Neuroimaging Flashcards
what is BOLD signal
bloody oxygen level dependent
what happens when neurons become active
blood flows to the part of the brain to provide oxygen to fuel the cells
What differs in how it responds to magnetic fields, depending on whether it has a bound oxygen molecule
Hemogblobin
What is more attracted to magnetic fields
deoxygenated hemoglobin
What is the net effect in BOLD signal MRI
• Net effect is a reduction in field inhomogeneity, and an increase in MR signal
What are the steps in using an MRI scanner?
- design a task to be used in the scanner
- collect some data- from raw data to functional brain ‘activation’ maps
- Preprocess the data
- analyse the data
- interpret your results
What is meant by the BOLD signal being arbitrary?
No stable baseline
What makes a good baseline?
- One that differs from the experimental condition only by the process of interest
- E.g. Face processing
How slow is the BOLD signal?
peaks 4-5 seconds after stimulus onset and takes around 16 seconds to return to baseline
What did fMRI experiments originally employ
block designs - long periods of alternating task/baseline performance
Disadvantages of block design
- often group together lots of trials
- highly predictable occurrence of stimuli
- inflexible for more complex tasks
- cant seperate trails by performance
What is event-related
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.
advantages of event-related designs?
- flexibility and randomisation
- post hoc sorting
- can look at novelty and priming
- can look at temporal dynamics of response
What are the preprocessing steps?
- high pass filtering
- motion correction
- slice time correlation
- coregistration
- normalisation
- spatial smoothing
HMSCNS
What is slice time correlation?
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.
What is montreal neurological institute (MNI)
– Combination of 352 MRI scans on normal controls
• All right-handed subjects
What is the talairach space?
original standard brain of a 60-year-old french lady
Why is smoothing done?
because neurons do not fire in isolation
Why multiple regression used
to determine the effect of a number of independent variables, x1, x2, x3 etc, on a single dependent variable, y
Limitaion of fMRI
- correlative so cant say one thing causes another
- need convering evidence
- temporal resolution of fMRI is low
- need converging evidence from EEG
What are the two approaches to data analysis?
Whole brain analysis and region of interest (ROI) analysis
What is whole brain analysis?
examine effects on a voxel by voxel basis across the whole brain
What is region of interest (ROI) analysis?
who knows
Eval of whole brain analysis
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
Eval of ROI analysis
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.
What are limitations of a block design?
- highly predictable
- inflexible
- lack ecological validity
- cant seperate trials by performance
advantages of event-related design
- flexible
- post hoc sorting
- can look at novelty and priming
- can look at temporal dynamics of response
What does a BOLD signal measure?
the magnetic properties of oxygenated v. deoxygenated blood
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?
- Complex algorithms to warp each subject’s brain into the shape of a TEMPLATE BRAIN.
What was the original standard based on?
The brain of a 60-year old French woman
Why do we no longer use the original standard?
May not be representative of all brains
What standard do we now use?
- Montreal Neurological Institute (MNI) space: This is now the international standard for neuroimaging.
How was the montreal neurological institute space created?
- 352 MRI scans on normal controls, all right-handed subjects.
Advantages of whole brain analysis?
■ No prior hypothesis about areas involved needed.
■ Includes the whole brain.
Disadvantages of whole brain analysis?
■ 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.
Advantages of region of interest analysis?
■ Hypothesis driven – avoids meaningless lists.
■ Avoids multiple comparison problem.
■ Simple – Data can be exported requiring no extra software.
■ Generalisable – Data can be easily compared.
Disadvantages of region of interest analysis
■ Easy to miss things going on elsewhere in the brain.
■ Not always simple how to define ROIs.
Limitations of fMRI
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.
