Neuroimaging

Question 1

What are the different brain imaging techniques?

Answer 1

• structural (take image of brain at one point in time): CT and MRI
• functional (take image of active brain at multiple points in time): PET and fMRI

Question 2

What is CT?

Answer 2

• moderately invasive
• inexpensive
• widely available
• low spatial

Question 3

What is MRI?

Answer 3

• non-invasive (use radio frequency fields)
• extremely high spatial resolution
• number one choice for structural brain imaging in neuroscience research
• extreme magnetic field so no metal

Question 4

What is PET?

Answer 4

• moderately invasive (radioactive tracer)
• measures indirect metabolic correlates of neural activity
• can measure synaptic transmission directly
• high spatial resolution
• extremely low temporal resolution
• extremely expensive

Question 5

What is fMRI?

Answer 5

• non-invasive
• measures indirect metabolic correlates of neural activity
• high spatial resolution
• low temporal precision as it measures processes
• moderately expensive

Question 6

What is the BOLD (blood oxygen level dependent) signal?

Answer 6

• when neurons become active, blood flows to part of the brain to provide oxygen to fuel the cells
• haemoglobin (iron-containing oxygen transporting protein present in blood) differs in how it responds to magnetic fields depending on if it’s bound to oxygen molecule
• MRI scanner detects these changes in the magnetic field

Question 7

How is raw data converted into functional brain ‘activation’ maps?

Answer 7

• design a task to be used in the scanner
• collect some data
• pre-process the data
• analyse the data
• interpret results

Question 8

What is the experimental design of fMRI?

Answer 8

• BOLD signal is arbitrary (has no stable baseline) so needs experimental and baseline condition
• good baseline is one that differs from experimental only by process of interest

Question 9

What are block designs and their limitations?

Answer 9

• BOLD signal is slow (peaks 4-5 seconds after stimulus, 16 seconds to return to baseline
• often group together lots of trials
• all fMRI experiments originally employed block designs
• limitations: highly predictable, inflexible for more complex tasks, ecological validity, can’t separate trials by performance

Question 10

What are event-related designs and their advantages?

Answer 10

• trials of different conditions are randomly intermixed and occur close together in time
• BOLD signal relating to different trial times can be disentangled, can see which one increases the BOLD signal
• advantages: allows for complex and novel experiments, flexible, can be randomized, post hoc sorting, can look at novelty and priming, and at temporal dynamics of response

Question 11

How is data collected?

Answer 11

• scanner collects data in slices
• 2 to 3 seconds to collect single volume
• to reference points in the brain the image is divided into cubes or voxels

Question 12

What are the preprocessing steps?

Answer 12

• high pass filtering: remove low frequency oscillations
• motion correction: correcting where it may have moved
• slice time correction: top slice is collected seconds before nose slice so are aligned temporarily
• coregistration: overlay any functional activations onto a structural image to see which parts are activated
• normalization: warp brains into standardized space
• spatial smoothing: model data according to this property of neurons

Question 13

What occurs in analysis?

Answer 13

• multiple regression used to determine effect of number of IVs on single DV
• for each voxel multiple regression is used to estimate how closely the BOLD signal correlates with timecourse of each condition
• then perform a contrast

Question 14

How are multiple comparisons corrected?

Answer 14

• brain images divided into up to 130,000 voxels so 130,000 individual t-tests
• with that many t-tests and alpha level of 0.05 it’s guaranteed to find some significant voxels by chance
• alpha level needs to be adjusted to correct for multiple comparisons

Question 15

What are the different approaches to data analysis?

Answer 15

• whole brain analysis: examine effects on a voxel-by-voxel basis across the whole brain
• region of interest analysis: restrict our analysis to a particular brain region

Question 16

What are the advantages and disadvantages of whole brain analysis?

Answer 16

• advantages: requires no prior hypotheses, includes entire brain
• disadvantages: can lose spatial resolution, can produce meaningless laundry list of areas that are difficult to interpret, depends on statistics and the threshold selected, multiple comparisons problems

Question 17

What are the advantages and disadvantages of region of interest analysis?

Answer 17

• advantages: hypothesis driven, avoids multiple comparisons problem, simple, generalisable
• disadvantages: easy to miss things going on elsewhere in brain, not always simple to define ROIs

Question 18

What are the limitations of fMRI?

Answer 18

• data is correlative

- temporal resolution of fMRI is low