Functional Connectivity Overview - Neurometrika...Global signal: introduces apparent negative...
Transcript of Functional Connectivity Overview - Neurometrika...Global signal: introduces apparent negative...
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Seed Driven ‘Resting State’Functional Connectivity
Susan Whitfield-GabrieliMIT
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Outline• Seed driven functional connectivity• Resting state seed driven functional connectivity:
rs-fcMRI
• Treatment of noise • Current debates in the field (respiration/anticorrelations)
• Compcor method• Anticorrelation experiment• Erosion• Motion and Artifacts• Introduction to functional conn toolbox
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Structure – Function RelationshipsFunctional Segregation
• Univariate analyses of regionally specific effects
Identification of local regions thatare specialized for a particular task
Functional integration• Multivariate analyses of regional
interactionsIdentification of interactionsbetween regions that allow integrated function
Experimentally designed input
SPM
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MODEL-FREE
Exploratory
Data Driven
No Causation
Whole brain connectivity
MODEL-DEPENDENT
Confirmatory
Hypothesis driven
Causal directions
Reduced set of regions
Functional connectivity= the temporal correlation between
spatially remote areas
Effective connectivity= the influence one area exerts over another
Functional integrationsubdivided into:
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Connectivity Analysis Methods• Functional Connectivity
- PCA/ICA (principal/independent component analyses)- Pairwise ROI Correlations- Whole brain seed driven connectivity **- Graph analyses
• Effective Connectivity
- PPI (psycho-physiological interactions)- SEM (structural equation models)- MAR (multivariate autoregressive models)- Granger Causality - DCM (dynamic causal models)
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Outline
• Seed driven functional connectivity• Resting state seed driven functional connectivity:
rs-fcMRI
• Treatment of noise • Current debates in the field (respiration/anticorrelations)
• Compcor method
• Introduction to functional conn toolbox
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“The Brain's Dark Energy”
Maurizio Corbetta, Washington University St. Louis
“Brain regions active when our minds wander may hold a key to understanding neurological disorders and even consciousness itself”
Marcus Raichle, SciAm, 2010
Neuroscientists have long thought that the brain’s circuitsare turned off when a person is at rest…..
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Resting State Networks
Spontaneous, low-frequency fluctuations in the fMRI BOLD that exhibit specific networks of the human brain in the absence of over task.
(Biswal 1995, Lowe 2000, ,Greicius 2003, Fox 2005)
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Frequency Bands
Buzsaki , Science 2004[.025 .06], toolbox cutoff [.009 .08]
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Resting State NetworksSpontaneous, low-frequency fluctuations in the fMRI BOLD that exhibit specific networks of the human brain in the absence of over task.
(Biswal 1995, Lowe 2000, ,Greicius 2003, Fox 2005)
• Reliable, robust and exists in infants, anesthesia, primates (Damoiseaux et al. 2006, Shehzad et al. 2009, Van Dijk et al. 2010, Zuo et al. 2010a)
• Task-independent: Relevant for understanding clinical & pediatric populations (bypass ceiling, floor & practice effects)
• Allows exploration of individual differences (e.g., Predict clinical outcome: identify which patients will respond to which treatments -Used as surrogate outcome measure for drug development or assessment of interventions)
• Easy to acquire and share data (Biswal in press PNAS, NYU )
(Large data sets can provide quantitative phenotypes for molecular genetic studies and biomarkers of developmental and pathological processes in the brain)
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rs –fcMRI ApplicationsResting state functional connectivity can reveal intrinsic, spontaneous
networks which can help us understand the basic functional organization of the brain:
a) Used to illustrate that the brain is intrinsically organized into dynamic “anticorrelated” functional networks (Fox 2005)
b) ROIs based on correlations in spontaneous fluctuations of the BOLD signal can predict localization of task-related functional responses. (Vincent et al 2006).
c) Used to functionally dissociate functionally and anatomically herterogeneous regions of interest (Margulies 2007 (ACC) , Roy 2009 (Amygdala) )
d) Used to delineate functional topography of the brain: sharp transitions in correlation patterns may be used to determine functional boundaries across cortex (Cohen 2008).
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Default Network
The default network is hypothesized to mediate task-independent or intrinsic thought rather than task-dependent extrinsic stimulus processing
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Estimates maps showing temporal correlations between the BOLD signal from a given seed and that at every brain voxel
e.g. Fox et al., 2005. Proc. Natl. Acad. Sci. 102:9673–9678Vincent et al, 2006 J Neurophysiol 96:3517–3531.Whitfield-Gabrieli et al, 2009, Proc. Natl. Acad. Sci. 102:9673–9678
Seed driven functional connectivity
Fox PNAS 2005
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Time courses: Correlated:PCC (yellow), MPFC (orange)Anticorrelated: IPS (blue)
Fox et al., 2005. Proc. Natl. Acad. Sci. 102:9673–9678
Seed driven rs-fcMRI reveal defaultAND task related networks
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Connectivity Maps Depend on Seed Location:
Flaw or Feature?
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Functional segregation of ACC using seeded connectivity maps
Margulies 2007
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Development of ACC fc from childhood to adulthood
Kelly et al 2009
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Functional segregation of Amygdala
Roy 2009
LB: laterobasalCM: centromedial SF: superficial
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rs-fcMRI used to used to determine functional boundaries across cortex
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Cohen 2008
Sharp transitions in rs-fcMRI patterns wereused to determine functional boundaries,reliably on individual and group data.
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Clinical Characterization
Cingulate-Precuneus Interactions:A New Locus of Dysfunction in ADHD:ADHD decreases in Precuneus/ACCConnectivity. Also, ADHD decreases in connectivity among precuneus andother default network componentsCastellanos 2008
Increased default functional connectivityin patients with major depression*subgenual ACC
Greicius, Biological Psychiatry, 2007
Whitfield-Gabrieli, PNAS 2009
Increased default connectivityin schizophrenia, correlates with psychopathology
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Subgenual ACC Implicated in Depression:
• Brain Function
• Brain Anatomy
• Cellular Anatomy
MRI-based volumes of same regionshowed reductions in mean grey matter volume in depressed subjects.
(39% biplor/ 48% unipolar).
There are a reduced numberof glial cells ,subgenual ACC in depression
PET images showed decreased metabolismIn the subgenual ACC in depressed subjectsrelative to controls.
Drevets,Nature, 1997
Ongur,PNAS, 1998
Drevets,Nature, 1997
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Outline• Seed driven functional connectivity• Resting state seed driven functional connectivity:
rs-fcMRI
• Treatment of noise • Current debates in the field (respiration/anticorrelations)
• Compcor method
• Introduction to functional conn toolbox
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Physiological noise
Scanner drift
Subject motion
Nuisance effects
Confounding effects
In “activation” studies
In “connectivity” studies
Introduce bias in results (show apparent connectivity between unrelated areas)
Usually degrade power (lower statistical significance of the results)
BOLD noise in connectivity analyses
Noise sources
Need to appropriately characterize and remove noise effects to improve the validity of connectivity analyses
Non-neuronal contributions to BOLD signal
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Low freq rs-fcMRI - Respiration?
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Removing Global Signal
Birn 2006
The natural variation in breathing depth and rate during rest has a significant impact on rs-fcMRI analyses as the induced fMRI signalchanges can occur at similar spatial locations and temporal frequencies.
The global (whole brain) signal correlates with respiration-induced fMRI signal fluctuations
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Correlation processing steps
Vincent et al 2006
1) BPF Data: (.009<f<.08Hz) reduce effect of low freq drift and high freq noise2) Gaussian spatial smooth (6mm FWHM)3) Removal by regression of
a) movement b) mean global signalc) mean ventricled) mean white matter
Note: Each successive step increases the sensitivity and specificity of the correlations!!
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Consequences of Removing Global Signal
Consequence of removing the global signal: The distribution of correlation coefficients throughout the brain is shifted such that there are roughly equal numbers of positive and negative correlations.
Anticorrelations are an “artifactual” consequence of global regression.
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Anticorrelations - artifacts?
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After global signal regression, the sum of correlation values with a seed voxel across the entire brain is less than or equal to zero.
Murphy et al Neuroimage, 2009
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The Global Signal and Observed Anticorrelated Resting State Brain NetworksFox et al. J Neurophysiol. 2009; 101: 3270-3283
Correlations and anticorrelations in resting-state functional connectivityMRI: A quantitatige comparison of preprocessing strategiesWeissenbacher et al. Neuroimage, 2009
Anticorrelations - artifacts?
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CompCor Approach
Noise effects are not distributed homogenously across the brain (e.g. cardiac effects are particularly visible near vessels, respiratory effects appear more globally and stronger near edges in the image).
Compared to previous methods that subtract global signal across the brain and the mean signals from noise ROIs, the CompCor method is more flexible in its characterization of noise. It models the influence of noise as a voxel-specific linear combination of multiple empirically-estimated noise sources.
CompCor : Principal components (PCA) are derived from noise ROIS. These components are then included as nuisance parameters within general linear models.
(Behzadi et al 2007. NeuroImage 37 90–101)
BOLD noise treatment in connectivity analyses
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• Compared to whole-brain regression, CONN produces higher sensitivity and specificity
Chai et al., 2011
Specificity = (|ztarget| - |zref| ) / (|ztarget| + |zref|)Specificity: Weissenbacher, Neuroimage, 2009
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Anticorrelation StudyIn this study, we examined the correlations andanit-correlations in resting state connectivity with several differentmethods for the treatment of possible confounds.
Resting state data: One resting state scan of 10 minutes was collectedin 14 healthy subjects. TR = 2.5s. 42 slices
Data preprocessing: Functional images were slice-time corrected,realigned, normalized and smoothed with a 6mm kernel inSPM5. Individual anatomical images were segmented to extractwhite matter and CSF masks.
