Rapid-Presentation Event-Related Design for fMRI

1. Rapid-Presentation Event-Related Design for fMRI Douglas N. Greve

2. Outline • What is Event-Related Design? • Fixed-Interval Event-Related • Rapid-Presentation (Jittered) Event-Related • Efficiency and Event Scheduling • Mathematical Basis • optseq – a tool for RPER design • (http://surfer.nmr.mgh.harvard.edu/optseq) Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

3. Dispersion • Dispersion is the spreading out of the response over time, usually far beyond the end of the stimulus • How closely can one event follow another? Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

4. Event-Related fMRI • Estimate response from a single event type • cf Blocked Design (Habituation, Expectation, Set, Power) • Randomize Schedule (Order and Timing) • Post-Hoc Event Sorting • Multimodal Integration (EEG/MEG,Behavioral) • Fixed Interval and Rapid Presentation (Jittered/Stochastic) Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

5. Event vs Event Type • Three Event Types (yellow, red, green) • Number of Events (Repetitions) per Event Type • Yellow: 2 • Red: 2 • Green: 3 • Two events belong to the same Event Type if, by hypothesis, they have the same response (violations are treated as noise). • Event Type = Condition = Trial Type = Explanatory Variable • Event = Stimulus = Trial Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

6. Event Schedule • Description of which event is presented when time code duration label 4.0 2 4 yellow 20.0 1 2 red 36.0 1 2 red 52.0 3 6 green • Time is the accumulated time since onset of scanning run • Code unique numeric id • Output of optseq Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

7. 12-20s Fixed-Interval Event-Related • 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-Presenation Event-related Design for fMRI -- Douglas N. Greve

8. Rapid-Presentation Event-Related • Closely Spaced Trials (Overlap!) • Raw signal uninterpretable • Random Sequence and Schedule • Highly resistant to habituation, set, and expectation • Jitter = “Random” Inter-Stimulus Interval (ISI/SOA) Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

9. A: N=5 B: N=10 Scheduling and Efficiency C: N=10 • Efficiency is statistical power/SNR/CNR per acquisition • Efficiency increases with N (number of observations) • Efficiency decreases with overlap • Efficiency increases with differential overlap • Choose schedule with optimum efficiency before scanning Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

10. Mathematical Concepts Forward Model (X = design matrix) Inverse Model Residual Error Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

11. Contrast, Contrast Vector (or Matrix), Contrast Effect Size, COPE (FSL) t-Ratio Efficiency Variance Reduction Factor Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

12. 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 A + A B A + + B + B A Time Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

13. Design Parameters (optseq) • TR – time between volume acquisition (temporal resolution). • Ntp – number of time points (TRs, frames, volumes, …) • Nc – number of event types (conditions) • Npc – number of events/repetitions of each event type (can vary across event types) • Tpc – duration of each event type (can vary across event types) • Schedule – event onset time and identity • Event Response Model – FIR Post-Stimulus Delay Window (needed for optimization) Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

14. Time Constraints Total Scan Time = Ntp * TR A B C + Null Time Ta = Na*Tpa Tb = Nb*Tpb Tc = Nc*Tpc Total Stimulation Time • Total Stimulation Time Cannot Exceed Total Scan Time • How much Null Time is needed? Rule of thumb: same as any other task condition (or the average of the task conditions). Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

15. PSDMin PSD=0 PSDMax dPSD Event Response Model (FIR) • PSD: Post-Stimulus Delay (PSD = 0 = Stimulus Onset) • PSDMin: Response is zero for PSD < PSDMin • PSDMax: Response is zero for PSD > PSDMax • PSD Window should be long enough to capture response • Response can be anything in between (FIR model) • dPSD: sets basic temporal resolution for schedule • DOF Constraint: Nbeta = nPSD*Nc < Ntp Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

16. Other optseq Parameters/Options • Getting help: optseq2 --help • Search termination criteria: nsearch/tsearch • Output files (and format) • Optimizing over number of repetitions • Nuisance variables (polynomial drift terms) • Cost Functions • First-Order Counter-Balancing Pre-optimization • http://surfer.nmr.mgh.harvard.edu/optseq • To come: contrasts and non-FIR Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve

17. Rapid-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) • Schedule Optimization Tool (optseq) Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve