Biomedical Engineering Reference
In-Depth Information
17.2 Measuring Brain Activity
17. 2 .1 e e g
Electroencephalography (EEG) is a measure of the electrical activity of the brain.
It is usually measured using multiple disk-shaped silver-silver chloride electrodes
(Ag-AgCl electrodes) of dimensions in the range of 2-12 mm on the scalp. It usually
requires the use of a conductive paste to reduce skin impedance, although dry
electrodes that do not require a paste are in development (Popescu et al. 2007). The
EEG recorded on the scalp reflects the sum of synchronous electrical activity of
millions of pyramidal cells.
Neurons are excitable by voltage changes across their membranes. Because of the
attenuation of the neuronal signal, it is only possible to record signals coming from the
dendrites; therefore, EEG has a good temporal resolution but a poor spatial resolution.
Although it is possible to record signals on the order of milliseconds, the spatial resolution
is poor (around 1 cm) compared with other techniques (see Table 17.1).
17.2.2 Meg
Magnetoencephalography (MEG) is a measure of changes to the magnetic fields. This
means that although maintaining the temporal resolution of the EEG (they measure the
same signal), the spatial resolution is better. MEG is considered as a compromise because
it offers the same temporal resolution of the EEG, but a spatial resolution closer to other
techniques, such as fMRI.
The advantages of MEG are counterbalanced by its costs. Apart from the costs of the
machine itself, MEG requires a magnetically shielded room to shield from all magnetic
fields coming from outside of the head and to record only the magnetic fields of the brain.
These magnetic fields are on the order of few femtotesla (10 -15 T). Not only can the Earth's
magnetic field interfere with MEG, but so can any electric source nearby that produces
detectable fields. The brain's magnetic fields are sensed by superconducting quantum
interference devices (SQUIDs) positioned in the helmet of the MEG.
17.2.3 fMrI
Functional magnetic resonance imaging (fMRI) uses a very strong magnet located inside
of a horizontal tube, where a person can be positioned, to generate a stable magnetic field
TABLe 17.1
Comparison of Different Techniques to Measure Brain Activity
Temporal Resolution Spatial Resolution Portability Cost
EEG ~1 ms ~1 cm High Low
MEG ~1 ms ~1 mm Not portable Very high
fMRI 4-5 s <1 mm Not portable High
NIRS 4-5 s < 3 cm Low Moderate
EEG and MEG have better temporal resolution, although MRI has the best spatial
resolution. EEG is a quite portable technique, and NIRS might be considered as
acceptably portable, whereas MEG and fMRI cannot be moved.
 
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