*sources. Seed : MPFC (Fox et al., 2005). 10mm sphere centered at (-1, 47 -4).
Correction for physiological and other noise sources:Residual motion was regressed out in all three methods.A temporal band-pass filter of .009 to .08 Hz was applied.
. Chai et al. 2011
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Connectivity with MPFC
Chai et al. 2011
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Correlations / Anticorrelations
Chai et al. 2011
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Reference Regions
Wm+ventricle: does not eliminate apparent positive connectivity withreference regions (too small correction, there are other confounding effectsnot accounted for by the wm+ventricle average timeseries)
Global signal: introduces apparent negative connectivity with reference regions (too much correction, global signal correction forces the average connectivity to be approximately zer
Compcor: Increasing dimensions reduce the size of the apparent connectivity with reference regions. (intermediate solution, expands on the wm+ventricle method by adding increasing complexity to the characterization of noise within these ROIs without imposing artificial constraintson the expected distribution of connectivity values)
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Specificity
Low specificity
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Specificity:
Compared to global regression, specificity for both the correlated and anti-correlatedregions were improved when Compcor was used for noise-reduction.
Specificity = (|ztarget| - |zref| ) / (|ztarget| + |zref|)
This quantifies to what extent each method is reducing the effect size in the areasof interest (decreasing sensitivity) compared to how much each method is reducingthe effect size in the reference areas (increasing validity)..
Specificity: Weissenbacher, Neuroimage, 2009
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Movement regression
Whole brain regression
aCompCor(PCA5)
Bandpass filter
1.0
Correlation values
Num
ber o
f voxelsAnticorrelation: Conclusion
CompCor increases sensitivity/specificity and allowsfor the interpretation of anticorrelations Chai et al. 2011
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Conclusion
• Our results suggest that anti-correlations observed in rest-state functional connectivity are robust even without global regression and may suggest a biological origin.
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CompCor
Noise ROIs
CompCor: Behzadi et al., 2007
PCA components
Use as an estimate for physiological and other noise
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Noise ROIs, Partial Volume: Erosion
In Conn, the white matter, and CSF noise ROIS, and the resulting masks are eroded (one voxel erosion, isotropic 2mm voxel-size) to minimize partial volume effects. This is critical to ensure grey matter signals are not includedin the nuisance regressors.
Note: Signals from the WM and CSF ROIs are extracted from the unsmoothed functional volumes, so smoothing should not introduce an additional risk of contaminating WM and CSF signals with grey matter signals.
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Noise ROIs: Erosion
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Movement Confound
• Children move more than adults, • Older adults more than younger adults,• Patients move more than controls.• Head motion varies considerably among
individuals within the same population.
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The effect of motion
Comparisons between groups of individuals with subtly different levels of head motion yielded difference maps that could be mistaken for neuronal effects . These effects are important to consider when interpretingvariation between groups and across individuals.
N = 1000, binned accordingto degree of motion
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More motion tended to result in lower connectivity
(Van Dijk et al., 2011)
Mean motion (mm)
Functional correlations among regions within the default network decrease with Increased motion
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Group comparisonGroup 1 (lowest 10% motion) > Group 10 (highest 10% motion)
(Van Dijk et al., 2011)
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Development of resting-state network
• Primary visual, somato-motor, auditory networks in place in newborns and infants (Fransson et al., 2007)
• Default network partially present in infants (Gao et al. 2009)
• Within-network strength in default network increases with age? (Fair et al., 2008)
• Local to distribute developmental pattern? (Fair et al., 2009)
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Voxelwise resting-state functional connectivity maps for a seed region (solid black circle) in mPFC
Fair D A et al. PNAS 2008;105:4028-4032
©2008 by National Academy of Sciences
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Artifact rejection tends to augment long-distance correlations and to decrease
short-distance correlations(Power et al., 2011)
Is Regressing Motion Enough?
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Identifying outliers
(Power et al., 2011)
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Removing outliers
(Power et al., 2011)
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Effect of “scrubbing” artifact rejection
(Power et al., 2011) Increased long range ( anterior/posterior) correlations & reduced local correlaions
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Effect of “scrubbing” or artifactrejection
(Power et al., 2011)
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Artifact detection
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fMRI connectivity tool (beta)
SETUP
Covariates – first level: Defines within-subject covariates (e.g. artifacts and
realignment from the art toolbox)
(one .txt or .mat file per subject/session; files should contain as many rows as scans)
Save outliers in ART
Art_regression_outliers_movement.mat
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Effect of motion artifact% of BOLD variance explained
Motion parameters only Motion parameters + outliers
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Effect of motion artifactfirst-level connectivity map
Regressing out motion parameters only
Regressing out motion parameters + outliers
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Outline• Seed driven functional connectivity• Resting state seed driven functional connectivity:
rs-fcMRI
• Treatment of noise • Current debates in the field (respiration/anticorrelations)
• Compcor method
• Introduction to functional conn toolbox