W1L1 - fMRI Flashcards
What are the 2 ways neuron activity can be measured
- ) Directly - Electrical changes at the level of single neurons
- ) Indirectly - Metabolic changes (Cognitive processes = ATP = Changes in blood flow/oxygenation)
What is the function of fMRI. Is there a relationship between psychological states and __________ activity?
Localized hemodynamic changes from neural activity.
Often a relationship between psychological
states/performance measures and hemodynamic changes
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 most widely used hemodynamic index
Blood Oxgenation Dependent Level (BOLD) signal
What is the physics behind the basic MRI
Hydrogen has a single proton with random spin around axis.
When there is a radio pulse, it aligns parallel/anti-parallel and it spins back
Cognitive processes require energy. Where do we get energy and how does it relate to fMRI
Need to produce ATP > Use oxygen from hemoglobin for energy.
fMRI relies on difference between magnetic responses between oxygenated and deoxygenated blood.
Oxygenated vs Deoxygenated Blood. Difference in MRI signal
Oxygenated:
1.) Diamagnetic > No Distort
Deoxygenated:
1.) Paramagnetic > Distory
(Seiji Ogawa: Blood vessels more visible as Oxygen decreases)
What is the Standard practice in analysis of BOLD. Why?
Spatial smoothing by 8mm to allow for group averaging by correspondence across brains
How do we quantify BOLD changes
General linear model:
Correlate 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
What are the purported neural correlates of BOLD. In the past and recently (Amptitude)
Amplitude of the BOLD signal associated with
- ) local field potential (Large no. of active neurons)
- ) increases in gamma-band electrical (EEG)
- ) quite often correlated with spike frequency (Animal Studies)
Recent spatial localization of BOLD signal has been
associated with
- electrocorticographic (ECoG) at mm accuracy
What are the 6 limitations of fMRI
- ) Mislocalisation
- ) Slow Changes
- ) Uncertainity
- ) Causality
- ) Spatial Limitations (Localising and Resolving)
1.) Mislocalisation of hemodynamics
Local changes in oxygen use and blood volume are carried downstream
2.) Slow changes in hemodynamics
Disallow precise neural coupling
3.) Uncertainty of the type of neurons involved
Excitation or Inhibitory unclear
4.) Causation between 2 coactive areas
Correlation only
5.) Spatial limitations on localising function activity
Vague despite being much better than EEG. Cannot explain layer-dependent activity
6.) Spatial limitations on resolving functional activity
Spares Encoding (No): Population Encoding (Yes) 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 2 fundamental limitation of fMRI
- ) Some nerual activity (Magnetic Field) are too small to be localized with fMRI
- ) Conclusions are ambiguous because it could reflect (blood velocity? volume? oxygen?)
What are recent advances in fMRI
- ) Multivoxel pattern analysis: More complex models (whole brain view) > Voxel encoding & Population field mapping: Better view of functional property of neurons
- ) Hi Resolution (7T) Scanning: Isolate activity in single cortical column
What are recent advances in structural MRI
- ) New methods such as CBV Changes allows analysis of neural activity at the resolution of the cortical layer
- ) Diffusion fMRI: Connectivity between brain regions (Map of how different brain regions are associated and correlated with one another)
