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Event-related fMRI (er-fMRI)
Methods & Models for fMRI Data Analysis06.11.2015
With many thanks for slides & images to:FIL Methods group, particularly Rik Henson and Christian Ruff
Jakob Heinzle & Klaas Enno Stephan
Translational Neuromodeling Unit (TNU)Institute for Biomedical Engineering, University of Zurich & ETH Zurich
Laboratory for Social & Neural Systems Research (SNS), University of Zurich
Wellcome Trust Centre for Neuroimaging, University College London
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Overview of SPM
Realignment Smoothing
Normalisation
General linear model
Statistical parametric map (SPM)Image time-series
Parameter estimates
Design matrix
Template
Kernel
Gaussian field theory
p <0.05
Statisticalinference
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Overview
1. Advantages of er-fMRI
2. BOLD impulse response
3. General Linear Model
4. Temporal basis functions
5. Timing issues
6. Design optimisation
7. Nonlinearities at short SOAs
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Advantages of er-fMRI
1. Randomised trial orderc.f. confounds of blocked designs
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Pleasant (P)Unpleasant (U)
Blocked designs may trigger expectations and cognitive sets
…
Intermixed designs can minimise this by stimulus randomisation
… … ………
er-fMRI: Stimulus randomisation
Unpleasant (U) Unpleasant (U) Unpleasant (U)Pleasant (P)
Pleasant (P)
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Advantages of er-fMRI
1. Randomised trial orderc.f. confounds of blocked designs
2. Post hoc classification of trialse.g. according to performance or subsequent memory
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er-fMRI: post-hoc classification of trials
Gonsalves & Paller (2000) Nature Neuroscience
Items with wrong memory of picture („hat“) were associated with more occipital activity at encoding than items with correct rejection („brain“)
„was shown as picture“
„was not shown as picture“
Participant response:
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Advantages of er-fMRI
1. Randomised trial orderc.f. confounds of blocked designs
2. Post hoc classification of trialse.g. according to performance or subsequent memory
3. Some events can only be indicated by the subjecte.g. spontaneous perceptual changes
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eFMRI: “on-line” event-definition
Bistable percepts
Binocular rivalry
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Advantages of er-fMRI
1. Randomised trial orderc.f. confounds of blocked designs
2. Post hoc classification of trialse.g. according to performance or subsequent memory
3. Some events can only be indicated by the subjecte.g. spontaneous perceptual changes
4. Some trials cannot be blockede.g. “oddball” designs
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…
er-fMRI: “oddball” designs
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Advantages of er-fMRI
1. Randomised trial orderc.f. confounds of blocked designs
2. Post hoc classification of trialse.g. according to performance or subsequent memory
3. Some events can only be indicated by subjecte.g. spontaneous perceptual changes
4. Some trials cannot be blockede.g. “oddball” designs
5. More accurate models even for blocked designs?“state-item” interactions
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P1 P2 P3
“Event” model may capture state-item interactions
U1 U2 U3
“Epoch” model assumes constant neural processes throughout block
U1 U2 U3 P1 P2 P3
er-fMRI: “event-based” model of block-designs
DataModel
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Convolved with HRF
Series of eventsDelta
functions
• Designs can be blocked or intermixed, BUT models for blocked designs can be epoch- or event-related
• Epochs are periods of sustained stimulation (e.g, box-car functions)
• Events are impulses (delta-functions)
• Near-identical regressors can be created by 1) sustained epochs, 2) rapid series of events (SOAs<~3s)
• In SPM, all conditions are specified in terms of their 1) onsets and 2) durations… epochs: variable or constant duration… events: zero duration
“Classic” Boxcar function
Sustained epoch
Modeling block designs: epochs vs events
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Disadvantages of er-fMRI
1. Less efficient for detecting effects than blocked designs.
2. Some psychological processes may be better blocked (e.g. task-switching, attentional instructions).
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• Function of blood volume and deoxyhemoglobin content (Buxton et al. 1998)
• Peak (max. oxygenation) 4-6s post-stimulus; return to baseline after 20-30s
• initial undershoot sometimes observed (Malonek & Grinvald, 1996)
• Similar across V1, A1, S1…
• … but differences across other regions (Schacter et al. 1997) and individuals (Aguirre et al. 1998)
BOLD impulse response
BriefStimulus
Undershoot
InitialUndershoot
Peak
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• Early er-fMRI studies used a long Stimulus Onset Asynchrony (SOA) to allow BOLD response to return to baseline.
