5. Brain Imaging Flashcards
fMRI better … resolution?
MEG/EEG better …resolution?
fMRI has better spatial resolution
MEG/EEG has better temporal resolution
why is EEG temporally highly sensitive?
electromagnetic waves propagate almost simultaneously
wat are 2 disadvantages of EEG?
solution?
EEG:
- is sensitive to neurons near skull, but deep
- electrical activity is distorted by tissue (skull, CSF etc)
possible solution: Electrocorticography
> record electrival activity directly from cortex surface
how to read an EEG
by Fourier transformation:
> transform complex waves into underlying sin/cosin waves
> count
MEG:
- temporal resolution?
- spatial resolution?
two advantages?
MEG:
- temporal resolution: milliseconds
- spatial resolution: few millimeters (beter than EEG)
by using magnetic source imaging
- entirely silent
- non invasive
what is a key difference in measurement between MRI and EEG/MEG?
EEG/MEG measures electrical/magnetic changes in neural activity directly
MRI measures indirectly the changes in bloodflow due to neural activity
wat is BOLD?
BOLD
> Blood Oxygen Dependent Activity
3 changes in blood oxygen levels due to neural activity
- blood oxygen levels drop due to consumption of oxygen
- overshoot of blood oxygen levels, oxygen rich blood is transported to that region
- blood oxygen levels go back to baseline
why do most of the research combine multiple measuring intruments?
it is not possible to get a good spatial AND temporal resolution using only one instrument
good spatial: fMRI, PET
good temporal: EEG/MEG, single cell recordings
wat is “co-registration”?
combining functional and structural imaging
fMRI: what is a disadvantage of subtracting conditions?
subtracting conditions: subtract baseline measurement from experimental measurement
disadvantage: if there is important brain activity occuring in both measurements, this data is lost
fMRI: what is an alternative to the subtracting conditions method?
parametric variation:
> gradually increase experimental task
> observe: does neural activity increase gradually as well?
“the resting brain is not silent”
> what does that mean?
> implications for fMRI measurements?
“the resting brain is not silent”
> in baseline conditions, where participants are told not to do something (e.g. speak), the brain still is active (e.g. inner speech)
> creating a valid baseline condition in fMRI research is difficult but important
what is DTI?
DTI: diffusion tensor imaging
> is a MRI method using water flow in the white matter to measure relative directions of white matter tracts
what is binocular rivalry?
binocular rivalry
> only one visual stream of input (left or right eye) is conscious at any time
3 main characteristics of MEG/EEG
MEG/EEG
- measures simultaneous activation of large populations of neurons
- this populations of neurons must have same orientation
- is sensitive to gradual changes in synaptic potentials
gamma activity in the brain is indication of what?
gamma activity
> is a signal for exchange between cortical and subcortical regions
what is “coding by synchrony”?
coding by synchrony
> random spiking of two neurons can be synchronized by an inhibitory cell
> by then firing at the same rate, they can activate a downstream cell
MEG measures magnetic fields where?
MEG measures in sulci
> only there the magnetic fields can be measured
3 steps of TMS
TMS
- coils change of current, creating a magnetic field
- magnetic field induces current in nearby neuron
- current disrupts cognitive function
> temporal lesion
MEG
> what is the difference between a
- tangential dipole
and a
- radial dipole
MEG
tangential dipole: magnetic field leaves and re-enters head
radial dipole: electric current direction “leaves” head
what are the advantages/disadvantages of TMS?
(4)
TMS
- is temporary - no brain reorganization takes place
- its reversible
- effects are focal (ca. 1cm), but it cant stimulate deep regions
- it can investigate the time course of cognition
(effects are brief)
which brain property uses
- EEG
- single cell recordings
- TMS
- MEG
- PET
- fMRI
- EEG - electrical
- single cell recordings - electrical
- TMS - electromagnetic
- MEG - magnetic
- PET - hemodynamic
- fMRI - hemodynamic
MRI:
what is T2?
what is T1?
MRI
T2: time it takes for protons to lose phase coherence
> transverse relaxation (spin-spin)
T1: time it takes for protons to realigne to magnetic field
> longitudinal relaxation (spin-lattice)
MRI
T1 used for?
T2 used for?
MRI
T1 is used for structural imaging
T2 is used for functional imaging
wat is T2*?
T2* is T2 + dephasing due to local inhomogeneity
BOLD:
what kind of blood has what magnetic property?
> implications for T2 intensity?
high oxygen blood: not magnetic
low oxygen blood: slightly magnetic
> low oxygen blod has less T2 intensity
fMRI: when is a brain regions active?
“activation” is always a statistical decision
> threshold has to be set
