True or false: Everyone has brain waves that can be measured using EEG
true
True or false: The fMRI-scanner uses magnetic resonance to make pictures of brain cells that light up
not true
True or false: EEG sends weak electrical pulses through the brain
not true
True or false: In scientific research using fMRI and EEG, data are being analysed via standardized protocols
not true
True or false: fMRI measures changes in the oxygen levels in the brain’s blood vessels
True
True or false: In scientific research using fMRI and EEG, data are being analysed via standardized protocols
Not true
True or false When using EEG, one is never certain about where in the brain the signal comes from
true
Techniques complement each other: Temporal and spatial resolution
Temporal resolution
•Precision of a measurement with respect to time
Spatial resolution
•Precision of a measurement with respect to space
When to use which technique?
- EEG/MEG:
- fMRI/EEG/MEG:
- MRI:
- TMS/TES (“brain stimulation”):
•EEG/MEG: temporal resolution
- fMRI/EEG/MEG:
- functional measures
- correlational relations brain-behavior
- MRI: anatomical measures
- TMS/TES (“brain stimulation”): causal relations brain-behavior
What can we do with an MRI scanner?
- MRI (magnetic resonance imaging) – a picture of your brain’s anatomy
- fMRI (functional MRI) – a “movie” of your brain’s activity
- DTI (diffusion tensor imaging) – pictures of the pathways of communication
MRI versus fMR
MRI, fMRI and DTI scans are made using the same machine, with different settings.
MRI: high resolution
•Shows structure/ anatomy in high detail
•Tissue contrast
fMRI: low resolution
• Time series of 3D picture (movie), fewer details
•E.g.: every 2 sec during 5 minutes
•Functional contrast: hemodynamics (blood oxygen level)
Functional MRI: 4 steps
- Person in scanner performs a task that activates a certain process, like inhibition, calculating, reading, etc… •Difficult to isolate complex processes
- Activated process > activated neurons > changed oxygen level in surrounding blood vessels
- Scans during the experimental condition are compared to scans during a control condition •Brain activity is always relative: shows a difference! •Choice of control condition influences results
- Colored “blobs”: areas in which the statistical value of
Origin of fMRI signal (4)
- Researcher provides stimuli / task (e.g. Stop task – lecture 5A)
- Stimuli/task activates neurons in certain brain areas
- Brain areas with activated neurons need oxygen
- Fresh blood (= oxygen rich) will be supplied (seconds later!)
Blood influences MRI signal
- Oxygenated vs. deoxygenated blood (Hb) have different magnetic properties
- BOLD signal: Blood Oxygen Level Dependent
Summary (f)MRI
- The colored “blobs” are in fact a (thresholded) statistical map
- The task probably activates the intended cognitive process
- Or actually: the intended cognitive process probably stronger than the control task
- “Activates” means in fact that the blood composition changed, probably as a result of increased activity of the neurons at the same location•In sum, probably, the colored brain parts in an fMRI picture reflect the neural activity specific to the investigated process.
EEG: electro-encephalogram
vs.
MEG: magneto-encephalogram
BOTH methods: uncertain localization due to volume conduction
•MEG slightly less sensitive to volume conduction
ERP components
ERP = Event Related Potential
- Well described components like N400
- Some have OK localization due to a lot of research
- Shape + timing informative for normal + impaired processing & brain development
Interpreting neuro-imaging results
1.Correlation vs. causation
fMRI/EEG/MEG: correlation methods
- Brain activity in area X correlates with performance on task A (vs control task B)
- We can’t conclude that area X is necessary for (causally related to) performance on task A
Interpreting neuro-imaging results
2.Reverse inference
Forward inference (= justified): what brain activity/region is associated with a given experimental condition/contrast?
- ‘if cognitive process X is engaged, then brain area Z is active’ Reverse inference (= unjustified): going backwards from the presence of brain activation to the engagement of a particular cognitive function: 1.In the present study, when task/contrast A was presented, brain area Z was active.
- In other studies, when cognitive process X was putatively engaged, brain area Z was active.
- Thus, the activity of area Z in the present study demonstrates engagement of cognitive process X by task comparison A.
Interpreting neuro-imaging results
3.Individual differences
- Neuro-imaging findings of different brain function in a disorder: “group” results
- Average of group 1 different from average of group 2
•Brain anatomy and fMRI/EEG signals known to differ across subjects, as well as task-related activity (strategies)
•Individual diagnoses not possible!
>More sophisticated analysis-techniques: classifier algorithms
>Never 100% accurate!
Neuroscience advantage
- Neuro-imaging enables studying the living human brain in action •Understanding brain-behavior relations
- Techniques complement each other
- Longitudinal studies
- Imaging development: e.g. adolescent brain
- Typical and atypical development
- Shedding light on mechanisms:
- Adding a level in understanding influences on educational outcomes (week 1)
Neuroscience limitations
- fMRI signal is indirect (measures blood, not neurons) and slow
- EEG/ERP will never tell you where the activity originates
- fMRI maps/ERP’s are relative to a control condition importance of a good experimental design
- Complex behavior is hard to study with neuro-imaging experiments: task design, scanner/lab environment, etc
- Interpretation caveats
- Correlation vs. causation
- Reverse inference
- Individual differences
