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1、Dynamic Causal Modeling(DCM)A Practical Perspective,Ollie HulmeBarrie RoulstonZeki lab,Disclaimer,The following speakers have never used DCM.Any impression of expertise or experience is entirely accidental.,Structure,1.Quick recap on what DCM can do for you.2.What to think about when designing a DCM
2、 experiment3.How to do DCM.What buttons to press etc.,A Re-cap for DummiesYou can ask different types of questions about brain processing.Questions of WhereQuestions of How,Functional Specialization is a question of Where?,Where in the brain is a certain cognitive/perceptual attribute processed?What
3、 are the Regionally specific effects,your normal SPM analysis(GLM),Functional Integration is a question of HOW,Experimentally designed input,How does the system work?,What are the inter-regional effects?How do the components of the system interact with each other?,MODEL-FREE,MODEL-DEPENDENTHypothesi
4、s driven,DCM!,Functional connectivity=the temporal correlation between spatially remote areas,Effective connectivity=the influence one area exerts over another,2 Categories of Functional integration analysis,PPI,DCM overviewDCM allows you model brain activity at the neuronal level(which is not direc
5、tly accessible in fMRI)taking into account the anatomical architecture of the system and the interactions within that architecture under different conditions of stimulus input and context.The modelled neuronal dynamics(z)are transformed into area-specific BOLD signals(y)by a hemodynamic forward mode
6、l().,The aim of DCM is to estimate parameters at the neuronal level so that the modelled BOLD signals are most similar to the experimentally measured BOLD signals.,Planning a DCM-compatible study,Experimental design:preferably multi-factorial(e.g.at least 2 x 2),1.Sensory input factor At least one f
7、actor that varies the sensory input changing the stimulus a perturbationto the system,2.Contextual factor At least one factor that varies the context in which the perturbation occurs.Often attentional factor,or change in cognitive set etc.,Planning a DCM-compatible study,TR should be as short as pos
8、sible 2 seconds,Possible corrections for longer TRs1.slice-timing 2.Restrict model to proximate regions.The closer they are along z axis the lower the temporal discrepancy,1,2,slice acquisition,visualinput,Timing problems in DCM:Due to the sequential acquisition of multiple slices there will be temp
9、oral shifts between regional time series which lie in different slices.This causes timing misspecification.At short TRs this is not too much of a problem since the information in the response variable is predominantly contained in the relative amplitudes and shapes of hemodynamic response rather tha
10、n their timings.Consequently DCM is robust against timing errors up to 1 second,Hypothesis and model:define specific a priori hypotheses.DCM is not exploratory!,Specify your hypotheses as precisely as possible.This requires neurobiological expertise(the fun part)read lots of papers!Look for converge
11、nt evidence from multiple methodologies and disciplines.Anatomy is your friend.,Parietal areas,V5,Hypothesis A attention modulates V5 directly,V1,Hypothesis BAttention modulates effective connectivity between PPC to V5,Defining your hypothesis,+,When attending to motion.,+,Which parameters do you th
12、ink are most relevant?Which parameters represent my hypothesis?Which are the most relevant intrinsic anatomical Connections?Which are the most relevant changes in effective connectivity/connection strength?Which are the relevant sensory inputs?2.Defining criteria for inference:single-subject analysi
13、s:What statistical threshold?What contrasts?group analysis:Which 2nd-level model?Paired t-test for parameter a parameter b,One-sample t-test:parameter a 0 rmANOVA(in case of multiple sessions per subject)3.Ensure that the model you generate is able to test yourhypothesesThe model should incorporate
14、every component of the hypothesis,Parietal areas,V5,Direct influence,V1,DCM cannot distinguish between direct and indirect!Hypotheses of this nature cannot be tested,4.Evaluate whether DCM can answer your questionCan DCM distinguish between your hypotheses?,In case of,1.Specify your main hypothesis
15、and its competing hypotheses as precisely as possible using convergent evidence from the empirical and theoretical literature2.Think specifically about how your experiment will test the hypothesis and whether the hypothesis is suitable for DCM to test.3.Klaas emphasises that you should Test your mod
16、el before conducting the experiment using synthetic data.Simulation is the key!4.DCM is tricky,ask the experts during the design stage.They are very helpful.,A DCM in 5 easy steps,Specify the design matrixDefine the VOIsEnter your chosen modelLook at the resultsCompare models,Specify design matrix,N
17、ormal SPM regressors-no motion,no attention-motion,no attention-no motion,attention-motion,attention DCM analysis regressors-no motion(photic)-motion-attention,Defining VOIs,Single subject:choose co-ordinates from appropriate contrast.e.g.V5 from motion vs.no motion RFX:DCM performed at 1st level,bu
18、t define group maximum for area of interest,then in single subject find nearest local maximum to this using the same contrast and a liberal threshold(e.g.P0.05,uncorrected).,specify,NB:in order!,Can select:effects of each conditionintrinsic connectionscontrast of connections,Output,Latent(intrinsic)connectivity(A),Modulation of connections(B),Input(C),Comparing models,See what model best explains the data,e.g.,Original ModelAttention modulates V1 to V5,Alternative ModelAttention modulates V5,?,The read-out in MatLab indicates which model is most likely,
