Transcript Understanding Differential Responses in fMRI Through

Rapid-Presentation Event-Related
Design for fMRI
Douglas N. Greve
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
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
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
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
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
Fixed-Interval Event-Related
12-20s
• 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-Presenation Event-related Design for fMRI -- Douglas N. Greve
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
Scheduling and Efficiency
A: N=5
B: N=10
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
Mathematical Concepts
y  X n
Forward Model
(X = design matrix)
T
1
ˆ
  ( X X ) Xy
Inverse Model
ˆ
ˆ
e  y y  y X
Residual Error
Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve
Contrast, Contrast Vector
(or Matrix), Contrast Effect
Size, COPE (FSL)
  Cˆ
tDOF 

 (C ( X X ) C )
2
e
T
1
T
1
eff 
T
1 T
trace(C ( X X ) C )
t-Ratio
Efficiency
Variance Reduction Factor
1
T
1 T
VRFi  , d  diag (C ( X X ) C )
di
Rapid-Presenation Event-related Design for fMRI -- Douglas N. Greve
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
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
Time Constraints
Total Scan Time = Ntp * TR
A
B
Ta = Na*Tpa Tb = Nb*Tpb
C
Tc = Nc*Tpc
+
Null Time
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
Event Response Model (FIR)
PSDMin PSD=0
PSDMax
dPSD
• 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
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
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