precisely than previously possible, the relationship between regions
of the cortex and various functions, such as sensory perception, mus-
cle movement, language generation, and language comprehension.
This direct-from-the-brain recording has continued to be devel-
oped into a highly refined technique called electrocorticography, or
ECoG. This technique is used in some situations before brain surgeries
in which epileptogenic tissue is to be removed. At the same time as the
precise location of epileptogenic tissue is being mapped with an array
of ECoG electrodes, neurosurgeons and neuroscientists may collabo-
rate to address interesting questions about the function and dynamic
activity of the cortex.
Electricity and magnetism are intimately connected. Electric cur-
rents, for example, generate magnetic fields. The magnetic fields
induced by the electric currents associated with neural activity can
be measured in the vicinity of the head. The result is another kind of
brain wave, called a magnetoencephalogram, named by analogy with
the electroencephalogram. The technique used to make the measure-
ment is called magnetoencephalography, or MEG.
The magnetic fields associated with neural activity in the human
brain measure about 1 picotesla (10-12 tesla) at the surface of the
skull. This is a very small amplitude—Earth’s magnetic field, ap-
proximately 50 microteslas (50 x 10- tesla), is about fifty million
times stronger. Ambient magnetic noise in an urban environment
(due primarily to electricity moving through power lines and other
wiring) is in the range of 0.1 microtesla (10~’ tesla), about 100,000
times stronger than the magnetic fields generated by brain activity.
Thus, the two primary hurdles that must be negotiated in MEG are
constructing very sensitive detectors of magnetic fields and shielding
the whole measurement process from ambient electromagnetic noise.