Brain maps old and new
The first technique used for brain mapping was the electroencephalogram
(EEG), which measured the oscillating electrical potentials
over various parts of the scalp. This has good time resolution,
but it is difficult to interpret and has poor spatial resolution,
since electrical fields are severely distorted in passing
through the brain and especially through the skull. A more
sophisticated version of EEG, coming into use in the last
decade, maps correlations between EEG patterns in different
parts of the brain — that is, it shows which parts of the
brain are in phase with another at a given frequency of oscillation.
Since magnetic fields produced by currents in the brain are
less distorted, magnetoencephalograms (MEG), which measure these
fields, give more precise three-dimensional maps but still do
not have resolution finer than a few centimeters.
In positron emission tomography (PET) scans, chemicals are
labeled with radioactive materials which emit positrons, the
positively charged anti-particles to ordinary electrons. These
travel only a very short distance before they encounter electrons
and are annihilated, releasing tiny amounts of gamma rays. Since
these travel in straight lines, their origin can be mapped very
accurately, showing where the chemical is concentrated. Often
this technique is used to measure change in blood flow, and
thus brain activity.
The most recently developed mapping technique, functional
magnetic resonance imaging, like ordinary MRI, detects radio
signals from atomic nuclei as they oscillate in a strong electronic
magnetic field. One of the more sensitive fMRI techniques,
flow-sensitive inversion recovery, measures relaxation times
in flowing blood — the time it takes for a signal from a batch
of nuclei to decline. These times are faster when flow is
increased because of greater turbulence in the blood. At high
magnetic fields (up to four T), this technique can deliver
spatial resolution of less than one millimeter in three dimensions.
However, since it takes a second or two for the brain to increase
blood flow, time resolution is still far less than for EEG
and MEG.
Also:
Two models of the brain
Brain maps old and new
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