1. Advances in Event-Related fMRI Design
Douglas N. Greve

2. Outline What is Event-Related Design? Blocked Design
Fixed-Interval Event-Related
Rapid-Presentation (Jittered) Event-Related
Efficiency and Event Scheduling
Blocked Design gives context for ER. RP-ER brings problem of scheduling.

3. Fact of (fMRI) Life #1: Dispersion
Not like EEG/MEG where response is finished after a few hundred ms. How closely can trials/events be placed and still be able to differentiate their responses? This data is the average of 212 responses.
How closely can trials/events be spaced?

4. Fact of (fMRI) Life #2: Noise
Ultimately, noise is the rate limiting factor. The further away from baseline, the easier it is to detect the response. These are stddev bars. Error bars will shrink as more data are collected. To detect a response, need to (1) create a bigger signal, and/or (2) collect many trials. Down side to many trials: (1) cost, (2) wear and tear on subjects, (3) need all that stimuli.
How much data needs to be collected?

5. Fact of (fMRI) Life #3: Time
Collect lots of observations to reduce noise
Time is Money
Subjects won’t work forever
Stimuli – famous faces, non-words, syntactically anomalous sentences.

6. Blocked Design Consecutive, rapid presentation for long duration.
fixation
Consecutive, rapid presentation for long duration.
Use overlap to build a larger signal.
Simple analysis.
Optimal for detection.
Each color represents a different type of stimulus. May be a stimulus or task of long duration instead of multiple rapid presentations. Example of stimulus types: global/local, novel/repeated, happy/sad faces. Assumption: invariant asymptotic. Use long dispersion to create overlap and generate a larger signal.

7. Using Overlap to Increase Amplitude
The effect of rapid presentation on the amplitude of the response. Don’t get 3 times the amplitude with 3 trials. The greater the amplitude, the more it stands out from the noise. 1 2 3

8. Blocked Design Drawbacks
Lose ability to distinguish individual responses
Confounding psychological and physiological effects
Habituation/Adaptation
Expectation
Set (Strategy)
Experimenter may be trying to measure set. Highly constrained design. Not good for Multi-modal. Little info about hemodynamics. Habituation – that face AGAIN. Expectation/Anticipation.
Reminder: efficient.

9. 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
Already described an event in first slide. Single trial.

10. 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)
Blue bar is length of typical block from blocked design. Raw signal still interpretable. Assumption: time-invariant response. Not very many presentations. Most efficient without overlap is fixed at minimum time needed to prevent overlap.

11. 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)
It is possible to present stimuli so close to each other that they overlap and still distinguish between responses. Must observe all levels of overlap. Raw signal uninterpretable. ISI/SOA is a randomly chosen multiple of the TR.

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
The null condition is not a condition. Differential overlap and gender/handedness. Conditions A and B with fixation (+). Yellow bars indicate the ISI between A trials. Red bars indicate ISI between B trials. Green bars indicate ISI from A to B. Differential overlap can come from non-adjacent presentations. Zero ISI. No bars between null and A or B because null is not a condition.
Time

13. 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?

14. Scheduling and Efficiency
B: N=10
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
Three designs: A, B, and C all within a fixed scanning interval. A has no overlap but only N=5. B has N=10 but no differential overlap. C has N=10 and differential overlap. Larger the N, the lower the ISI. Differential overlap: cf gender/handedness test.

15. 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
Blocked uses dispersion to create a bigger signal at the cost of distinguishing individual responses (and hab, exp, and set).