fMRI: L8

Question 1

what does oxygenated blood, oxyhemoglobin (Hb) do to the signal?

Answer 1

it is diamagnetic, enhances the signal

Question 2

what does deoxyhemoglobin (Hb) do to the signal?

Answer 2

introduces field distortions, decreasing the signal

Question 3

what do neurons need?

Answer 3

highly oxygenated blood for glucose metabolism

Question 4

what is neural activity accompanied by?

Answer 4

a local oversupply in oxygenated blood and therefore a better blood oxygen-level dependent (BOLD) signal

Question 5

What is being measured?

what does this conclude?

Answer 5

• the oxygen content NOT the neurons themselves

- something about neurons in brain regions

Question 6

what do large blood vessels cause in scans

Answer 6

brighter areas = better signal

Question 7

1. areas of enhanced activity have…

2. this can be…

Answer 7

1. more oxygenated blood

2. mapped onto a structural image of the brain called statistical parametric mapping

Question 8

what can statistical parametric mapping say about the brain?

Answer 8

that there is a significantly strong activation in a particular region during a cognitive task or for task A compared to task B

Question 9

red indicates

blue indicates

Answer 9

• more oxygen

- less oxygen

Question 10

what is measured during a cognitive task?

Answer 10

BOLD signal = blood oxygen-level dependent

Question 11

what is required for the whole brain while performing experimental tasks and why?

Answer 11

repeated measures because the signal is very noisy

Question 12

what does the BOLD signal tell us?

-> doesn’t tell us

Answer 12

whether a brain region is engaged in the task or not

-> doesn’t directly measure neural activity, it is an INDIRECT measure of neural activity

Question 13

what does enhanced neural activity impact? (3)

Answer 13

1. how much oxygen is in the blood
2. how fast the blood flow is
3. how much oxygen can be extracted

Question 14

when blood flow is faster how much oxygen can be extracted?

Answer 14

LESS because in total more oxygen is extracted per unit time

Question 15

what is the temporal lag? & how many s?

Answer 15

lag between neural activity and the peak of the BOLD response - 8 seconds

Question 16

how long does the BOLD signal need before reaching baseline?

baseline reduction is therefore…

Answer 16

• 16 seconds

- slow

Question 17

Why can we not compare signals between different regions of the brain?

Answer 17

the signal change is different therefore it would be invalid to compare them

Question 18

the measured response is described by the what?

what does it look like across regions?

Answer 18

• Haemodynamic response function (HRF) -> line on the graph

- looks v similar across different regions

Question 19

the BOLD signal DOESN’T reflect

Answer 19

neurons firing/ neuron output

Question 20

what does Logothetis (2008) describe local cortical excitation-inhibition networks (EIN) as?

Answer 20

small and highly interconnected functional microunits, which show massive recurrent feedback

Question 21

What does feedback processing in EINs account for?

Answer 21

most activity measured by BOLD fMRI

Question 22

Limitations of BOLD fMRI (2)

Answer 22

1. we do not see the full networks involved during cognitive functions (we only see the rip of the iceberg)
2. fMRI might not always map the units that matter

Question 23

Limitations of BOLD cont. (1)

Answer 23

3. poor temporal resolution = HRF is slow & fast processes are difficult to image. it takes 2s to measure the brain (we cannot explore processing in this time). We have to wait for HRF to reach baseline every time (16s)

Question 24

Limitations of BOLD cont. (2)

Answer 24

4. smallest measurement unit is a “voxel” = 3D pixel. 1 voxel contains >100,000 neurons!
5. risk of rejecting the Ho for each test is 1% ( t-test conducted), we may therefore have to calculate loads of tests (est. 50,000 x) to find false positives

Question 25

what is the bonferroni-correction?

Answer 25

divide the significance level by number of tests -> use for each test
0.01/50,000 = p < .0000002