Binocular Instability in Dyslexia

Document Sample
Binocular Instability in Dyslexia Powered By Docstoc
					The Brain Basis of Dyslexia
John Stein, University Laboratory of Physiology, Oxford, UK
• • • • • •

Why reading is difficult – visual and auditory requirements
Dyslexia – diagnosis and incidence, overlap with other neurodevelopmental conditions Impaired visual magnocellular development  unstable binocular control  visual confusion Impaired auditory development  phonological confusion Impaired motor development  speech impediments, clumsiness, poor handwriting Heredity – dyslexia genes on Chromosomes 1,2,6,15,18 Environment – immune system, nutrition –fish oils

Reading is difficult because it requires:
1. Rapid visual identification of letters, even in experienced good readers
2. Rapid sequencing of their order 3. Rapid translation into the sounds they stand for

4. Background knowledge of splitting words down into constituent phonemes (phonology)

Reading is difficult!
• 20% of UK and US adults cannot find the word „plumber‟ in the yellow pages
• Difficulty learning to read is one of the greatest causes of individual and family misery

• Loss of self-confidence, depression, alcoholism, drug addiction, suicide
• Anti- social behaviour, aggression, crime

• 3/4 of prisoners in gaol are illiterate; ½ dyslexic
• Commonest cause of disability in University students

Developmental dyslexia
0037 Valerius Maximus 1897 Berlin – „Dyslexia‟ 1898 Pringle Morgan „Word blindness‟ 1930s Samuel Orton strephosymbolia 1950s McDonald Critchley Parietal lobe 1960s Social explanation “Middle class children are dyslexic,working class children are thick!”

1980s Linguistic theory phonological deficit 1990s Development of the brain is different: planum temporale, ectopias, problems with all kinds of timing and sequencing 2000s Magnocellular theory impaired development of visual, auditory and motor magno-neurones

Percy F (Dr Pringle Morgan, 1896)
„Percy F has always been a bright and intelligent boy, quick at games and in no way inferior to others. His great difficulty is his inability to learn to read. This inability is so remarkable and so pronounced that I have no doubt that it is due to some congenital defect. In spite of laborious and persistent training, at the age of 13 he still cannot even spell his own name, often writing Precy for Percy‟

What is Developmental Dyslexia?
Reading and spelling significantly below that expected from age and IQ
Symptoms 1. 2. 3. 4. 5. 6. 7. 8. 9. Reading/IQ discrepancy 80% males Poor phonology Speech impairments (lisps, spoonerisms, mispronuciations Unstable vision, visual confusion Poor spelling Mixed handedness, left/right confusions Sequencing problems Clumsiness & incoordination – „soft‟ cerebellar signs History 11. Family History of language, literacy and psychiatric problems. 12. Difficult birth 13. Delayed milestones (crawling, walking, speech) 14. Developmental dyspraxia, dysphasia hyperactivity 15. Otitis media @ age 1-3 16. Asthma, eczema, hayfever

Brain Differences in Dyslexia
Ectopias („brain warts) –outgrowths through surface, particularly in left language areas
Symmetrical planum temporale Thinner axons in left hemisphere Smaller visual magnocells Less activity in visual motion areas, left angular gyrus, Wernicke‟s and Broca‟s areas when reading Delayed visual evoked potentials

Your Brain
• 100 billion (1011) nerve cells (neurones) • 10,000 contacts (synapses) between neurones •  1 million billion (1 quadrillion – 1015) connections • More possible combinations of connections than there are particles in the Universe • Your brain is the most complex organised entity in the whole Universe • Reading is the most complex skill that most of us learn

The Brain‟s Reading System
• Input from eyes via relay in LGN to primary visual cortex at the back of the brain • Further visual processing moving forwards in secondary visual cortical areas • Links with auditory processing in angular gyrus to form lexicon (representation of visual and auditory form of words and their meaning) • Projection forwards to speech areas, for internal speech as well as reading out loud

Overlap (comorbidity) between developmental dyslexia, dysphasia, dyspraxia, ADHD, autism – abnormal magnocellular neurones?
Dyslexia - reading and spelling difficulties, unsteady eyes, visual and auditory inattention, incoordination Dysphasia (specific language impairment) speech impediments (mispronounciation, lisps, stuttering) auditory inattention, incoordination Dyspraxia – incoordination, poor motor planning/ execution, inattention ADHD -hyperactivity/impulsivity, inattention, incoordination Autism - inattention, incoordination, absent social & communication skills All caused by impaired development of magnocells?

The main problem for many dyslexics is visual instability: letters & words appear to move around and wobble.

This is because their eyes wobble when they try to read

The Visual Problem of Reading When reading the eyes are on the move most of the time. Words can only be identified during very brief „fixations‟ lasting 1/3rd sec.

