neuroimaging

Question 1

function of an MRI scanner?

Answer 1

developed for structural imaging
also used for functional brain imaging

1.5-7 tesla magnet

Question 2

what is a BOLD signal?

Answer 2

Blood Oxygen Level Dependent signal

blood flows to active neurons in the brain to provide oxgen to fuel the cells

hemoglobin differs in how it responds to magnetic field, depending on whether it has a bound oxygen molecule (reduces signal if no oxygen bound)

MRI scanner detects these changes in the magnetic field

Question 3

describe fMRIs?

Answer 3

functional imaging
doesn’t directly measure brain activation but blood flow
measures the magnetic properties of oxygenated which is going to active regions of brain vs. deoxygenated blood

Question 4

what is the order of the steps to get from raw data to functional brain ‘activation’ maps

Answer 4

1. design a task to be used in scanner (experimental and control condition)
2. collect some data
3. preprocess the data (clean it)
4. analyse the data (whether getting results expected)
5. interpret your results

Question 5

what is meant by the BOLD signal as being ‘arbitrary’?

how to control for this?

Answer 5

it has no stable baseline : if you scan someone one day their singal may differ to the next day

so have to have both experimental and baseline condition

Question 6

good baseline?

Answer 6

differs from the experimental condition only by the process of interest

e.g face processing

Question 7

how long does it take from stimulus presentation to activation in brain area?

Answer 7

4-5 secs after stimulus presented

Question 8

what is a block design?

disadvantages?

Answer 8

clumping all trials together in one block

• stimuli very predictable so alter strategies
• inflexible
• ecological validity problems
• can’t separate data by performance

Question 9

what is an event related design?

advantages?

Answer 9

trials of different conditions are randomly intermixed rather than alternating

• can know which BOLD signal associated with which condition
• flexibility
• avoid practice events and predictability
• post hoc sorting
• temporal dynamics (how response changes over time)

Question 10

what is meant by a volume?

how long to collect a single volume?

Answer 10

a scan of the brain from top to bottm
om

2-3 secs to collect single volume

Question 11

what is a voxel?

and what are they used for?

Answer 11

a segment of the brain having been divided into cubes

so can easily reference specific parts of the brain

3x3x3 mm

Question 12

what are the steps of preprocessing data?

and what does this mean

Answer 12

to ‘clean up the noise’ (non-task-related variables)

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

Question 13

what is high pass filtering?

Answer 13

remove low frequency oscillations from data

Question 14

what is motion correction?

Answer 14

correcting images of head movements while in scanner

Question 15

what is slice time correction?

Answer 15

correcting for delay in different slices imaging in the brain

Question 16

what is coregistration?

Answer 16

overlay functional images on structural image to see where in brain activation is occuring

Question 17

what is normalisation?

Answer 17

account for size variability in subjects brains by warping each brain in standardised space (average brain)

in order to compare brains

Question 18

what is spatial smoothing?

Answer 18

modify BOLD signal in a voxel according to the activation in a neighbouring voxel by using the Gaussian kernel function

as if a neuron fires, neurons close to it will fire as well so data fits this model

Question 19

what is used to analyse data?

Answer 19

multiple regression
determine effect of IVs on DV (brain activation)

perform a contrast (t test) in each voxel
yellow indicates higher t values

then apply a threshold (p<0.05) to determine whether significant

Question 20

what is the type 1 error issue?

how to correct?

Answer 20

with 100 independent t tests have 99.4% chance of type 1 error

so very important to correct for multiple comparisons (correct p value to fit data) BIG PROBLEM FOR REPLICATIONS AS UNCORRECTED RESEARCH (many occur by chance)

Question 21

what are the 2 approaches to data analysis?

Answer 21

1. whole brain analysis

2. region of interest analysis

Question 22

what is whole brain analysis?

advantages?
disadvantages?

Answer 22

examine effects on a voxel by voxel basis across whole brain

advantage:
no need for prior hypothesis

disadvantages :
can lose spatial resolution and produce loads of areas that are difficult to interpret

Question 23

what is region of interest analysis?

advantages?

disadvantages?

Answer 23

restrict analysis to a particular brain region

advantages:
avoids “laundry lists” of activated regions
hypothesis driven
avoids multiple comparisons problem so simple
generalisable

disadvantage:
easy to miss things going on elsewhere in the brain
not simple how to define regions of interest

Question 24

limitations of fMRI?

Answer 24

• correlative (can’t say region activated is essential for that function) - need converging evidence
• temporal resolution is lower than EEG/ERP- need converging evidence

Question 25

what does MRI measure?

Answer 25

measures the magnetic signal coming from hydrogen atoms (small dipole magnets - magnetic fields have axis and orientation)

MRI scanners have strong magnetic fields which makes hydrogen atoms in the body respond with a magnetic signal