Neuroimaging

Question 1

what is BOLD signal

Answer 1

bloody oxygen level dependent

Question 2

what happens when neurons become active

Answer 2

blood flows to the part of the brain to provide oxygen to fuel the cells

Question 3

What differs in how it responds to magnetic fields, depending on whether it has a bound oxygen molecule

Answer 3

Hemogblobin

Question 4

What is more attracted to magnetic fields

Answer 4

deoxygenated hemoglobin

Question 5

What is the net effect in BOLD signal MRI

Answer 5

• Net effect is a reduction in field inhomogeneity, and an increase in MR signal

Question 6

What are the steps in using an MRI scanner?

Answer 6

1. design a task to be used in the scanner
2. collect some data- from raw data to functional brain ‘activation’ maps
3. Preprocess the data
4. analyse the data
5. interpret your results

Question 7

What is meant by the BOLD signal being arbitrary?

Answer 7

No stable baseline

Question 8

What makes a good baseline?

Answer 8

• One that differs from the experimental condition only by the process of interest
• E.g. Face processing

Question 9

How slow is the BOLD signal?

Answer 9

peaks 4-5 seconds after stimulus onset and takes around 16 seconds to return to baseline

Question 10

What did fMRI experiments originally employ

Answer 10

block designs - long periods of alternating task/baseline performance

Question 11

Disadvantages of block design

Answer 11

• often group together lots of trials
• highly predictable occurrence of stimuli
• inflexible for more complex tasks
• cant seperate trails by performance

Question 12

What is event-related

Answer 12

The alternative to a block design is an event-related design, in which trials of different conditions are randomly intermixed and occur close together in time.

Question 13

advantages of event-related designs?

Answer 13

• flexibility and randomisation
• post hoc sorting
• can look at novelty and priming
• can look at temporal dynamics of response

Question 14

What are the preprocessing steps?

Answer 14

1. high pass filtering
2. motion correction
3. slice time correlation
4. coregistration
5. normalisation
6. spatial smoothing

HMSCNS

Question 15

What is slice time correlation?

Answer 15

Because it takes a few seconds to collect a whole brain volume, different slices may be acquired up to a couple of seconds apart. This means that our estimate of the hemodynamic response will be incorrect for slices acquired later. Slice timing simply corrects this by moving them earlier in time.

Question 16

What is montreal neurological institute (MNI)

Answer 16

– Combination of 352 MRI scans on normal controls

• All right-handed subjects

Question 17

What is the talairach space?

Answer 17

original standard brain of a 60-year-old french lady

Question 18

Why is smoothing done?

Answer 18

because neurons do not fire in isolation

Question 19

Why multiple regression used

Answer 19

to determine the effect of a number of independent variables, x1, x2, x3 etc, on a single dependent variable, y

Question 20

Limitaion of fMRI

Answer 20

• correlative so cant say one thing causes another
• need convering evidence
• temporal resolution of fMRI is low
• need converging evidence from EEG

Question 21

What are the two approaches to data analysis?

Answer 21

Whole brain analysis and region of interest (ROI) analysis

Question 22

What is whole brain analysis?

Answer 22

examine effects on a voxel by voxel basis across the whole brain

Question 23

What is region of interest (ROI) analysis?

Answer 23

who knows

Question 24

Eval of whole brain analysis

Answer 24

Advantages
• Requires no prior hypotheses about areas involved
• Includes entire brain
Disadvantages
• Can lose spatial resolution with intersubject averaging
• Can produce meaningless “laundry lists of areas” that are difficult to interpret
• Depends highly on statistics and threshold selected
• Multiple comparisons problem
• Very important

Question 25

Eval of ROI analysis

Answer 25

Advantages of ROI approach: • Hypothesis driven – avoids meaningless ‘laundry lists’ of activated regions. • Avoids multiple comparisons problem – data summarised in a single number per subject reflecting mean activation across voxels in ROI. • Simple – data can be exported and treated as any other type of data, requiring no special software for further analysis • Generalisable – data can easily be compared across studies, e.g. for meta analysis. Disadvantages: • Easy to miss things going on elsewhere in the brain. • Not always simple how to define ROIs.

Question 26

What are limitations of a block design?

Answer 26

- highly predictable - inflexible - lack ecological validity - cant seperate trials by performance

Question 27

advantages of event-related design

Answer 27

- flexible - post hoc sorting - can look at novelty and priming - can look at temporal dynamics of response

Question 28

What does a BOLD signal measure?

Answer 28

the magnetic properties of oxygenated v. deoxygenated blood

Question 29

Ultimately we want to do statistics on GROUP activation maps. Thus, we must get all of the brains into a ‘standard space’. How do we do this?

Answer 29

1. Complex algorithms to warp each subject’s brain into the shape of a TEMPLATE BRAIN.

Question 30

What was the original standard based on?

Answer 30

The brain of a 60-year old French woman

Question 31

Why do we no longer use the original standard?

Answer 31

May not be representative of all brains

Question 32

What standard do we now use?

Answer 32

1. Montreal Neurological Institute (MNI) space: This is now the international standard for neuroimaging.

Question 33

How was the montreal neurological institute space created?

Answer 33

1. 352 MRI scans on normal controls, all right-handed subjects.

Question 34

Advantages of whole brain analysis?

Answer 34

■ No prior hypothesis about areas involved needed. | ■ Includes the whole brain.

Question 35

Disadvantages of whole brain analysis?

Answer 35

■ Can lose spatial resolution  due to inter-subject averaging. ■ Can produce meaningless lists of areas. Hard to interpret. ■ Depends highly on statistics and selected threshold. ■ Multiple comparisons problem.

Question 36

Advantages of region of interest analysis?

Answer 36

■ Hypothesis driven – avoids meaningless lists. ■ Avoids multiple comparison problem. ■ Simple – Data can be exported requiring no extra software. ■ Generalisable – Data can be easily compared.

Question 37

Disadvantages of region of interest analysis

Answer 37

■ Easy to miss things going on elsewhere in the brain. | ■ Not always simple how to define ROIs.

Question 38

Limitations of fMRI

Answer 38

 Data is correlative: we cant say an activated region during a task is essential for the task.  SO… converging evidence needed for causal inferences.  Temporal resolution is low.  SO… converging evidence from EEG/TMS to provide finer grained temporal information.