The visual magnocellular system controls reading eye movements
10% of retinal ganglion cells are

magnocellular (large); they
respond rapidly to time visual events accurately – they detect visual motion, direct attention, control eye movements and enable steady eye fixation 80% are parvocells (small) : for colour, fine detail, slower responses.

The visual magnocellular system is impaired in dyslexics
• • • • • • • Smaller LGN magnocells post mortem Delayed motion evoked brain waves Lower sensitivity to visual motion Reduced activation of V5/MT Lower contrast sensitivity Slower direction of visual attention Unstable binocular control

Homophone test of orthographic skill
“Which of rane and rain is the proper spelling?” This task demands accurate memory of the word‟s visual form orthography. It can‟t be solved by sounding out the letters because both spellings sound the same. This homophone test is the best predictor of reading skill in 10 yearold children Visual magnocellular motion sensitivity predicts homophone/orthographic performance. Over 1/5th of the differences between people‟s orthographic reading ability can be explained just by their visual motion sensitivity. Visual magnocellular sensitivity affects how well children can develop visual orthographic skill.

Why should weak visual magnocellular function impede reading?
• Many researchers believe that the only cause of dyslexia is poor phonological skill • True but why can‟t dyslexics acquire good phonological skill? • Because they have visual and auditory processing problems • Letters tend to blur and jump around, and the letter sounds mix up.

The visual magnocellular system stabilises the eyes
Retinal motion, „slip‟ Feedback to eye control Steady eye fixation

Visual stability

Identify letter order

Orthographic skill

Visual Reading Problems
Children‟s quotes reveal their unstable vision oscillopsia
“The letters go all blurry” “The letters move over each other, so I can‟t tell which is which” “The letters seem to float all over the page” “The letters move in and out of the page” “The letters split and go double” “The c moved over the r, so it looked like another c” “The p joined up with the c” “d‟s and b‟s sort of get the wrong way round” “The page goes all glary and hurts my eyes” “I keep on losing my place”

• Unsteady binocular fixation (wobbly eyes), hence unstable visual perception • Jerky pursuit eye movements • Restricted & jerky vergence control • Slower mental rotation • Inaccurate dot localisation • Lose place following vertical lines • Slower visual search - less „pop out‟

Fixation Exercises
Double blind, randomised controlled trial

Reading increased 6 months in 3 months, ie 2 months/month Without special help these dyslexics would probably only have gained 9 months in 18 ms (ie ½ m/month) Best phonic remediation programmes achieve 1 month per month – also much more expensive than a patch

Vergence control
• The eyes have to converge for near vision when reading
• The vergence eye movement control system is highly vulnerable (eg alcohol makes you see double!) • Control of vergence eye movements is dominated by the visual magno system • Dyslexics have very unstable vergence control

Patching the left eye
Double blind, randomised controlled trial

Reading increased 3 yrs in 18 months, ie 2 months/month, 25 scale points
Without special help these dyslexics would probably only have gained 9 months in 18 ms (ie ½ m/month)

Phonic remediation programmes usually achieve only 1 month per month improvement – also much more expensive

Yellow Filters
• Retinal magnocellular ganglion cells receive 45% from red (LW) light receptors (cones); 45% from green (MW) and 10% inhibition from blue cones • Deep yellow filters eliminate the inhibitory blue cone input • Hence yellow enhances magnocellular sensitivity - visual motion, binocular control • 1/3rd of dyslexics benefit from wearing yellow filters for 3 months; their eye control improves and their reading progress can increase four fold

Blue Filters
• c. 10% of dyslexics seem to have too weak blue input to magnocells • These children often complain of glare and letters moving around • Blue filters can improve their reading dramatically (8 months in 3 ms)

Coloured Filters
• In summary we find that the vision of c.40% of dyslexics can be helped by giving them simple, cheap, yellow or blue filters for 3 months • These correct imbalanced input to visual magnocellular neurones • Other colours & expensive systems do not help so much because they do not selectively affect the input to the visual magnocellular system

• Understanding the role of the visual magnocellular system in the acquisition of orthographic skill has led to significant improvements in treatment • Coloured filters can help 40% of dyslexics to quadruple their reading progress • Binocular exercises or monocular patching can help another 1/3rd to quadruple their reading progress

Auditory/phonological errors
due to poor auditory discrimination?
• • • • • • • • • • • Whisk/wisp, deaf/death, effect/affect They came down on the food like a flock of vouchers The gossip spread like wild flies This strike will bring the country to a Stanstead Dry as a door nail; dead as rust; a cartoon of soup Ears (eves) dropping; an endearment (endowment) policy; Sort (saute) of potatoes; customs and exile After a time I syphoned (deciphered) it Concord is so noisy – too many decimals I torned and tussed all night Its just like flogging a brick wall

Does auditory sensitivity determine phonological ability?
High auditory sensitivity to changes in sound frequency (FM) and amplitude (AM) is required to distinguish letter sounds

Dyslexics have lower than normal sensitivity to frequency and amplitude modulated sounds

Auditory sensitivity & Phonology
• Nonsense words - „tegwop‟, „blint‟, „plomt‟, „peltip‟,„visht‟ can only be read if the letters can be translated into their sounds quickly and accurately;
• Nonword reading tests phonological ability • Individuals‟ FM & AM sensitivity predicts their ability to read these nonsense words:

Sensory Basis of Reading Skills
• In summary nearly 2/3rds of the differences in children‟s reading abilities can be explained by their non reading IQ, and how sensitive their visual and auditory magnocellular systems are • Teaching quality and other sociocultural influences account for less than one third • Nevertheless good teaching is crucially important!

