fMRI

Question 1

fMRI basics

Answer 1

• does not measure neural activity directly, but it is a hemodynamic neuroimaging method.
• Ogawa changed the blood-oxygen level experiementally and found that this impacts the signal

Question 2

how fMRI works

Answer 2

• local neural activity requires energy which is ghenerated in the form of adenosine triphosphate (ATP)
• to produce ATP, glucose is metabolized, for which oxygen is required
• oxygen is constantly transported through the brain arteries and a network of arterioles
• in the capillaries, oxygen molecules are removed from the hemoglobin, turning oxyhemoglobin into deoxyhemoglobin
• deoxygenated hemoglobin is transported away by the venules and larger veins.

Question 3

what happens when there is local neural activity?

Answer 3

• first, there is a slightly delayed increase of glucose and oxygen consumption
• this triggers an increase in cerebral blood flow, to supply more oxygen
• the consequence is a local increase in blood oxygenation

Question 4

As a result of the increase blood oxygenation

Answer 4

• the increase in blood oxygenation is much larger than the initial dip, meaning that shortly after the neural activity, there is an oversupply of oxygen in the blood
• the increase of blood oxygenation causes our signal to get better- this is the BOLD signal we measure
• oxygenated blood is diamagnetic, enhancing the signal.

Question 5

BOLD signal

Answer 5

• Blood oxygen level dependent signal

Question 6

what is reflected in the BOLD signal?

Answer 6

• the relationship between the bold signal and the neural activity is complicated.
• the bold signal correlates best with the local field potentials ( LTPs), which reflect the neurons input at the synapses, by the synapses, but recent studies show it may actually be related to action potentials
• in sum, the BOLD signal might be a mixture of activity within the local cortical excitation inhibition networks, small and highly interconnected functional microunits, which show recurrent feedback.

Question 7

changes in blood oxygenation- HRF

Answer 7

• the change in signal is best described by the Hemodynamic response function (HRF), which is similar (but not identical) in different brain regions
• the peak of the HRF is reached in 4-8 seconds after the neural activity occured
• it takes the signal up to 16 seconds to go back to baseline levels
• if several neural events take place, the HRF’s will add up linearly.

Question 8

from measuring the BOLD signals to getting results

Answer 8

• Due to the presence of deoxygenated hemoglobin, which is paramagnetic )Ie. It has a magentic momentum, protons (hydrogen atoms),. Experience an additional speeding up od the decay of transversal magnetization
  This means the total decay is faster than predicted by the T2 constant.
• the total decay is referring to as T2 decay. BOLD fMRI mostly uses T2 weighted pulse sequences, which give a better signal when blood is oxygenated

Question 9

Op da Beeck

Answer 9

• statistical parametric mapping used for the analysis, using a general linear modelling approach, we search for regions in which the signal increase fits to the predictions of our model
• the program is given the mapping between conditions and recorded brain images and then estimates the model fit
• irrelevant variables, which might impact the estimate of the model, can also be factored in but not analyses
• this is done first for each voxel in each participants data
• group statistical analyses are then performed on the sample

Question 10

results of the statistical mapping

Answer 10

• the resulting statistical maps is then overlaid on a structural image of the brain
• significant effects in a brain region for task A compared to task B, is interpreted as involvement of this region
• we cannot compare activation between regions because the HRG is different
• activation ‘blobs’ are statistical effects in experiment, often color coded for activation (red) and deactivation (Blue)

Question 11

bold signal results (P2)

Answer 11

• we need to repeat the measurement of brain activity many times while participants perform the experimental task because the signal is noisy, and the final brain maps are always averaging across many trials