• However, if the BOLD response is explicitly modelled, overlap between successive responses at short SOAs can be accommodated…
• … particularly if responses are assumed to superpose linearly.
• Short SOAs can give a more efficient design (see below).
BOLD impulse response
BriefStimulus
Undershoot
InitialUndershoot
Peak
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General Linear (Convolution) Model
For block designs, the exact shape of the convolution kernel (i.e. HRF) does not matter much.
For event-related designs this becomes much more important.
Usually, we use more than a single basis function to model the HRF.
GLM for a single voxel:
y(t) = u(t) ⊗ h(τ) + ε(t) Design Matrix
convolution
T 2T 3T ...
u(t) h(τ)=∑ ßi fi (τ)
sampled each scan
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Temporal basis functionsFinite Impulse Response (FIR) model Fourier basis set
Gamma functions set Informed basis set
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Informed basis set
• Canonical HRF (2 gamma functions)plus Multivariate Taylor expansion in:
time (Temporal Derivative)width (Dispersion Derivative)
• F-tests allow testing for any responses of any shape.
• T-tests on canonical HRF alone (at 1st
level) can be improved by derivatives reducing residual error, and can be interpreted as “amplitude” differences, assuming canonical HRFis a reasonable fit.
CanonicalTemporalDispersion
Friston et al. 1998, NeuroImage
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Temporal basis sets: Which one?
+ FIR+ Dispersion+ TemporalCanonical
• canonical + temporal + dispersion derivatives appear sufficient• may not be for more complex trials (e.g. stimulus-delay-response)• but then such trials better modelled with separate neural components
(i.e. activity no longer delta function) (Zarahn, 1999)
In this example (rapid motor response to faces, Henson et al, 2001)…
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Penny et al. 2007, Hum. Brain Mapp.
right occipital cortexleft occipital cortex
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Timing Issues : Practical
• Assume TR is 4s
• Sampling at [0,4,8,12…] post- stimulus may miss peak signal
Scans TR=4s
Stimulus (synchronous)
Sampling rate=4s
SOA = Stimulus onset asynchrony(= time between onsets of two subsequent stimuli)
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Timing Issues : Practical
• Assume TR is 4s
• Sampling at [0,4,8,12…] post- stimulus may miss peak signal
• Higher effective sampling by:
– 1. Asynchrony, e.g. SOA = 1.5×TR
Scans TR=4s
Stimulus (asynchronous)
Sampling rate=2s
SOA = Stimulus onset asynchrony(= time between onsets of two subsequent stimuli)
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Timing Issues : Practical
• Assume TR is 4s
• Sampling at [0,4,8,12…] post- stimulus may miss peak signal
• Higher effective sampling by:
– 1. Asynchrony, e.g. SOA = 1.5×TR
– 2. Random jitter, e.g. SOA = (2 ±0.5)×TR
• Better response characterisation (Miezin et al, 2000)
Scans TR=4s
Stimulus (random jitter)
Sampling rate=2s
SOA = Stimulus onset asynchrony(= time between onsets of two subsequent stimuli)
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Slice-timing
T=16, TR=2s
Scan0 1
o
T0=9 oT0=16
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Slice-timing
• Slices acquired at different times, yet model is the same for all slices=> different results (using canonical HRF) for different reference slices
• Solutions:
1. Temporal interpolation of data… but less good for longer TRs
2. More general basis set (e.g. with temporal derivatives)
… but more complicated design matrix
Derivative
SPM{F}
Interpolated
SPM{t}
Bottom sliceTop slice
SPM{t} SPM{t}
TR=3s
Henson et al. 1999
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Design efficiency
1122 ))(ˆ(),,ˆ( −−= cXXcXc TTσσε
)ˆvar(
ˆ
β
βT
T
c
cT =• The aim is to minimize the standard error of a t-contrast
(i.e. the denominator of a t-statistic).
cXXcc TTT 12 )(ˆ)ˆvar( −= σβ
• This is equivalent to maximizing the efficiency ε:
Noise variance Design variance
• If we assume that the noise variance is independent of the specific design:
11 ))((),( −−= cXXcXc TTε
• This is a relative measure: all we can say is that one design is more efficient than another (for a given contrast).
