fMRI

Question 1

Hemodynamic signal

Answer 1

Indirect measure of neural activity
- complex relationship between neural activity and hemodynamics –> can it be trusted?
- deep understanding allows discriminating (in)valid ways of using it

Question 2

Relation between hemodynamics and neural activity

Answer 2

Neurons require energy to function (ATP), from glucose
- continuous sypply via blood circulations:
–> via arteries, exchange glucose and oxygen in capillaries, deoxyhemoglobin and blood leaes again
Hemodynamic response function

Question 3

Deoxyhemoglobin

Answer 3

Oxygen gets removed from hemoglobin

Question 4

HRF in neural activity

Answer 4

• delayed local increase in oxygen and glucose consumption
• ratio oxygenated en deoxygenated heoglobin decreases
• signal through neurovascular coupling mechanism, triggeren increase in blood supply
• peak increase in blood oxygenation several seconds after
• blood volume and oxygenation decay again
• expand across larger territory than region of neural activity

Question 5

Short step by step HRF

Answer 5

Rest –> stimulus –> inc neural activity –> inc oxygen and glucse consumption –> big inc cerebral blood flow –> fMRI BOLD response inc.

Question 6

Three components HRF

Answer 6

• intial dip: regular oxygen consumption
• primairy response: bring lots of oxygen
• negative overshoot: signal decreases
  –> additivity assumption

Question 7

Additivity assumption

Answer 7

In case of multiple stimulu, total hemodynamic response is sum of HRF’s to individual stimuli

Question 8

Hemodynamic repsonse and electrical potential changes

Answer 8

Measure action potentials = measure output of a neuron
- HR of a region: not sure whether it represents overall action potential output of that region
- situations possible where energy consumption increases, while neuron output stays the same
- difference in HR –> twwo conditions are different in neural activity, but action potential output does not need to be

Question 9

HR vs. other electrophysiological measures

Answer 9

• multi-unit activity MUA: number of action potentials
• local field potentials LFP: synaptic input of neurons
  –> typical situation: everything correlates
  –> with long stimulation: HR slightly more correlated with LFP than with MUA

Question 10

T2 and T2*

Answer 10

Mainfactors contributing to decay of transverse magnetization
Molecular interactions (=> T2 decay):
- field inhomogeneity
- tissue succeptibility
Total dephasing = T2* decay (always faster, because there are more factors to the decaying)

Question 11

BOLD

Answer 11

Blood oxygenation level dependent signal
- blood carriers hemoglobin
- hemoglobin contains iron atoms
- iron atoms can distort the magnetic field
- the iron has slightly different magnetic properties depending on whether it is bound or unbound to oxygen

Question 12

BOLD signal

Answer 12

Inhomogneities are modulated by variable ratio of deoxygenated/oxygenated blood
- level of inhomogeneity of magnetic field affects speed of dephasing
- more DeoxyHb, strong field inhomogeneities (fast dephasing)
- more OxyHb, weak field inhomogeneities (slow dephasing)

Question 13

Is fMRI worth it

Answer 13

It is very expensive and will take a long time
- formulate relevant hypotheses
- design study to provide evidence for or against some hypothesis
- expensive machines do not think of you
–> need to know the methods and the psychological theories of mental functioning

Question 14

Subtraction

Answer 14

Isolate behavior by substracting conditions that only differ in 1 mental process
- realted to behavioral method of Donders: mental chronometry

Question 15

problems substraction

Answer 15

What if other mental states are included?
Underlying assumption: pure insertion or additivity assumption
- conflict monitoring
- priming

Question 16

Pure assumption

Answer 16

Two (or more) oncditions can be congnitevely added, implying no interactions among the cognitive components of a task
- In most cases, this assumption is invalid
- if assumption fails => compasrison between conditions is confounded

Question 17

Conflict monitoring

Answer 17

SLower in incongruent trials and activaiton increase in incongruent trials

Question 18

Priming

Answer 18

Faster in congruent trials and activation decrease in congruent trials

Question 19

Factorial design for pure assumption

Answer 19

Additivity assumption violations visible through interaction effects in factorial design
Principle: task where cognitive components are intermingles in one moment and separated in another instance of the paradigm => allows testing for interactions

Question 20

Parametric variation

Answer 20

Principle: increase cogntive demand without modifying intrinsic nature of task
- BOLD increase implies heavy association of area with nature of manipulated parameter

Question 21

Conjuntion analyses

Answer 21

Cognitive conjuntion studies are designed such that two or more distinct task pairs each share a common processing difference
- task
- stimuli
- senses
Disjunction analyses

Question 22

Results conjunction analyses

Answer 22

Conjunction: both production and perception
Disjunction: production, no perception
Disjunction: perception, no production

Question 23

Recap contrasting ecperimental conditions

Answer 23

• substraction: effect = A - B
• Factorial: addition and interaction
• parametric
• conjunciton

Question 24

Correlational studies

Answer 24

Between subjects design –> relate individual differences in behavior to brain differences

Question 25

Forward inference

Answer 25

If cognitive process X is manipulated, particular brain region R is activated => activation of brain region R is related to cognitive process X

Question 26

Reverse inference

Answer 26

Engagement of particulalr cogntive process is inferred by activation of particular brain area - based on literature

Question 27

Overlapping HRFs

Answer 27

HRF peak delayed with 6 sec, total duration more than 12s from onset to back to baseline - if you show multiple simuli within those 12 s they will overlap 3 options: 1: ISI of 16 sec -> inefficient 2: ISI of 2 sec -> signal reaches asymptomic level (low sensitivity for differences) 3: Block design: trials are blocked per condition

Question 28

Block design

Answer 28

- alternate blovks of different conditions - within vlock: strong HRF because additivity signal - after a block: signal goes down again before next block - condition-associated ups and downs in BOLD signal are late - very efficient

Question 29

Drawbacks to block design

Answer 29

- predictabilty of conditions (tendency to prepare) - Impossible for some experimental questions - impossible to estimate single-trial response function

Question 30

Slow event-related design

Answer 30

Long ISI (with or without jitter) - ideal for estimating HRF function by the event related response - inefficient use of time and borign

Question 31

Rapid counterbalanced event-related design

Answer 31

- ISIS = 0 or not longer than the trial duration - conditions alternate in pseudo-random order <-> alternating event-related design - peak is signal difference between conditions when particular condition occurs frequently in short period of time - reasonable sensitivity and power - rest - trial order similar to that of most behavioral experiments

Question 32

Rest condition

Answer 32

Studies often include a rest condition as a baseline - makes comparison between different studies easier - helps to disambiguate more activation from less deactivation

Question 33

Default mode network

Answer 33

Very predictable set of deactivations in the contrast of an active task minus a passive no-stmulus baseline: - number of subtraction task - one-back repetition detection of object images - one-back repetition detection of texture patterns --> thus, these regions are active when you are at rest