Lecture 21 fMRI & BOLD Flashcards
(34 cards)
What does fMRI measure?
It measures hemodynamic responses
Who discovered the BOLD signal?
Seiji Ogawa in the early 1990s.
What did Ogawa find about blood oxygenation?
More oxygenated blood leads to brighter MRI signals.
What energy form do neurons need for activity?
ATP (adenosine triphosphate).
How is ATP produced in the brain?
By metabolizing glucose
Where does oxygen exchange happen in the brain?
In the capillaries
What removes deoxygenated hemoglobin?
Venules and veins carry it away.
What happens first during local neural activity?
Slightly delayed increase in glucose and oxygen consumption.
What compensatory process follows neural activity?
Increased cerebral blood flow and volume
What is the BOLD signal?
Blood-Oxygen-Level Dependent signal—higher oxygenation improves MRI signal.
Why does oxygenated blood enhance the signal?
Oxyhemoglobin is diamagnetic and aligns with the magnetic field.
What is the Hemodynamic Response Function (HRF)?
The typical time course of the BOLD signal following neural activity.
When does the HRF peak?
4–8 seconds after the stimulus.
How long does it take for HRF to return to baseline?
Up to 16 seconds.
What happens if several neural events occur?
Their HRFs add up linearly.
What neural activity does the BOLD signal best correlate with?
Local Field Potentials (LFPs),
reflect the neurons’ “input” at the synapses, but recent studies have shown that it is also correlates with action potentials
What influences the fMRI signal most strongly?
Changes in excitation-inhibition balance affecting metabolic demand.
Why can’t EEG/MEG detect action potentials?
Opposing charges cancel out; BOLD is more sensitive to LFPs.
Why does T2* decay matter for fMRI?
It reflects faster dephasing due to deoxyhemoglobin’s paramagnetic properties.
What type of pulse sequence does fMRI use?
T2*-weighted sequences
What does deoxygenated hemoglobin do to the MRI signal?
It increases T2* decay
What is the difference between T1 and T2?
T1 is longitudinal relaxation; T2 is transverse (spin-spin) relaxation.
Why do we care about oxygenation in MRI?
It alters magnetic properties that affect signal strength.
Why can’t we directly measure HRF in fMRI?
Temporal resolution is low; HRF must be estimated.
