advances in event related fmri design l.
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Advances in Event-Related fMRI Design. Douglas N. Greve. Outline. What is Event-Related Design? Blocked Design Fixed-Interval Event-Related Rapid-Presentation (Jittered) Event-Related Efficiency and Event Scheduling. Fact of (fMRI) Life #1: Dispersion.

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Presentation Transcript
slide2

Outline

  • What is Event-Related Design?
  • Blocked Design
  • Fixed-Interval Event-Related
  • Rapid-Presentation (Jittered) Event-Related
  • Efficiency and Event Scheduling
fact of fmri life 1 dispersion
Fact of (fMRI) Life #1: Dispersion
  • How closely can trials/events be spaced?
fact of fmri life 2 noise
Fact of (fMRI) Life #2: Noise
  • How much data needs to be collected?
fact of fmri life 3 time
Fact of (fMRI) Life #3: Time
  • Collect lots of observations to reduce noise
  • Time is Money
  • Subjects won’t work forever
blocked design
Blocked Design

20-60 sec

20-60 sec

fixation

  • Consecutive, rapid presentation for long duration.
  • Use overlap to build a larger signal.
  • Simple analysis.
  • Optimal for detection.
blocked design drawbacks
Blocked Design Drawbacks
  • Lose ability to distinguish individual responses
  • Confounding psychological and physiological effects
    • Habituation/Adaptation
    • Expectation
    • Set (Strategy)
  • Reminder: efficient.
slide9

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 Blocked
fixed interval event related
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 presentation event related
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
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 presentation properties
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)
  • How to schedule stimulus onsets?
scheduling and efficiency

A: N=5

B: N=10

Scheduling and Efficiency

C: 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 before scanning
summary
Summary
  • 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 MEG