fmri journal club september 28, 2004 andy james and jason craggs evaluation of mixed effects in...
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fMRI Journal ClubSeptember 28, 2004
Andy James and Jason Craggs
Evaluation of mixed effects in event-related fMRI studies: Impact of first-level design and filtering
M. Bianciardi, A. Cerasa, F. Patria, and G.E. Hagberg
Neuroimage 22 (2004) 1351-1370
Problem: What is the best design and analysis approach for event-related fMRI (er-FMRI) studies?
DesignsBlock
BimodalGeometric
Latin squareBimodal
Fixed
AnalysisSPM99SPM2FSL3.0
We are primarily interested in fixed effects (factors we control) and not random effects (factors varying by subject).
We want results that are sensitive and specific.
What is an event related design?
• An fMRI experiment where stimuli are presented as individual discrete trials, which can vary both time and sequence of stimuli
• In contrast, block design experiments typically have groups or “blocks” of trials
Event Block
Binomial
Hayberg 2001
Different event related fMRI designs
Fixed, Random, and Mixed
• Fixed effects: factors and levels that the experimenter is “arbitrarily and systematically” choosing to analyze
• Random effects: factors the experimenter is not attempting to control, but will use to test external validity
• Mixed effects: analyses that incorporate fixed and random effects
Random effects analysis
In performing a standard GLM analysis, the resulting significant results are strictly speaking only valid for the group(s) of subjects or patients included in the analysis because subjects are treated as a fixed effect in a standard GLM. In order to generalize the obtained fMRI results to the population level, a random effects analysis has to be performed. This means that the studied sample of subjects are treated as a random selection from the population of all people. Note, that for generalization to the population level, many subjects should be included, i.e. 50 or more (per experimental group!). With a few subjects, it is simply impossible to estimate general population effects. The recommended minimum for random effects analysis are 10 subjects per experimental group.
Other Key concepts
• Sphericity– Is an extension of homogeniety of variance, but
with a repeated measures twist• That is, we expect the covariances between
groups/regions/whatever to be roughly equal across multiple measurements
Sensitivity
• How well can the hardware detect an fMRI signal elicited from the paradigm
Precision
• How well can you specify the origin of the detected signal.– How much is it really related to the
experimental manipulation
Anatomic ROI masks
• Red: M1 (Active) Green: S1 (not active)
• Used for 2nd level (random effects) analyses
• Also depict ROIs for comparing designs and programs
Sensitivity vs Specificity/Precision
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