Information Technology Reference
In-Depth Information
this way, the electrical activity in different Regions Of Interest (ROIs) can be
obtained by averaging the current density of the various dipoles within the
considered cortical area.
10.1.1. Head models and regions of interest
In order to estimate cortical activity from conventional EEG scalp recordings,
realistic head models reconstructed from T1-weighted MRIs are employed.
Scalp, skull and dura mater compartments are segmented from MRIs and
tessellated with about 5000 triangles. Then, the cortical regions of interest (ROIs)
are drawn by a neuroradiologist on the computer-based cortical reconstruction of
the individual head model by following a Brodmann's mapping criterion.
10.1.2. Estimation of cortical source current density
The solution of the following linear system:
Ax = (10.1)
provides an estimation of the dipole source configuration x which generates the
measured EEG potential distribution b . The system includes also the
measurement noise n , assumed to be normally distributed. A is the lead field
matrix, where each j-th column describes the potential distribution generated on
the scalp electrodes by the j-th unitary dipole. The current density solution vector
ξ of Eq. (10.1) was obtained as:
b
n
(
)
2
2
2
ξ
=
arg min ||
A
x
− +
b
||
λ
||
x
||
(10.2)
M
N
x
where M , N are matrices associated to the metrics of data and source space,
respectively; λ is a regularization parameter; || … || M represent the M-norm of the
data space b and || … || N the N-norm of the solutions space x . The formula (10.2)
represents a minimization problem also known as linear inverse problem.
As a metric of the data space the identity matrix is generally employed.
However, the metric in the source space can be opportunely modified when
hemodynamic information is available from recorded fMRI data. This aspect can
notably improve the localization of the source activity. An estimate of the signed
magnitude of the dipolar moment for each one of the 5000 cortical dipoles was
then obtained for each time point. The instantaneous average of all the dipoles'
magnitude within a particular ROI was used to estimate the average cortical
activity in that ROI during the whole time interval of the experimental task.
Search WWH ::




Custom Search