4. Experimental Design and Analysis in fMRI Flashcards
BOLD (___) signal is an ___ measure of ____
Blood oxygenation level dependent signal is an indirect measure of neural activity
What is cognitive subtraction (fRMI design)?
Choosing two tasks that only differ in one critical respect, then compare the BOLD signals generated by each task
- regions showing a difference in avg activity may be contributing to the varied element of the task
Which experimental design should fMRI experiments use?
Because of individual variation in the BOLD response and anatomy, fMRI experiments should use a WITHIN-SUBJECTS design wherever possible
Why is a block design very important for investigating activity with fMRI?
The responses to individual trials cannot be distinguished, but add-up to produce a robust signal change.
What is the differences between longer and shorter blocks in fMRI experiment designs?
- At longer interblock intervals, the hemodynamic response can return to baseline during the control condition
- Shorter blocks allow more repetitions, but the sluggish hemodynamic response means less signal change – harder to distinguish from noise.
What are the advantages and disadvantages of a block design in fMRI experiments?
+ excellent power to detect active voxels; improved signal to noise (relative to event related design)
- responses to individual trials cannot be analyzed
What is an Event-related design in fMRI experiments?
- Each event (typically a single experimental trial or part of it) is modelled separately.
- Trials are carefully spaced and ordered so that time-locked responses can be estimated for different classes of trial
What are the advantages and disadvantages of event-related design in fMRI?
+ data can be analyzed based on responses (e.g. sorting trials into correct and incorrect and comparing them)
+ Timecourse of the BOLD response can be accurately estimated
- power to detect active voxels is reduced
fMRI preprocessing: Motion correction
Successive images are aligned with one-another
fMRI preprocessing: Spatial smoothing
Blurring (increases signal to noise)
fMRI preprocessing: High pass temporal filtering removes
Gradual changes in signal intensity
What do statistical maps indicate? (fMRI)
maps indicate the probability of a type 1 error (false positive), not absolute signal change
- the lower this probability (p-value) the brighter/hotter the colour
What is the Bonferroni correction?
takes account of the number of comparisons made (too conservative for fMRI)
What is the FWE (Family Wise Error) voxel correction?
Takes account of number of INDEPENDENT comparisons
What is the FDR (False discovery rate) correction?
Controls false-discovery rate
What is the cluster-size correction? (fMRI)
Takes account of contiguity (the state of bordering or being in contact with something) of active voxels
What are whole brain analysis useful for?
which brain regions are selectively engaged in a given task:
- spatial pattern of activity associated with different tasks: gives an indication of which tasks overlap in terms of the underlying brain processes.
- identify range of tasks which activate specific brain regions: gives an indication of what specific brain regions do.
One way to avoid the problem of multiple comparisons? (fMRI)
Restricting analysis to data from a particular part of the brain, IDENTIFIED in ADVANCE
ROIs can be defined using ____ or ____ criteria
anatomical or functional
What are the advantages and disadvantages of ROI analysis?
+ Increased statistical power
+ more targeted approach - leads to better specified hypotheses?
+ can be combined with whole brain analysis
- misses potentially important patterns outside the ROI
- danger of biased post-hoc selection of ROI
stats in a combined statistical map take into account ____ ____
individual variation
Whole-brain group analysis demands that different brains…
are aligned with one another in the same space
fMRI design needs to consider:
- Timecourse (event related vs block design)
2. Intersubject variability of the hemodynamic response (Within vs between subjects)
After spatial and temporal preprocessing, fMRI analysis uses a ____ to estimate task related changes in the BOLD signal
general linear model (GLM)
(In fMRI) The statistical significance of differences between conditions is determined by…
mapping statistical parameters to brain voxels, using appropriate thresholds to avoid false positives (Type 1 errors)
To support robust statistical inferences which will/can be extended to a wider population, multi-subject fMRI experiments typically employ:
Random/mixed-effects, within-subjects design
In the analysis of a typical fMRI experiment, BOLD signal changes are modelled by combining predicted timeseries for each distinct class of event (or block) which are fitted to the fMRI data…
to minimize residual variation
If BOLD responses to individual experimental trials of the same task are to be compared (e.g., correct versus incorrect trials) in an fMRI study, it is essential to use:
an event related design