Motor Disorders in Dyslexia
• Cerebellum is brain‟s autopilot for prediction • Magnocellular Systems project to the cerebellum • Cerebellar neurones stain for CAT 301; hence it is part of the magno system • Cerebellum is underactive in Dyslexics • Explains their balance and coordination problems, but not their reading difficulties • Therefore expensive balance exercises unlikely to help dyslexics‟ reading

Genetic Studies
• In over 300 Oxford families we have analysed the DNA of father, mother and at least one dyslexic child
• To find out whether particular chromosomal sites are associated with poor reading • Strong associations with C6 and C18 found • We are now searching these sites to identify genes that may contribute to the differences in brain development found in poor readers. • This will clarify why the development magnocellular neurones is impaired in dyslexia

Dyslexia, Genetics and the Immune System
• Development of magnocellular neurones is regulated by the immune system recognising magno- surface antigen, CAT 301 • Linkage of poor reading to immune control genes on Chromosomes 6 &18 • High incidence of immune anomalies in dyslexics: allergies, asthma, eczema, lupus • Evidence for antimagno antibodies in serum of some mothers with dyslexic children

Dyslexic Mice!
• All mothers develop antibodies to their foetus • Some cross the placenta (eg rhesus incompatibility) • A few cross into the brain and damage it (eg Rasmussen‟s encephalitis) • Mouse model of placental transmission (Angela Vincent, Oxford) • Serum from mother with 2 dyslexic children injected into pregnant mice • Coordination of pups impaired • Cerebellar metabolism (MRS) abnormal • Antibodies bound to mouse cerebellar Purkinje cells • ie mothers‟ antibodies can attack foetal brain magnocells • But its not mum‟s fault! Placenta is foetus tissue and vulnerable

Why Fish Oils?
• Hugh Sinclair, Magdalen College, Oxford, persuaded wartime government to give cod liver oil to all pregnant mothers and children • Highly unsaturated fatty acids (HUFAs) constitute 20% of the weight of the brain • HUFAs particularly important for magnocellular function • Treatment with HUFAs can improve reading • My brother is a well known fish chef in Britain, Rick Stein!

Fatty Acid Deficiencies in Dyslexia?
• Some dyslexics have signs of HUFA deficiency: dry & bumpy skin, dry & scurfy hair,
brittle nails, excessive thirst, frequent urination

• Low blood and brain HUFAs • Elevated levels of PLA2 enzyme • Treatment with HUFAs can improve attention, coordination and reading

Magnocellular Neurones
• A rapid reaction system of large neurones with fast transmission

• They time changes in intensity, frequency, position
• Found throughout the nervous system: visual, auditory, skin, muscles, cerebellum • They recognise each other because they have same surface antigen, CAT 301

Magnocellular Neurones
• Development controlled by immunological (MHC) regulation site on chromosome 6 • Impaired development in developmental dyslexia, dyspraxia, dysphasia, ADHD, autism, prematurity, Williams syndrome • Association of these conditions with autoimmunity: asthma, eczema etc • High dynamic sensitivity requires high membrane flexibility provided by highly unsaturated fatty acids (fish oils) • Hence they are vulnerable to HUFA deficiency (we eat too little fish!)

Fast magnocellular neurones are especially vulnerable to lack of highly unsaturated fatty acids (HUFAs – omega - 3 fish oils) Their fast ionic channels expand when they open; so they need flexible HUFAs in the surrounding membrane

• Dyslexics‟ reading, spelling, attentional and coordination problems may all result from impaired development of magnocellular neurones in the brain; these are specialised for tracking transients, visual, auditory, motor Visual magnocellular weakness causes visual instability, hence letter position confusions  fuzzy orthographic representations, leading to poor orthographic skill. Auditory magnocellular weakness impedes letter/sound translation  low phonological skill



Conclusions 2
• Weak magnocellular function may result from: Genetic vulnerability Immunological attack HUFA deficiency

• However do not be downhearted! These weaknesses can be remedied: eye exercises, coloured filters, phonological training, fish oil supplements
BUT remember Einstein, Churchill were dyslexic!

Neurones, Genes & Fish, the Brain Basis of Dyslexia
John Stein, University Laboratory of Physiology, Oxford, UK

Support the Dyslexia Research Trust! Become a member

Shared By: