The Anatomy of Reading · Bob Dougherty Stanford Institute for Reading and Learning. SIRL...
Transcript of The Anatomy of Reading · Bob Dougherty Stanford Institute for Reading and Learning. SIRL...
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The Anatomy of ReadingThe Anatomy of ReadingBob DoughertyBob Dougherty
Stanford Institute for Stanford Institute for Reading and LearningReading and Learning
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SIRL Longitudinal Study of Reading Development
• Behavioral assessment
• Anatomical + Diffusion Tensor Imaging
• Functional MRI
• 50 7-11 yr olds
• 3 years (4 measurements)• Completed 1st and 2nd measurement (1yr)
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Overview• Proficient reading is an impressive skill• Garden-variety brains• The anatomy of reading
• Key cortical regions• Connected by important white matter pathways• But- some brains aren't optimized for reading
• Conclusions
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Name the ink colors:
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Name the ink colors:
redgreenblue
orangepurple
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Reading Numbers
Typical reading rate: 250 words/minuteFixation duration: 225 ms (skewed distribution)Saccade distance: 7-10 letters (2 deg)Duration of 2 deg saccade: 30 ms
Regressive saccades: 10-15%, ↑ with difficulty
Minimal saccade latency: 150 msProbability of fixation: 2-3 letter words: 0.25
> 7 letter words: ~1.0
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Sensory Aspects of Reading
• Maintain fixation• Identify currently fixated word• Preprocess parafoveal words
• Program saccades• Execute saccades• Ignore irrelevant retinal motion
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Visual Processing of Words is Fast
• ~150 ms to program a saccade• Visual info influences fixation duration and
subsequent saccade target• Relevant info extracted and processed
within ~75ms (+ overlap w/ saccade program?) during fluent reading
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Explaining Reading Development
• Most variance is explained by social factors and general cognitive ability
• Significant variance remains...• What are the neurological factors?
• Identify biological correlates • Explain individual differences in reading
development with variation in anatomy and physiology
• Predict & intervene before reading failure
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Neural Basis of Reading
LR, 11yrs old
• Behavior• Gross Anatomy
• Brain shape/size analysis
• White matter structure• Diffusion imaging
• Cortical activity• Functional MRI
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Every Brain is Different...
n=55n=55
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What's the Matter?
• Gray matter: the connections (synapses)• Site of functional activity (EEG, MEG, fMRI)
• White matter: the wiring (myelinated axons)• Looks white due to high lipid content (myelin)• Long-range connections• Connections develop early and limit plasticity• Connections define cortical modularity (Müller's
law of specific nerve energies)
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Why You See with the Back of Your Brain
Brain dissection image from: The Virtual Hospital (http://www.vh.org)
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Diffusing Water ProbesMicroscopic Tissue Structure
• Tissue structures affect water diffusion• Diffusion through white matter probes:
• Axon density & myelination, principal fiber direction and directional coherence
• MR Diffusion weighting measures diffusion• Fiber tracking in diffusion data
• Hints at axonal connectivity
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Water Diffusion in the Brain
Unimpeded direction- higher diffusion rate
Impeded direction- lower diffusion rate
White matter fibers
H2O
H2O
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• Water molecules move in Brownian motion
• 3D Gaussian (3x3 covariance matrix) model• Eigenvalues & vectors define ellipsoidal
isodiffusion surface
The Diffusion Tensor:3x3 Covariance Matrix (Ellipsoid)
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DTI Reveals White Matter StructureT1 DTI (FA) DTI (PDD)
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Trace Paths Through Tensor Field• Connecting the dots to make
fiber tract estimates
• Stream-tubes tracking (STT)• Assume PDD is tangent to fiber tract
estimate
• Go where PDD leads (Runge-Kutta path integral)
• Tri-linear interpolation of tensors
• 1 mm step size
• Stop at FA<0.15 or angle > 30º
From Watts et. al. (Cornell)
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Occipital Fibers
1cm scale bars
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Occipital Callosal Fibers
1cm scale bars
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Occipital Callosal Fibers
1cm scale bars
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Left-Right Convergence:Mean-Centered Density
Left Occipital Fibers Right Occipital Fibers
5 mm
Dougherty et. al. 2005 PNAS; Dougherty et. al. 2005 NYAS
N=53 children (7-12yr)
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Left and Right Fibers Converge
Mean-shifted (r2 = 0.94)Unshifted (r2 = 0.88)
Dougherty et. al. 2005 PNAS; Dougherty et. al. 2005 NYAS
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Splenium Map in 53 Children
LD LV LLRD 0.87 0.73 0.21RV 0.47 0.89 0.38RL 0.37 0.50 0.77
left/right convergence (r2):
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Previous WM Findings in Reading
Deutsch et. al. 2005 Beaulieu et. al. 2005
Some overlap in extent; voxels of maximal difference <1cm apart
Klingberg et. al. 2000
adults children
• Decreased FA in low readers in temporal-parietal WM region (esp. on left)
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Principal Diffusion Direction:Group Means
Poor Readers
Good Readers
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A PDD Difference in Anterior WM(Schwartzman, Dougherty, Taylor, 2005, MRM)
Poor ReadersGood Readers Bipolar Watson Distribution
cm
FA difference
Ages 8-12; N = 14
Significant at FDR<0.01
z=23
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Limits of the SPM Analysis
• Statistical power varies greatly across brain regions
• Interpretation is often ambiguous• Differences may be due to
WM properties or structural differences
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Statistical Power for FA SPMs
• Assumptions:• T-test (mean difference
between groups)• N=10 in each group• Mean FA difference = 0.12• Uncorrected p=0.001• Spatial normalization to
MNI T1 template
Z=-4
Z=6
Z=16
Z=26
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Statistical Power for FA SPMs
• Assumptions:• T-test (mean difference
between groups)• N=15 in each group• Mean FA difference = 0.12• Uncorrected p=0.001• Spatial normalization to
MNI T1 template
Z=-4
Z=6
Z=16
Z=26
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Low Power in CC with FA SPM
X=-2 X=0 X=+2
N=10
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Low Power in CC with FA SPM
X=-2 X=0 X=+2
N=15
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Low Power in CC with FA SPM
X=-2 X=0 X=+2
N=25
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ROI-based Methods
• More statistical power• Eliminate much anatomical variance• Many fewer statistical tests
• Easier to interpret• But:
• Labor-intensive• ROI boundaries are subjective• Need a-priori hypostheses
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Tracing Virtual Fibers
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Why The Callosum?
• CC in alexia• Mid-splenial lesions can cause
alexia
• CC in developmental dyslexia• Morphological differences in
shape and size• Reduced hemispheric
asymmetry in anatomy and function
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Defining Callosal ROIs
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Defining Callosal ROIs
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Defining Callosal ROIs
Genu/RostrumGenu/Rostrum BodyBody IsthmusIsthmus SpleniumSplenium
1/3 2/3 1/5
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FA Negatively Correlated with Phonological Awareness in Splenium
r = -0.44 (p=0.001)
N = 53
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FA Negatively Correlated with Phonological Awareness in Splenium
r = -0.44 (p=0.001)
N = 53
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Phonological Awareness is Correlated with Reading
r = 0.67 (p<0.0000001)
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FA and Reading in the Splenium
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Grand Unification Hypothesis
• More left-right connections in low readers• larger CC• higher FA in CC pathways
• Increased CC connections cause decreased FA in other pathways• More crossing fibers, esp. in corona radiata
• Increased hemispheric connectivity causes more anatomical and functional symmetry
• But- is increased hemispheric connectivity a cause or an effect?
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Conlcusions
• Splenium FA is lower in skilled readers• Consistent with previous studies of dyslexia
• Less lateralized language• Enlarged posterior callosum• Greater callosal bending angle (?)
• Posterior callosum crucial for skilled reading• Lesions there result in alexia• But which lobe? Occipital? Parietal? Temporal?
• Segment callosum by projection zone
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AcknowledgementsBrian Wandell (Psychology)Gayle Deutsch (Neurology)
Michal Ben-Shachar (Psychology)Roland Bammer (Radiology)Polina Potanina (Psychology)
Arvel Hernandez (Psychology)Armin Schwartzman (Statistics)
Alyssa Brewer (Psychology)All our subjects (kids and parents)
Funding: Schwab Foundation for Learning & NIH EY015000