What is the difference between oxygenated and deoxygenated blood?
What is neural activity accompanied by?
- Oxygenated (Hb)
- Diamagentic
- Enhance Signal
- Deoxygenated (dHb)
- Paramagnetic: Distortions/artefacts
- Decrease Signal
- Neural Activity = Need glucose = Local oversupply in oxygenated blood = Better BOLD
Who discovered the science behind fMRI?
- Seiji Ogawa
- Large blood vessels (with lots of oxygen) cause brighter areas on scans (better signal)
BOLD fMRI technique make use of what fact?
All neurons need oxygen from blood
Does fMRI looking at action potentials?
No. fMRI reflects signal differences due to different oxygen levels
What do we use to map areas of enhanced activity into the structural image of the brain?
Statistical Parametric Mapping.
- A General Linear Model is fitted to brain activity at each measurement point/voxel (50,000-100,000x)
- (i.e. fit model to explain the data. how well does the model explain the data)
- Significantly stronger activation in region X for task A compared to task B is interpreted as involvement of the region X in task A
What are blobs in fMRI?
Activation “blobs” are statistical effects in experiment, often colour-coded for “activation” and “de-activation” (They are not activation per se)
In a typical fMRI experiment, how do we get BOLD?
- While participants engage in a cognitive task
- Repeated measurements to reduce noise
- Differences in BOLD tell us whether a brain region ‘is engaged’ in a task
What is the time between neural activity and the peak of the BOLD response?
How long does the BOLD signal need to reach back baseline after activation?
- Neural activity and peak
- Substantial temporal lag of about 8 seconds
- Neural activity and baseline
- About 16 seconds
What does the BOLD actually tell us about neural activity?
What does enhanced neural activity depend on?
Indirect measure of neural activity
- Enhanced neural activity impacts
- (a) Blood oxygen volume
- (b) Blood velocity
- (c) Extraction of oxygen
Complex interplay suggest that neural activity = oversupply of oxygen
Can we compare BOLD signals across different regions of the brain?
- Not valid to compare between different brain regions because the signal change is different
- Measured response function of BOLD across regions will look similar
- Hemodynamic Response Function (HRF)
- Modelled after data is analysed
- Only valid to compare within the same brain region between experimental conditions
What neural processes drive the BOLD signal? What does it tell us and not tell us?
Logothetis (2008)
Feedback processing within excitation-inhibition networks (EIN)
EIN
- Determines output (i.e. net excitation or inhibition) of the microunit.
- Local cortical small and highly inter-connected function microunits, which show massive recurrent feedback
- Does not tell us driving process
What are the 4 limitations of BOLD in Bio Psych?
- Blobs suggest brain is modular (Might not be true)
- Poor temporal resolution
- Good, but not great spatial resolution
- Multiple Comparisons Problem
Limitation BOLD #1?
- Blobs does not suggest brain is modular
- Might not be modular
- We do not see connectivity and information flow of full network involved
- fMRI may only show “Tip of the Iceberg” and might not always map functional units that matter
Limitation BOLD #2? How do we solve this?
Poor Temporal Resolution
- Slow HRF
- 2 Seconds to measure brain once
- Very fast processes (within the 2 seconds) are difficult to image
- We need to use “tricks” in experimenetal design to separate “events of interest” if we don’t want to wait for HRF to reach baseline
- Makes task artifical
Limitation BOLD #3?
Good, but not great spatial resolution
- Smallest measurement unit is a “voxel”
- 3x3x3mm
- Can’t learn processes within
- but one voxel still contains >100k neurons
Limitation BOLD #4? What is the solution (?)
Multiple Comparisons Problem
- Running a t-test for each voxel leads to too much false positives. At p < .01, 1% false positive x 50,000 (approx. voxels in the brain) adds up
- Bonferroni-correction (divide sig level by no. of test)
- Makes Sig findings difficult
What is the spatial resolution of fMRI?
1 - 3 mm
What is the typical temporal resolution of fMRI?
2 seconds
