Biomedical Engineering Reference
In-Depth Information
Applications of MEG. Magnetoencephalography (MEG), in which magnetic
fields generated by brain activity are recorded outside of the head, is now in rou-
tine clinical practice throughout the world. MEG has become a recognized and vital
part of the presurgical evaluation of patients with epilepsy and patients with brain
tumors. The big advantage of the MEG technique is that the magnetic field gener-
ated inside the brain is affected to a much lesser extent by the conductivities of the
skull and scalp than the electric field generated by the same source. In the limiting
case of spherical head model, concentric inhomogeneities do not affect the mag-
netic field at all, whereas they have to be taken into account in the analysis of EEG
data [Hamalainen et al., 1993]. This property of MEG is very advantageous when
localizing sources of activity. A current review showing an improvement in the post-
surgical outcomes of patients with epilepsy by localizing epileptic discharges by
means of MEG can be found in [Stufflebeam et al., 2009]. Magnetic evoked fields
(EF) are a counterpart of ERP; they are an effect of event-related brain activity which
can be elicited by visual, auditory, sensory, or internal stimuli. Their analysis usually
concerns identification of the brain region responsible for their generation.
The MEG sensors are not directly attached to the subject's head. This implies
that during the MEG measurement the subject's head should not move in respect to
the sensors. This requirement limits the possibility of long-session measurements, or
long-term monitoring.
4.1.1 Generation of brain signals
In the brain there are 10 11 nerve cells. Each of them is synaptically connected with
up to 10 4 other neurons. Brain cells can also communicate by means of electrical
synapses (gap junctions) transmitting current directly. Electric activity of neurons
is manifested by generation of action potentials and post-synaptic potentials (PSP).
Action potentials occur when the electrical excitation of the membrane exceeds a
threshold. The generation of action potentials is connected with rapid inflow of Na +
ions to the cell, which changes the polarization of the inside of the neuron from about
-80 mV to about +40 mV; the repolarization of the cell is connected with the outflow
of K + ions to the extracellular space. In this way an action potential of a characteristic
spike-like shape (duration about 1 ms) is created. The exact time course of the action
potential is shaped by the interplay of many ionic currents, specificforthegiven
neuron type.
Postsynaptic potentials are sub-threshold phenomena connected with the pro-
cesses occurring on the postsynaptic membrane. When the action potential arrives at
the synapse, it secretes a chemical substance called a mediator or transmitter, which
causes a change in the permeability of the postsynaptic membrane of the target neu-
ron. As a result, ions traverse the membrane and a difference in potentials across the
membrane is created. When the negativity inside the neuron is decreased, an excita-
tory postsynaptic potential (EPSP) is generated. An inhibitory postsynaptic potential
(IPSP) is created when the negativity inside the neuron is increased, and the neuron
becomes hyperpolarized. Unlike the action potential, the PSPs are graded potentials;
their amplitudes are proportional to the amount of secreted mediator. Postsynaptic
 
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