Resting State & Event-Related fMRI

Question 1

What are the two experimental conditions?

Answer 1

1. Block/Epoch Design

2. Intermixed/Event-Related Design

Question 2

What is the relevance of event-related fMRI?

Answer 2

Post hoc categorization of trial-types according to the subject’s behavioure.g. recognition memory task
Modelling events whose occurrence is beyondexperimental control, such as those that canonly be indicated by the subjecte.g. perceptual transitions in the vase-faceillusion
Some trials cannot be blockede.g. “Oddball” paradigms, epileptic events (on EEG)

Question 3

What are the disadvantages of event-related designs?

Answer 3

1. Less efficient for detecting effects than are block designs
2. Some psychological processes may be better blocked e.g. task-switching, attentional instructions

Question 4

What are advantages of event-related fMRI

Answer 4

• Randomised trials order
• Post-hoc subjective classification of trials
• Some events can only be indicated by participants
- Some events cannot be blocked due to stimulus context
- More accurate model even for epoch/block designs

Question 5

Example of Event-related Resting-state FMRI: BOLD correlates of the Alpha rhythm:

Answer 5

• Put electrodes on the back of the head
• Ask subjects to close eyes – block alpha
• Record EEG and calculate the power in the alpha band – fluctuate naturally
• Convolute with the canonical HRF and get the desired  post-hoc design

Question 6

What part of the brain have this kind of blood flow variations?

Answer 6

• predicting based on the ongoing EEG
• Put in GLM and SPM analyses
• region in the brain in which signal correlates strongly to predictor

Question 7

BOLD of Alpha rhythm (continued):

Answer 7

• The brain regions were more active when the alpha power was reduced – get a negative correlation
• EEG will tell you about the different waves of the scalp, but it is not easy to infer from that which part of the brain will be more active

Question 8

What is alternative design for BOLD of Alpha rhythm?

Answer 8

1. Berger effect

2. Alternating blocks of eyes closed and open

Question 9

What is the spike-triggered FMRI?

Answer 9

• Patient with epilepsy
• 43 spikes on EEG during FMRI scan
• Record EEG during FMRI and look back at EEG recorded in the scanner
• Identify which scan occurred during the spike and which scan occurred when there was no spike
• Build regressors/predictors based on that
• Classify scans as being in the resting or active state
• If there is an epileptic spike at time T=0 – which scans do you expect to be different from the baseline

Question 10

The BOLD Impulse Response

Answer 10

• Represents the BOLD response to a brief stimulus
• Is a function of blood oxygenation, flow, volume?
• Its typical features are:
- Peak at 4-6s post-stimulus
- Undershoot at ~15s post-stimulus
- Return to baseline after 20-30s
• Its common mathematical representation is the Canonical hemodynamic response function (Canonical HRF)
• It varies across:
- Brain regions
- Individuals

Question 11

What is a good experimental design?

Answer 11

A design that gives you results that are straight-forwaed to interpret

Question 12

What can designs be?

Answer 12

1. Blocked or intermixed

2. But models for blocked designs can be epoch - or event-related

Question 13

What are epochs?

Answer 13

Periods of sustained stimulation (e.g. box-car functions)

- Events are impulses (delta-function)

Question 14

What can near-identical regressors be created by?

Answer 14

1. Sustained epochs

2. Rapid series of events

Question 15

What are all conditions specified in SPM9?

Answer 15

1. Onset
2. Durations
   Epochs = variable or constant duration
   Events = Zero duration

Question 16

What can blocks of trials be modelled as?

Answer 16

Boxcars or runs of events

Interpretation of the parameter estimate may differ

Question 17

What does epoch model estimate?

Answer 17

Parameter that increases with rate because the parameter reflects response per block

Question 18

What does event model estimate?

Answer 18

Parameter that decreases with rate because the parameter reflects responses per word

Question 19

What is the BOLD impulse response?

Answer 19

1. Function of blood oxygenation, flow, volume
2. Peak (max. oxygenation) 4-6 seconds post-stimulus: baseline after 20-30s
3. Initial undershoot can be observed
4. Similar across V1, A1, S1
5. But possible differences across:
   - Other regions
   - Individuals

Question 20

What is a typical haemodyanmic response observed?

Answer 20

Primary sensory areas such as visual cortex/ auditory cortex

Question 21

What can the shape of the BOLD impulse response impose?

Answer 21

Constraints on design efficiency

Question 22

What is the linear convolution model?

Answer 22

The predicted fMRI series is obtained by convolving a neural function (e.g. stimulus function) with an assumed IR

Question 23

What is the basic idea behind maximising efficiency for linear convolution model?

Answer 23

Maximise energy of the predicted fMRI timeseries

Question 24

What is the benefit of maximising the variability of signal?

Answer 24

Detect the signal in the presence of background noise

Question 25

What is the temporal basis function?

Answer 25

1. Fourier Set
   - Windowed sines & Cosines
   - Any shape (up to frequency limit)
   - Inference via F-test
2. Finite impulse Response
   - Mini ‘timebins’ (selective averaging)
   - Any shape (up to bin-width)
   - Inference via F-test

Question 26

What is the timing issues: sampling?

Answer 26

Typical TR for 60 slice EPI at 3mm spacing is ~3-4seconds

Sampling at [0,4,8,12] post-stimulus may miss peak signal

Question 27

What can be done to improve timing issues: sampling?

Answer 27

Higher effective sampling by:

• Asynchrony e.g. SOA=1.5TR
• Random Jitter e.g. SOA= (2+/- 0.5)TR

Question 28

What is timing issues: slice timing?

Answer 28

The model of the haemodynamic response that is built in the convolution model is of a very high temporal resolution

Every measure that you take of the whole brain takes seconds

in the model of the haemodynamic response - only 1 value is assigned to each of the slices

Which particular time point during volume acqusition is the time point during which i want to sample - Reference slice

Question 29

What is the slice-timing problem?

Answer 29

1. Slices are acquired at different times, yet model is the same for all slices
2. Different results (using canonical HRF) for different reference slices

Question 30

What is the acqusition (partial) solutions?

Answer 30

1. Middle slice as reference

2. Short TR

Question 31

What are the modelling solutions

Answer 31

Temporal interpolation of data: “Slice timing correction”

Use a more general basis set (e.g., with temporal derivatives)

Question 32

What is the design effficency?

Answer 32

1. HRF can be viewed as a filter
2. We want to maximise the signal passed by this filter
3. Dominant frequency of canonical HRF is ~0.04 Hz

Question 33

What is the most efficient design?

Answer 33

Sinusoidal modulation of neural activity with period ~24 seconds
(e.g. boxcar with 12s on/ 12s off)

Question 34

What is block design

Answer 34

Alternate between blocks of two types of stimuli - A rest and a stimulus with a temporal property that has a dominant frequency

Question 35

What is design efficiency?

Answer 35

A measure of how well an experiment can be expected to reveal the effect of interest

Question 36

Block designs

Answer 36

Generally efficient but often not appropriate.

Optimal block length 16s(beware of high-pass filter).

Question 37

Event-related designs:

Answer 37

Efficiency depends on the contrast of interest
With short SOAs the inclusion of ‘null events’ (jittered ITI) can help optimise efficiency across multiple contrasts.
Non-linear effects start to become problematic at SOA<2s