W1L1 - fMRI Flashcards
What direct and indirect ways neural activity can be measured
Directly
- AP (Single neruon)
- Local Field Potentials (Summed activity)
Indirectly
- Metabolic changes
- Blood flow
- Cerebral blood volume
- Blood volume
- PET/fMRI
What is the function of fMRI
Localise hemodynamic changes from neural activity.
Why is fMRI popular, contrast with other methods
- Non-Invasive
- No needles, unlike PET
- Enable human studies
- Focal
- High precision, unlike EEG
What is the physics behind the basic MRI (fast)
- Hydrogen has a single proton which preccesses around an axis
- RF pulse, aligns parallel/anti-parallel
- After swtich off, spins back
Cognitive processes require energy. Where do we get energy and how does it relate to fMRI
- Cognitive Processes =
- ATP =
- Use oxygen from hemoglobin =
- Reverse ion influxes underlying synaptic potentials and action potentials
- fMRI relies on difference in magnetic responses between oxyhemoglobin and deoxyhemoglobin blood
Oxygenated vs Deoxygenated Blood. Difference in MRI signal
Oxygenated
- Weakly diamagnetic
- Does not distort magnetic field
Deoxygenated
- Paramagnetic
- Distorts magnetic field
What is the standard practice in analysing BOLD?
-
Spatial smoothing by 8mm
- Allow for group averaging by correspondence across brain
- Use g_eneral linear mode_l to quantify BOLD changes
- Correlation between time course of the BOLD signal change in each voxel of the smoothed images with the measure of cognitive function.
What is the implication of the BOLD response (thus far)
Cortical Segregation/Modularity
- Explains spatial structure of brain responses
- ‘Neo-phrenology’
What is the amptitude of BOLD signal correlated with?
Amplitude of the BOLD signal associated with
- Local field potential
- Large no. of active neurons responsive together
- Increases in gamma-band electrical
- EEG
- Quite often correlated with spike frequency
- Animal Studies
- Electrocorticographic (ECoG) at mm accuracy
> Confidence that BOLD is associated with activity
What does magnetic suspectibility of blood depend on?
- Blood oxygenation, but also depend on
- Regional cerebral blood volume (CBV)
- Not independent
What are the main limitations of fMRI (together with elaborations)
1.) Mislocalisation of hemodynamics
- Local changes in oxygen use and blood volume are carried downstream
- Mislabel brain region
- CBV is useful but most still use BOLD
2.) Slow Changes in hemodynamics
- Might not capture true response
- Precise neural coupling invisible to fMRI
- ) Uncertainity in type of neurons involved
* Positive BOLD signal could be excitation or inhibitory - ) Direction of causation is unclear
* Separate region co-active, but does not say how it influence one another - ) Spatial Limitations (localising)
- Vague despite being much better than EEG.
- Cannot explain layer-dependent activity
6.) Sparse encoding vs population encoding (resolving)
- Spares Encoding: Poor
- Population Encoding: Good
- A BOLD signal driven by a stimulus does not mean that the entire area is used to process that stimulus, or even that class of stimuli
What are the 3 fundamental limitations of fMRI
- Some nerual activity (Magnetic Field) are too small to be localized with fMRI
- MRI only shows vascular responses to neural activity
- Conclusions are ambiguous because it could reflect (blood velocity? volume? oxygen?)
What are recent advances in fMRI
Multivoxel pattern analysis (Statistical techniques)
- Whole Brain View
- Does not require spatial smoothing
Voxel encoding and population field mapping (Statistical techniques)
- Functional property of neurons
- Not possible with group averaging
Hi Resolution (7T) Scanning
- Isolate activity in single cortical column (sub-mm)
What are recent advances in structural MRI
-
CBV Changes
- __Allows resolution of the cortical layer
-
dMRI tractograhy
- Connectivity between brain regions using density of fibres
- Map of how different brain regions are associated and correlated with one another
- Connectivity between brain regions using density of fibres
Movement away from modularity to connectivity
