Lecture 5: What is fMRI?

What is fMRI and how does it work?
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What is fMRI and how does it work?
a technique used to measure brain activity
When a brain area is more active, it consumes more oxygen and to meet this increased demand, blood flow increases to that area
What are the three advantages to an fMRI?
1. non-invasive and doesn't use radiation (safe)
2. excellent spatial and good temporal resolution
3. easy for experimentation to use
What is the key to fMRI?
the signal from hydrogen nuclei varies in strength depending on surroundings. Provides a means of discriminating between gray matter, white matter, and CSF in structural images of the brain
BOLD signal
-an indirect measure of neural activity
-since blood oxygenation varies according to the levels of neural activity, differences in magnetic properties of hemoglobin (diamagnetic when oxygenated and paramagnetic when deoxygenated) can be used to detect brain activity
computerized tomography (CT)
construed according to x-ray absorption in different types of tissue

typically used only in clinical settings (diagnosing tumors, identifying hemorrhaging, etc.)
advantages of MRI
-Does not use ionizing radiation, is safe
-Provides better spatial resolution (individual gyri discrete)
-Provides better discrimination between white and gray matter ○ -Can be adapted for use in detecting blood oxygenation changes associated with neural activity (fMRI)
Structural MRI is used to study brain ________ while functional MRI is used to study brain ________.
anatomy, function
Diffusion tensor imaging (DTI)
MRI based methods for imaging the axon tracts that form the white matter of the brain

Measures density and motion of water traveling along the myelinated axons

Water normally diffused in all directions but since myelin has a lipid boundary, that boases water to move along the length of the axon

measuring this movement can create images of white matter tracts in the brain
basic physiology underpinning functional imaging
-brain consumes 1/5 of the body's oxygen uptake, does not store oxygen and little glucose
-blood supply to an activated region must increase to meet the demand
-brain is always physiologically active
PET
-uses a radioactive tracer injected into the bloodstream
-greater blood flow in a region indicated by a greater signal being emitted by the racer in that region
-usually oxygen-15 or fluorine-18
HRFthe way the BOLD signal evolves over time in response to an increase in neural activity is called the hemodynamic response function3 phases of HRF1. initial dip 2. overcompensation 3. undershootcognitive subtraction theoryby comparing the activity of the brain in a task that utilizes a particular cognitive component to the activity of the brain in a baseline task that doesn't, it is possible to infer which regions are specialized for that particular cognitive component Problem: assumption of pure insertions and interactions make imagine data ambiguousfunctional integrationrefers to the way in which different regions communicate with each other; likely to be essential for a full understanding of how cognition is linked to the brainsports and head injury-concussions and subconcussive hits in many sports are suggested to be associated with chronic traumatic encephalopathy (CTE) -Buildup of tau protein, problems with memory, concentration, depression, anger -Stricter policies implemented in some sportsparametriccontinuous dimension, measuring associations between brain activity and variablesassumption of pure insertionassumes brain regions can be inserted without affected functions of other regionsHow does structural MRI work?magnetic pulse briefly aligns H atoms in the brain and records when they return to normal positionA longer neural signal means that the BOLD response is ______ and ______.taller and longer