can track changes in the pitch of a sound and can segment words into their constituent phonemes to match them to symbol. Insensitivity to temporal auditory changes correlates with poor phonological awareness which affects reading of irregular words, non-words, homophones, delay in sensitivity to rhythm of speech — kissing fish, kissing fish (meaning is added to by prosody, for initial parsing analysis)

Therefore the best test of phonological awareness is can they read non-words. These have the same sound structure as the English language but the child has never seen them before so cannot rely on visual memory.

 

Orthographic skill

Orthographic skill correlates with ability to use and identify familiar letter sequences with minimal phonological information — letter order, frequency, spatial position, (spelling)

Orthographic sensitivity is independent of phonological sensitivity and can contribute to poor reading even when phonological skills are normal.
(Good motion detectors are less likely to mistake anagrams for real words (xepi for pepsi) Even in non-dyslexic children these sensory abilities correlate with reading and spelling.

The best test of orthographic skill is homophones — can they distinguish reign from rain, sale from sail etc.When auditory and visual sensitivity are analyzed together they can explain 93% of variance in reading.

Both types of discriminatory sensitivity, orthographic and phonological, have 40/50% heritability.

Evidence is based on studies reported in;

Talcott, J, Witton. C. et al. 2000 "Dynamic sensitivity and children’s word decoding skills" PNAS, 97, 6, 2952 — 57
 

The sensitivity of the magnocellular component of visual processing can be assessed psychophysically by using stimuli that selectively stimulate it. Flickering lights, low intensity, low contrast, coarse (low spatial frequency) gratings and moving targets stimulate the magno selectively. But less so in Dyslexics .. Motion sensitivity is tested by a random dot "kinematogram", a square of moving dots, and Dyslexics need to see 30% more dots move before they perceive movement, in relation to normal readers.

Similarly sensitivity to amplitude modulation (perception of change of auditory frequency) measured at 2Hz FM sensitivity is the best predictor of phonological Dyslexia, this reflects the phonemic range in language.

"Impaired Neuronal Timing in Developmental Dyslexia — The Magnocellular Hypotheis" Stein J. et al (1999) "Dyslexia" 5: 59 — 77


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