fMRI event-related design issues

fMRI event-related design issues - fMRI Basic Experimental...

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fMRI Basic Experimental Design – event-related fMRI.
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Block Designs Assumption: Because the hemodynamic response delays and blurs the response to activation, the temporal resolution of fMRI is limited. = trial of one type (e.g., face image) = trial of another type (e.g., place image) This is not entirely true.
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Assumption of steady-state dynamics. For block designs we assume that the BOLD effect remains constant across the epoch of interest. For PET this assumption is valid given the half-life of the tracers used to image the brain. But the BOLD response is much more transient and more importantly may vary according to brain regions and stimulus durations and maybe even stimulus types.
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Assumption of steady-state dynamics. Price et al. 1999, Neuroimage, 10, 36 – 44. Distributed sampling is ensured if the ISI and TR are not integer multiples of one another! Do not sample the responses as discrete peaks or valleys – means you should vary the ISI and TR relative to one another
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What are the temporal limits? What is the briefest stimulus that fMRI can detect? Blamire et al. (1992) – 2 sec Bandettini (1993): 0.5 sec Savoy et al (1995): 34 msec •With enough averaging, anything seems possible. •Assume that the shape of the HRF is predictable. •Event-related potentials (ERPs) are based on averaging small responses over many trials. Can we do the same thing with fMRI?
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SNR in block vs. ER-fMRI – trade-offs again! Block Design vs widely space ER-fMRI ~ 35% loss of SNR (Bandettini and Cox, 2000) Widely spaced ER-fMRI vs rapid ER-fMRI ~ 17-25% loss of SNR (Miezin et al. 2000) So from Block Design to rapid ER-fMRI ~ 50% loss of SNR!! Claim is that the power lost in SNR is made up for by increased numbers of trials for event-related averaging. This may differ across regions and for different tasks – yet to be determined.
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Why do an event-related design? Pros: multiple trial types in one run – randomization becomes possible greater temporal control can look for activation to a single specific trial types (usually the average of many trials) Cons: smaller SNR means smaller n – ramp up number of trials (~ 50 – 100 per condition is considered reasonable) more complex design and analysis (esp. timing and baseline issues)
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Thought Experiments for event-related fMRI. • What do you hope to find? (run through all the same Q’s you did for a block design
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fMRI event-related design issues - fMRI Basic Experimental...

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