Lecture 11: Source Localization in MEG Flashcards
So far in the module, we have discussed (sensor space) - (2)
activity in brain produces magnetic field which is picked up from sensors outside the head
We can plot activity across the sensors like this:
In sensor space, we detect the brain activity at sensors outside of the
head
In sensor space, we can perform assessments of
time and frequency of responses
In sensor space we can also look at approximately where the effects are happening - example
So yellow region show strongest effect happens towards the back of head but don’t have greater sense of ‘where’ it specifically is
Sometimes we want to know instead of sensor space , we want to know
where the activity is changing in the brain
What is source localization?
Tries to work out where the activity is changing in the brain by using a plausible model of the head and a set of assumptions about how signals propogate
Diagram of sensor space to (estimate) source space - (3)
No longer looking at…
In source space get better at…
This is an estimate source space meaning…
No longer looking at data per sensor but looking data due to positions within brain than the head
In source space, talk a bit better where responses are coming from - get this brain map like in MRI.
Its estimate of source space as difficult to transform sensor to source space so estimating based on what we know how signals move around and what we know about the anatomy of the participants head
Source localization gives our results in
source space
What does this diagram show? - (3)
Image things called dipoles in the brain
Has a positive charge on one side and a negative charge on other
This dipole produces our brain activity
We can model the sources in the brain as..
when the dipole turns on.. - (2)
dipoles in brain
When the dipole turns on it produces some brain activity that will be picked up at the sensors
Since we can model sources in the brain as dipoles we can consider the brain to include many
dipoles
Consider the brain to include many dipoles that are potential sources of
brain activity
Consider the brain to include many dipoles which are all potential sources of brain activity
what question are we then asking?
Which of these dipoles are turning on?
Consider the brain to include many dipoles which are potential sources of activity and
each dipole have a
location and an orientation
What does this image show? - (2)
The black dot shows the location of the dipole
Red arrow shows the orientation of dipole - which way is negative and which way is positive
Consider brain to have many dipoles and all potential sources of brain activity
To know where the possible sources of the brain activity (i.e., in diapoles) should be and likely orientation we need to know
the anatomy/structure of participants’ brain to see the possible sources of brain activity and likely orientation in cortex of grey matter
Participants have individual differences in how their
brain is structured
To know where the possible sources of the brain activity (i.e., in diapoles) should be and likely orientation we need to know the anatomy/structure of participants’ brain
how do we know the anatomy of the participants’ brain? - (2)
using their fMRI structural scan
or if we can’t then use average brain (e.g., MNI152 brain template)
Diagram of structural MRI and MNI
MNI is not as good as technique as structural MRI as..
but why is it used? - (2)
won’t know exact cortical folds and but some sense where their grey matter is
Used if some participants don’t show up for MRI session after EEG/MEG
From structural scan and MNI, we try to
produce 3D model of the brain using brain mesh
T1 Structural scans will take a series of
if we see an edge of scans then - (2)
2D slices that shows us the anatomy of the participants’ brain
compute a 3D model of brain
Intensity differences in 2D T1 anatomcial scan and T2 functional scans lets us work out the
3D shape of the brain , head and skull
From, Intensity differences in the 2D T1 and T2 scans let us work out the 3D shape of the brain, head and skull.
we can then build the
3D meshes of the brain