NB: efficiency depends on design
matrix and the chosen contrast !
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⊗ =
Fixed SOA = 16s
Not particularly efficient…
Stimulus (“Neural”) HRF Predicted Data
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⊗ =
Fixed SOA = 4s
Very inefficient…
Stimulus (“Neural”) HRF Predicted Data
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⊗ =
Randomised, SOAmin= 4s
More efficient …
Stimulus (“Neural”) HRF Predicted Data
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⊗ =
Blocked, SOAmin= 4s
Even more efficient…
Stimulus (“Neural”) HRF Predicted Data
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Another perspective on efficiency
efficiency = bandpassed signal energy
Hemodynamic transfer function (based on canonical HRF):neural activity (Hz) → BOLD
Highpass-filtered
Josephs & Henson 1999, Phil Trans B
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⊗ =
×
Blocked, epoch = 20s
=
Blocked-epoch (with short SOA)
Stimulus (“Neural”) HRF Predicted Data
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⊗ =
Sinusoidal modulation, f = 1/33s
× =
The most efficient design of all!
Stimulus (“Neural”) HRF Predicted Data
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× =
⊗
Blocked (80s), SOAmin=4s, highpass filter = 1/120s
Don’t use long (>60s) blocks!
=
Stimulus (“Neural”) HRF Predicted data(incl. HP filtering!)
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Randomised, SOAmin=4s, highpass filter = 1/120s
⊗ =
× =
Randomised design spreads power over frequencies.
Stimulus (“Neural”) HRF Predicted Data
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4s smoothing; 1/60s highpass filtering
Efficiency: Multiple event types• Design parametrised by:
SOAmin Minimum SOApi(h) Probability of event-type igiven history h of last m events
• With n event-types pi(h) is a nm × nTransition Matrix
• Example: Randomised AB
A BA 0.5 0.5 B 0.5 0.5
=> ABBBABAABABAAA...
Differential Effect (A-B)
Common Effect (A+B)
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4s smoothing; 1/60s highpass filtering
• Example: Alternating ABA B
A 0 1 B 1 0
=> ABABABABABAB... Alternating (A-B)
Permuted (A-B)
Efficiency: Multiple event types
• Example: Permuted ABA B
AA 0 1AB 0.5 0.5 BA 0.5 0.5BB 1 0
=> ABBAABABABBA...
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4s smoothing; 1/60s highpass filtering
• Example: Null eventsA B
A 0.33 0.33B 0.33 0.33
=> AB-BAA--B---ABB...
• Efficient for differential and main effects at short SOA
• Equivalent to stochastic SOA (null event corresponds to a third unmodelled event-type)
Null Events (A+B)
Null Events (A-B)
Efficiency: Multiple event types
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Design efficiency: Conclusions• Optimal design for one contrast may not be optimal for another
• Blocked designs generally most efficient (with short SOAs, given optimal block length is not exceeded)
• However, psychological efficiency often dictates intermixed designs, and often also sets limits on SOAs
• With randomised designs, optimal SOA for differential effect (A-B) is minimal SOA (>2 seconds, and assuming no saturation), whereas optimal SOA for main effect (A+B) is 16-20s
• Inclusion of null events improves efficiency for main effect at short SOAs (at cost of efficiency for differential effects)
• If order constrained, intermediate SOAs (5-20s) can be optimal
• If SOA constrained, pseudorandomised designs can be optimal (but may introduce context-sensitivity)
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But beware: Nonlinearities at short SOAs
Friston et al. 2000, NeuroImage Friston et al. 1998, Magn. Res. Med.
stim. presented alone
stim. when preceded by another stim. (1 s)
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Thank you