HC 5

Question 1

What is a hemodynamic signal?

Answer 1

It is an indirect measure of neural activity.

Question 2

What is the relation of hemodynamics with neural activity?

Answer 2

It is measured with the hemodynamic response function (HRF).

When neural activity occurs, there is a slightly delayed local increase in oxyen and glucose consumption. The ratio between oxygenated and deoxygenated hemoglobin decreases. The signal goes through neurovascular coupling mechanism, triggering an increase in supply of blood.

There is a peak increase in blood oxygenation several seconds after initial oxygen consumption. Blood volume and oxygenation then decay again. Then it expands across larger territory than the region of neural activity.

Question 3

What are the three components of HRF?

Answer 3

1. Initial dip= oxygen consumption
2. Primary (strongest) response= influx oxygenated blood –> strong increase in signal
3. Negative overshoot= signal decreases

Question 4

What additivity?

Answer 4

The assumption that in case of multiple stimuli, the total hemodynamic response is the sum of HRF’s to individual stimuli.

Question 5

What is the problem when measuring HR and action potentials?

Answer 5

Sometimes, energy consumption increases, while the output of the neuron stays the same. Differences in HR means that two conditions can be different in neural activity, but action potential output does not need to be.

Question 6

What is multi-unit activity (MUA)?

Answer 6

Number of action potentials.

Question 7

What are local field potentials (LFP)?

Answer 7

Synaptic input of neurons.

Question 8

What is T2* decay?

Answer 8

Total dephasing.

Question 9

What is the difference between T2 and T2* decay?

Answer 9

While both involve the loss of transverse magnetization, they represent different physical processes and have different implications in MRI. T2 is due to spin-spin interactions and affects contrast, while T2*decay is influenced by that and magnetic field inhomogeneities that can lead to signal loss and distortion in images, especially in regions with suscepitibility variations.

T2* is also faster, because it incorporates additional sources of signal decay.

Question 10

What is fMRI?

Answer 10

Functional magnetic resonance imaging.

It uses BOLD signals= blood-oxygenation-level dependent signal.

Blood carries hemoglobin, hemoglobin carries iron atoms, iron atoms can distort magnetic field.

Iron has different magnetic properties depending on whether it is bound (OxyHb) or unbound (DeoxyHb) to oxygen.

Question 11

How does inhomogeneity of the magnetic field affect speed of dephasing?

Answer 11

More deoxyHb means strong field inhomogeneities, means fast dehapsing, faster T2*.

More oxyHb means weak field inhomogeneities, means slow dephasing, means slower T2*.

Question 12

What are inhomogeneties?

Answer 12

Variations or irregularities in the magnetic field strength across the imaging volume.

Question 13

What are two misconception of fMRI?

Answer 13

“fMRI shows what parts of the brain lights up when one performs a certain task.”

1. Light up implies electrical changes, but fMRI does not measure that. It tells us what regions are active due to firing of cells.
2. It also implies areas to be turned on and off. However, the brain is active all the time. fMRI shows what parts are more active than others.

Question 14

What is subtraction?

Answer 14

Isolate behavior by subtracting conditions that only differ in 1 mental process. Related to behavioral method of Donders: mental chronometry.

Question 15

What is mental chronometry?

Answer 15

It involves reaction time and other timing measures to infer underlying cognitive processes.

Question 16

What is the problem with subtraction?

Answer 16

What if other mental states are included?

The underlying assumption is pure insertion or additivity assumption.

Question 17

What is pure insertion?

Answer 17

The assumption that two or more conditions can be cognitively added, implying no interactions among the cognitive components of a task.

In most cases, this assumption is invalid.

If the assumption fails, the comparison between conditions is confounded. There would be no way to determine what cognitive processes are reflected in the activation observed in the subtraction experiment.

Question 18

What is a factorial design?

Answer 18

Additivity assumption violations are visible through interactions effects in factorial design.

The principle is: task where cognitive components are intermingled in one moment and seperated in another instance of the paradigm, allows testing for interactions.

Question 19

What is parametric variation?

Answer 19

Increase cognitive demand without modifying intrinsic nature of a task. BOLD increase implies heavy association of area with nature of manipulated parameter.

Question 20

What are conjunction analyses?

Answer 20

Cognitive conjunction studies are designed such that two or more distinct task pairs each share a common processing difference.

Question 21

What is forward inference?

Answer 21

If cognitive process X is manipulated and particular brain region R is activated, this means that activation of R is related to X.

Question 22

What is reverse inference?

Answer 22

Engagement of particular cognitive process is inferred by activation of particular brain area (based on literature). This is used mostly, but is not the correct way. We should use forward inference.

Question 23

How should we present conditions in fMRI?

Answer 23

With a block design. Trials are blocked per condition.

Question 24

How does a block design work?

Answer 24

Alternate blocks of different conditions.

Within blocks: strong HRF, because of additivity signal.

After a block: signal goes down again, before the next block.

This is efficient, because many trials can be presented per unit of time and there is no long waiting time between trials.

Question 25

What are the drawbacks of a block design?

Answer 25

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

Question 26

What is a slow event-related design?

Answer 26

They have long ISI (interval in between stimulus). This is ideal for estimating HRF by the event-related response. However, it is inefficient use of time and boring.

Question 27

What is counterbalanced event-related design?

Answer 27

ISI= 0 or not longer than the trial duration.

Conditions alternate in pseudo-random order: each condition follows each condition an equal number of times.

Question 28

What is rapid counterbalanced event-related design?

Answer 28

There is a peak in a difference of signal between conditions when a particular condition occurs frequently in a short period of time.

There is a reasonable sensitivity and power. It is better than the alternating and slow design, but worse than the block.

Estimation of single-trial response function is possible when one of the conditions is at rest. It is worse than the slow design here.

Question 29

Why do we include baseline or rest conditions even though it rarely satifies the assumptions of subtraction?

Answer 29

Makes comparison between diferent studies easier and helps differentiate between more and less activation. Difference in activation may be due to anything. Rest is easier to interpret.

Question 30

What is active during rest and not when a task is performed?

Answer 30

Default mode network.