Biomedical Engineering Reference
In-Depth Information
of a millisecond to minutes and hours
(12)
. It thus becomes pos-
sible to extract from the same set of MEG data views of local and
global activity and patterns of connectivity in this wide spatiotem-
poral scales of bewildering complexity
(13)
.
Here, we provide an overview of MEG, covering basic prin-
ciples, measurements, source reconstruction, and finally examples
to demonstrate the different types of output that can be extracted
from the analysis of MEG recordings and by implication what
is the relevance of such output to neuroscience. The next sec-
tion,
Materials
, first describes in simple terms possible neural
mechanism that may be responsible for coherent electrical activ-
ity from many neurons. It then outlines how the well-understood
laws of electromagnetism and properties of the head explain the
generation of, and differences between the EEG and MEG sig-
nals. Finally, this section outlines the instruments that allow the
detection of the MEG signal outside the head. The
Forward and
Inverse Problems
section outlines how one can compute the sig-
nal generated by a given configuration of generators (the forward
problem) and how one can derive estimates for the generators
from the measurements (inverse problem). The next section,
Out-
put of MEG
, provides a flavor of the output of MEG analysis with
emphasis on aspects that are unique to this technology. We finish
with
Notes
where the advantages and disadvantages of MEG are
discussed.
2. Materials
This section first outlines the likely neuronal mechanisms respon-
sible for the generation of the MEG and EEG signal and then
describes the basic elements of the hardware used for the detec-
tion of the MEG signal.
2.1. Neural
Mechanisms
The smallest detectable MEG signal requires concerted action
from many neurons numbering at a minimum many hundreds.
These neurons must be arranged in a similar way in space and
they must be activated in near synchrony. The very presence of a
good size MEG and EEG signal is evidence for dual organization
of neurons: a spatial organization in the way they are grouped
together in space and large scale synchrony in the way their activ-
ity is organized in time. It is generally believed that relatively slow
changes in electrical activity associated with post-synaptic poten-
tials (PSP) at the apical dendrites of large pyramidal neurons are
the main contributors to the MEG signal. Large pyramidal neu-
rons are prime candidate generators of MEG signals because their
elongated shape is ideal for producing strong primary currents.
Furthermore they are arranged in parallel in the cortex so the
