MEG Flashcards
MEG uses the …
magnetic field (as consequence of electrical current flowing)
just diff way of measuring the same thing as EEG
the magnetic field coming from the brain can be measured by the MEG (3 points) …
> at a single point
with no reference needed
with no electrical circuit (not sensitive to conductivity sensitivities - better spatial resolution)
the magnetic field flows in which direction around the current flow?
anti-clockwise (remember right hand rule - thumb being flow)
because of the orientation of magnetic fields, MEG and EEG need an xxx arrangement
open field
picking up mostly from xxxx neurons
pyramidal
an area of aligned neurons firing electrochemical processes creates a…
dipole
what is a dipole ?
a magnetic (+ and - poles to it)
so MEG (and EEG) measure
aggregate dipoles (PSPs)
which dipole does MEG record?
1) Tangential
or
2) Radial
Tangential
What is a Tangential orientation?
the neurons face sideways and therefore the magnetic fields flow outside the brain….
radial point outward and therefore the mag fields run sideways through the brain
can be a benefit to only get tangential, why?
as EEG picks up both - so if only intersted in certain part of brain, then we can focus in
the procedure to find out where a signal originated ….
source localisation
source localisation because?
we can’t tell which source it has come from
the MEG scanner has to compete with…
the magnetic fields around like the earth / urban noise etc
the MEG scanner is situated in a ….
faraday cage
and the magnetic coils need….
SQUID
what is a SQUID?
SQUID superconducting quantum intereference device
how does squid work
Li£ Helium
what is the sensor design?
Axial gradiometer
what is an Axial gradiometer ?
a pick up coil close to head and a further away compensation coil to compensate for background field
3 Noise removal points?
Reference subtracted from SQUID
Filters (low pass / band etc)
averaging output ideally 100+ trials
how many sensors ?
up to 275
MEG sensors move, so need a
fiducial point
what is a fiducial point
a MEG reference for head movements
tactile - visual cueing study (like simon task) showed…
enhanced response and component (gain mechanism to spatial attention / processing)
found in MEG - that spatial conflict between crossmodal attention - the bias from one modality draws one away. MEG allowed the tangential recording of associated sensory areas.
what is an ERF
event related field
what does ERF reflect ?
changes over time
after ERF we can …
decompose time-frequency
and make a
frequency spectrum
it’s not clear from the data where something is happening … what can we do
source reconstruction - an estimate of where the signal came from
source and dipole are used interchangeably
in source reconstruction we?
estimate dipole positions from MEG signal
what is the problem source reconstruction?
> dipole could be anywhere along the two lines of dipole emanation
(can’t find out where along the line it came from, could be right next to surface, could be deep within the brain)
- so need to find the source (remember dipole is an aggregate resulting activity of lots of neurons – that aggregation is a dipole)
solution to the dipole location issue?
from the data on the scalp, we can measure the angle from dipoles, and from this we can figure the angle out (trigonometry)
but
actually we dont have a way of source localising accurately – always be critical – most often ‘the best possible solution’
what is ‘the best possible solution’
it is an iterative programme computing from all the possible data points, which one is most likely source configuration given certain information (shape head / hypothesis /
forward model = if certain dipole location - then outcome –over and over again until - the inverse solution
which fits with the experimental data from the study
least error is solution
What is a newer technique to source location?
Friston and bayesian expectation-maximisation http://bit.ly/2o2PY1Z
Voxel-based morphometry - fmri
What is the mismatch negativity
• Response to deviant in train of auditory stimulus, compared to response to standard, related to ShortTermMemory trace
where is the MMN?
• Located Planum temporale (within sylvian fissure)
Clinical abnormality?
• Schizophrenia: MMN shows lateralisation abnormalities
• May be related to structural superior temporal gyrus
differences in schizophrenia (related to hearing voices + symptoms)
• Can be observed in at-risk individuals and family members
in this study on schiz using MEG/(with fmri) ….. why?
to get a good temporal resolution from tangential sources
and to get god spatial to find location
what was the clever use of the limitation if MRI in auditory research?
they used scanner noise as their experimental stimulation
The scanner noise was
recorded and applied to the same subjects
in a whole-head magnetoencephalography
(MEG) device.
A novel design to measure mismatch negativity responses to deviant auditory stimuli was generated by using the switching noises from the functional magnetic resonance imaging (fMRI) scanner, thus avoiding any interfering background sound. Stimuli included deviants of amplitude (9 dB lower) and duration (76 msec shorter) presented in a random sequence.
what was results of the study on mismatch in schizo?
As expected, neuromagnetic mismatch
fields were smaller in the patient
group. More specifically, a lateralization to
the right for duration deviance was only
found in comparison subjects. For the relative
amplitude of the blood-oxygen-leveldependent
signal (measured with fMRI),
differences emerged in the secondary
(planum temporale), but not primary
(Heschl’s gyrus), auditory cortex. Duration
deviants achieved a right hemispheric advantage
only in the comparison group. A
significantly stronger lateralization to the
left was found for the deviant amplitude
stimuli in the patients.
another study she mentioned was the …
MEG study into hand representation in string players
what did the MEG study of hand representation find?
Increased Cortical Representation of the
Fingers of the Left Hand in String Players
enlarged somatosensory area
