Unformatted text preview: Magnetoencephalography
SQUID sensors Averaged magnetic fields recorded from the
left hemisphere of a subject in response to
stimulation of the digits of the right hand.
The magnetic field scale is 200 fT! The upper left panel shows the magnetic field change in each of 143
MEG sensors distributed uniformly about the head. Each time trace is
approximately 600 msec in duration and has been displayed on the surface
of a flattened head with the nose pointing upward. The data was collected
in an unshielded environment and is the result of 100 averages.
The upper right panel shows the same MEG channels overlaid upon each
other. The n100 response occurs, in this case, 97 msec after the onset
of the tone. The red line is chosen to be the time of interest and the
result is shown in the lower left map. The magnetic field distribution
in the map clearly shows a signal consisting of two dipolar sources.
After performing the inverse solution, the resultant equivalent current
dipoles may be overlaid on the MRI. The lower right image is the result
of this fusion, shown from the back of the head. All steps in this process
are conveniently carried out using the MEG/4 software package. MEG Source Detection
The field from a point like source in the brain produces a dipolar pattern in the MEG data.
Using an inverse solution, the magnetic field distribution may be described as an equivalent
current dipole. ...
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