Brain imaging methods Flashcards
spatial vs. temporal resolution
- spatial: precise in space, e.g. fMRI
- temporal: precise in time, e.g. EEG
correlational vs. causal
- fMRI/EEG/MEG: correlational methods
- TMS: “virtual lesions”: causal relations brain-behavior
When do you use fMRI?
- spatial resolution and whole brain coverage
- questions e.g., Where? Which areas cooperate? How does this differ between individuals?…
When do you use EEG?
- EEG/ERP/MEG: temporal resolution
- question: when? When is a certain process happening?
When do you use TMS?
- TMS: causal relations brain-behavior
- questions: Is a brain area necessary for a certain behavior? When is it involved?
What can we do with an MRI scanner?
- MRI (Magnetic Resonance Imaging): a picture of your brain’s anatomy
- static anatomical picture
- pretty high resolution
What can we do with an fMRI scanner?
- fMRI (functional Magnetic Resonance Imaging): a “movie” of your brain’s activity
- measures brain function/process
- colored parts are the fMRI picture
What can we do with DTI?
- DTI: diffusion tensor imaging
- pictures of the pathways of communication
- measures how axons are oriented
- static
MRI vs. fMRI vs. DTI
- MRI, fMRI and DTI scans are made using the same machine, with different settings
- MRI, DTI: participant lies still in the scanner
fMRI: participants usually do a task
What do you do in an fMRI scanner?
- person in scanner performs a task that activates a certain process, like calculating, remembering, etc…
- difficult to isolate/catch complex processes, like planning homework, complex social situations…
What is measured in fMRI?
- activated process –> activated neurons –> changed oxygen level in surrounding blood vessels (hemodynamics- BOLD signal, indirect)
- BOLD: indirect measure of neuronal activity: limitation bc it adds an extra step: not as localized as neurons themselves)
What is a contrast in fMRI?
- scans during the task are compared to scans during a “control” condition or rest (baseline)
- brain activity is always relative: shows a difference!
- choice of control task influences results
What are the colored ‘blobs’ in fMRI?
- areas in which the statistical value of this comparison is above a certain threshold
- we only color the voxels that are sign. lower/higher activated during experimental vs. control condition
anatomical MRI picture
- high resolution (1mm)
- one 3D image
- shows structure/ anatomy in high detail
- tissue contrast
functional MRI picture
- low resolution (2-3mm)
- time-series of 3D images (movie), fewer details
- functional contrast: hemodynamics
- resulting image: statistical map (fMRI) projected on anatomical image (MRI)
origin of fMRI signal
- researcher provides a task containing stimuli, e.g., pictures or sounds
- sense organs pass signals on to the brain and neurons are activated
- brain areas with activated neurons need oxygen
- fresh blood (=oxygen rich) will be supplied (seconds later)
- MRI scanner measures higher signal (oxygenated vs. deoxygenated blood have different magnetic properties)
What does DTI measure?
- measures the diffusion of water
- Looks at the structure of axons and axon bundles to figure out communication between different areas
- more specifically: how constrained this diffusion is in different orientations
- gives an “anisotropy” measure in each voxel (also called a tensor or Fractional Anisotropy)
- in axon bundles: diffusion constrained because of myelin sheath –> anisotropic
DTI: isotropic
equal diffusion in all orientations
DTI: anisotropic
more diffusion in 1 orientation (this is what happens in an axon)
- If the anisotropy is high (so that it is constrained) it is typically in myelinated axons because it can only diffuse in the same direction as the myelin sheath.
DTI: what can these “anisotropy” values and orientation be used to?
- assess white matter integrity (and e.g., compare across groups)
- reconstruct nerve fibers (fiber tracking)
! purely anatomical, not functional: no information about direction or intensity of communication
localization in EEG and MEG
- both methods: uncertain localization due to volume conduction (inverse problem)
- volume conduction: signal distorts because it has to travel a long way
- MEG slightly less sensitive to volume conduction
EEG
electro-encephalography
MEG
magneto-encephalography
ERP
= event-related potential
- evoked by certain task/stimulus
1. stimulus/task
2. EEG/MEG –> amplifier
3. raw data (a lot of repititions to cancel out noise)
4. cleaned, averaged data
ERP components
- well described components like N400 (negative ‘spike’ at 400ms)
- some have okay localization due to a lot of research
- shape + timing informative for normal + impaired processing and brain development
TMS
= transcranial magnetic stimulation
- causal relation brain-behavior
- more invasive
- only superficial brain areas
Good things neuroimaging
- neuro-imaging enables studying the human brain in action
- valuable for understanding brain-behavior relations in healthy brain and disorders
- fMRI: the entire brain in 1-2 seconds
- EEG: activity in milliseconds accuracy
- TMS: helps to understand causality
- methods complement each other: use methods that best answer your rq, combining methods to combine strengths
limitations neuroimaging
- fMRI: signal is indirect (measures blood, not neurons)
- EEG/ERP will never tell you exactly were the activity originates
- fMRI maps/ERP’s are relative to a control condition
- complex behavior is hard to study with neuro-imaging experiments: task design, scanner/lab environment etc.
needed orientation of neurons and neuronal activity in EEG and MEG
- you can only measure EEG or MEG signal if there are many neurons in the same orientation showing synchronous activation
- Same orientation refers to in the cortex and in some parts of the brain neurons are organized in a different way (e.g. more subcortical areas in which EEG cannot pick up this activity) and this means that EEG is only sensitive to certain kinds of neural activity
- Even if the neurons are organized in parallel but they are not synchronously activated then the activity is not strong enough to be picked up at the scalp
- -> EEG and MEG requires the synchronous activation of many neurons
