Download - Haskins fMRI Workshop Part III: Across Subjects Analysis - Univariate, Multivariate, Connectivity
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Haskins fMRI WorkshopPart III:
Across Subjects Analysis - Univariate, Multivariate, Connectivity
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Across-subjects “Composite” MapsRecall: two-stage analysis• Stage 1: extract subject maps for effects of interest (B weights)• Stage 2: at each voxel, test the values across-subjects versus zero
• t-test, ANOVA with planned comparisons or contrasts• each new composite map shows p-values for one subject-level
effect
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[ single-subject maps ] [ composite map ]
S1 S2 S3 S4 average
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data matrix layout for across-subjects analysis
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CRM study:“new” words
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CRM study:“old” words
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CRM study:contrast ofold-new words
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ThresholdsWhat is the appropriate threshold?• 902,629 voxels in standard MNI space image;
258,370 actually in-brain• Type I and Type II error
Approaches:• assume real activations are large (reduce number of actual tests)
control Family-Wise Error Rate “chance of any false positives”• alternatively: control False Discovery Rate
“proportion of false positives among rejected tests”• employ a priori regions-of-interest (ROIs)• multivariate analysis
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Correlational Analysis
• across subjects: at each voxel, correlate the activation level to some external subject variable like age:
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Correlational Analysis
• across subjects: at each voxel, correlate the activation level to some external subject variable like age...
or behavioral skill:
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Multivariate Analysis• PCA/SVD/Eigenimage analysis/ICA
• within subject: identify set of (1) spatial patterns with (2) associated timecourse
• across subject: identify spatial patterns with associated subject loadings
• data driven, work only on the input image data(not classified by condition, subject group, etc.)
• PLS (Partial Least Squares)• across subject: identify spatial patterns that change from task to
task• also data driven, but optimized to identify task-related changes• identifies the strongest possible contrasts among conditions
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Multivariate Analysis
Calhoun et al., Human Brain Mapping, 2001
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ConnectivityFunctional Connectivity:• df: correlations between spatially remote neurophysiological events• does not imply causality, but identifies covariation• subject to “third variable” explanations
Effective Connectivity:• df: the influence one neuronal system exerts on another• implies causality; requires something beyond correlations and
correlational analysis such as• tests of temporal relations (e.g. lagged autocorrelation analysis)• SEM - model testing
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Within- vs. Between-Subjects ConnectivityWithin-subject Connectivity:• df: correlations over time-course of a single study
activations by time point
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Within- vs. Between-Subjects ConnectivityWithin-subject Connectivity:• difficulties...
• low signal-to-noise• primarily reported in low frequencies <20sec/cycle• HRF response dissimilar across regions
Hampson et al., Human Brain Mapping, 2002
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Within- vs. Between-Subjects ConnectivityBetween-subject Connectivity:• df: correlations over subjects within a single task• cf Horwitz et al., 1984 (!)
activations by subject number
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Pugh et al., 2000;also Horwitz et al., 1992
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Functional Connectivity
activations in Shaywitz et al. 2002
older good readers
74 good readers 7-18 yrs70 dyslexic readers 7-18 yrs
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Functional Connectivity
seed voxel correlations
older good readers
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Functional Connectivity
selected univariate correlations
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Functional Connectivity
Older Non-Impaired
univariate correlations
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Older Non-Impaired
Older Dyslexics Younger Dyslexics
Younger Non-Impaired
Functional Connectivity
univariate correlations
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Functional Connectivity
First Component
Second Component
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Connecticut Longitudinal Study: ConnectivityConnecticut Longitudinal Study: Connectivity
Shaywitz et al., 2003
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24Worsely et al., 2005
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Dynamic System…