Lecture 4: Flashcards
What does MRI stand for and what is it used for?
Magnetic Resonance Imaging; originally for structural imaging, now also used for functional brain imaging (fMRI).
What are key advantages of MRI/fMRI?
Non-invasive, high spatial resolution, and reasonably affordable.
How strong is an MRI scanner compared to Earth’s magnetic field?
MRI: 1.5–7 Tesla; Earth: 0.5 Gauss; Exeter scanner = 30,000x Earth’s field.
Why must metal be avoided in MRI scanners?
Strong magnetic fields can cause metal objects to move or interfere with the scan.
What does BOLD stand for in fMRI?
Blood Oxygen Level Dependent signal.
What does the BOLD signal measure?
Differences in magnetic properties of oxygenated vs. deoxygenated hemoglobin—indirect measure of neural activity.
What does fMRI NOT directly measure?
It does not directly measure brain activation; it measures blood flow.
Why is a baseline important in fMRI experiments?
The BOLD signal has no stable baseline, so comparisons require both experimental and baseline conditions.
What makes a good fMRI baseline?
It differs from the experimental condition only by the process of interest (e.g., scrambled faces for face processing).
What is a block design in fMRI?
Long alternating periods of task and baseline; originally the standard design.
What are disadvantages of block designs?
Predictable, inflexible, poor ecological validity, and can’t isolate responses to individual stimuli or errors.
What is an event-related design in fMRI?
Trials of different types are randomly intermixed and close in time.
What are advantages of event-related designs?
Flexible, unpredictable, suitable for complex tasks, and allows trial-by-trial analysis.
What are the key preprocessing steps in fMRI data?
High pass filtering, motion correction, slice time correction, coregistration, normalization, spatial smoothing.
What is the purpose of spatial smoothing?
To account for neural activity occurring in clusters, improving signal detection.
What is Montreal Neurological Institute (MNI) space?
A standardized brain space created from 352 MRI scans for comparing across subjects.
What is the main method of fMRI data analysis?
Multiple regression to relate voxel BOLD signals to condition timecourses.
What are beta values in fMRI?
Measures of how strongly a voxel’s activity correlates with a specific condition’s timecourse.
What is a contrast in fMRI analysis?
A t-test comparing beta values between conditions to find significant differences.
Why apply thresholds in fMRI t-maps?
To determine statistical significance and control false positives (e.g., p < 0.05).
Why is correction for multiple comparisons important?
With 130,000 voxel tests, uncorrected analysis leads to high risk of false positives.
What is whole brain analysis in fMRI?
Voxel-by-voxel analysis across the entire brain without hypotheses.
Pros and cons of whole brain analysis?
Pros: Exploratory, includes full brain. Cons: Can be hard to interpret, requires multiple comparison correction.
What is ROI (Region of Interest) analysis?
Analysis focused on a specific brain region based on prior hypotheses.
Pros and cons of ROI analysis?
Pros: Hypothesis-driven, avoids multiple comparisons, simple. Cons: May miss unexpected effects elsewhere.
What are limitations of fMRI?
Correlational data (not causal), poor temporal resolution—requires TMS/EEG for timing or causality.
