Neurobiology of Dyslexia Elizabeth S. Norton, Sara D. Beach and John DE Gabrieli Presented by Michaela Cronin April 16, 2015

Why am I interested? ● BCS major on neurobiology track ● Doctors thought my twin sister was dyslexic as a child

Intro to Dyslexia ● Very common learning disability o “Unexplained difficulty in word reading accuracy and/or fluency” o “affects 5-12% of children” o Legos vs. dough example ● Outcomes o “Reduced educational attainment” o “Academic self esteem” o Less reading outside of school - poor reading skills ● Neuroimaging methods o Functional magnetic resonance imaging (fMRI) o Electroencephalography (EEG, and ERPs) o Magnetoencephalography (MEG)

Intro to Dyslexia

Typically Reading Adults ● Use left hemisphere for reading o Inferior frontal (Broca’s), superior and middle temporal, and temporo-parietal regions ● Use Visual Word Form Area (VWFA) to process written words o Had two papers on this recently! o Develops with experience ● Major tracts in reading o Left arcuate/superior longitudinal fasciculus o Corona radiata

Reading in the left hemisphere Inferior frontal gyrus (Broca’s) Inferior parietal area Arcuate Fasciculus (end = Wernicke’s) Fusiform Gyrus (VWFA) These areas look or work differently in dyslexia.

Probable causes of reading inaccuracies ● Weakness in phonological awareness (PA) for spoken language ● Weakness in rapid automatized naming (RAN) o Video ● Basic perceptual processes (mechanistic) o Ability to read words and connect text with speed and accuracy  Temporal sampling or processing, visual-spatial attention, perceptual learning deficits

RAN

Big question: Why does a dyslexic brain look the way it does? Because of the neurobiology of a dyslexic brain (before it is taught to read) Because it has had years of altered reading experience because of dyslexia OR

What this paper reports ● Brain differences before formal reading instruction that impact how well one learns to read ● Neural networks associated with specific psychological factors that are associated with dyslexia (PA, RAN, etc.)

Brain Differences in Dyslexia

Brain differences in dyslexia: What do they mean? ● Some areas show less activation o Classical left-hemisphere language areas: VWFA o Arcuate fasciculus, corona radiata fibers - major tracts in reading ● Some areas show more activation o Non-standard areas, e.g. in right hemisphere ● What does this mean? o Left side doesn’t work as well as normal → compensating for this on right side of brain o Dyslexics can still read, but will take much, much longer

How can we tell what causes these differences? ● Compare dyslexic children to both: o Age-matched typically reading children o ‘Ability matched’ children  Years younger than dyslexic children but read at same level (same amount of reading experience) If show reduced activations relative to both of these groups, these reduced activations are related to the underlying causes of dyslexia!

Using this technique ● Dyslexia: smaller motion perception neurons → reduced activation in area MT ● Children with dyslexia had MT activations equivalent to ability-matched younger children o Reduced MT activation = consequence of reading experience, not cause!

Determining brain differences underlying dyslexia: Another technique... ● Study pre-reading children o Brain differences cannot be consequence of altered reading experience ● Identifying children at risk for dyslexia o Family history (4x more likely) o Low performance on tests (PA, RAN)

Example: studying pre-reading children Using MRI Positive correlation between measures of PA, and size and structural white-matter organization of left arcuate fasciculus Brain differences in dyslexia occur before reading experience: They are causes, not consequences. Size and structure of white matter Phonological Awareness

Advances in Understanding the Brain Basis of Aspects of Dyslexia (PA, RAN, fluency, perceptual processes)

PA and RAN: brain basis PA (phonological awareness) deficits ● Phonetic representations are intact, but access to them may be impaired o Connections between auditory cortices and left inferior frontal gyrus reduced RAN (random automatized naming) deficits ●RAN speed related to volume in distributed network across all 4 lobes (not localized) o Activation in right cerebellar lobule VI showed a gradient with RAN ability

A1 VWFA Brain Basis

Reading Fluency and Perceptual Processes: brain basis ● Reading fluency deficits o Rapid reading → more activation in VWFA o Different findings:  Dyslexia: reduced VWFA activation despite no comprehension differences VS.  Dyslexia: worse comp. and less act. as a function of reading speed, but no group difference in VWFA ●Basic perceptual processes o Parsing speech: left aud. cortex to amplify phonetic info in low gamma range o Dyslexia: reduced synchronization of neural firing to low gamma freq. range o In animals: dyslexia genes linked to  atypical neural migration  impaired speech sound discrim.

Conclusions ● Understanding neurobio of dyslexia brings us closer to intervention with children ● Early and accurate identification is very important o Remediation is most effective in beginning readers ● Understanding specific components (PA, RAN, fluency) → personalized interventions