Advances in Event-Related fMRI DesignSlide 2Fact of (fMRI) Life #1: DispersionFact of (fMRI) Life #2: NoiseFact of (fMRI) Life #3: TimeBlocked DesignUsing Overlap to Increase AmplitudeBlocked Design DrawbacksSlide 9Fixed-Interval Event-RelatedRapid-Presentation Event-RelatedWhere does jitter come from? (What’s a Null Condition?)Rapid-Presentation PropertiesScheduling and EfficiencySummaryAdvances in Event-Related fMRI DesignDouglas N. GreveOutline• What is Event-Related Design?• Blocked Design• Fixed-Interval Event-Related• Rapid-Presentation (Jittered) Event-Related• Efficiency and Event SchedulingFact of (fMRI) Life #1: Dispersion•How closely can trials/events be spaced?Fact of (fMRI) Life #2: Noise•How much data needs to be collected?Fact of (fMRI) Life #3: Time•Collect lots of observations to reduce noise•Time is Money•Subjects won’t work foreverBlocked Design20-60 sec• Consecutive, rapid presentation for long duration.• Use overlap to build a larger signal.• Simple analysis.• Optimal for detection.20-60 sec fixationUsing Overlap to Increase Amplitude1 2 3Blocked Design Drawbacks• Confounding psychological and physiological effects• Habituation/Adaptation• Expectation• Set (Strategy)• Lose ability to distinguish individual responses • Reminder: efficient.What is Event-Related Design?(c.f. Blocked Design)• Measure Average Response to Single Event Type• Post Hoc Event Assignment based on Subject’s Response• Random Order of Events • Historical: EEG/Evoked Potentials• Less Powerful than BlockedFixed-Interval Event-Related12-20s• Push trials apart enough to prevent overlap.• Interval fixed at minimum is most efficient.• Random Sequence (Counter-balanced)• Allows Post-Hoc Stimulus Definition• Mitigates Habituation, Expectation (?), and Set• Inflexible/Inefficient/Boring• Good if limited by number of stimuli (not scanning time)Rapid-Presentation Event-Related • Closely Spaced Trials (Overlap!)• Raw signal uninterpretable• More Stimulus Presentations for given scanning interval• Random Sequence• Jitter = “Random” Inter-Stimulus Interval (ISI/SOA)Where does jitter come from?(What’s a Null Condition?)• “Null” condition – fixation cross or dot• By hypothesis, no response to null• Insert random amounts of null between task conditions• Differential ISI = Differential Overlap A + B AAAA ++ + +BB BTimeRapid-Presentation Properties• Efficient (not as efficient as blocked)• Can distinguish responses despite overlap • Highly resistant to habituation, set, and expectation• Flexible timing (Behavioral, EEG, MEG)• Linear overlap assumption• Analysis: Selective Averaging/Deconvolution (GLM)• How to schedule stimulus onsets?Scheduling and EfficiencyA: N=5B: N=10C: N=10• Efficiency: statistical power/SNR/CNR per acquisition• Efficiency increases with N• Efficiency decreases with overlap• Efficiency increases with differential overlap• Choose schedule with optimum efficiency beforebefore scanningSummary• Facts of Life: Dispersion, Noise, Time• Blocked - Habituation, Expectation, Set, No Post-Hoc• Fixed-Interval Event-Related – Inefficient/Boring• Rapid-Presentation Event-Related• Randomized inter-stimulus onsets• Overlap Linearity• Efficient - Optimization Tool• Identical designs for Behavioral, fMRI, EEG, and
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