The Missing Chapter
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bones, and scalp without being diffused, an MEG locates the current
source more accurately than EEG measurements. The NYU group has
since begun correlating magnetic events with well-known cerebral re-
sponses, such as the reaction of cells in the visual cortex to simple pat-
terns and flashes of light. When the brain reacts to any stimulus, it
produces a wave of electrical activity that's contained in the EEG. It's
invisible in a standard EEG recording, because so much else is always
going on in the brain at the same time. However, when one simple
stimulus is repeated many times and the EEG tracings are averaged by
computer, the particular electrical response to that one stimulus—called
an evoked potential—can be teased out. Several research groups have
slowly built up a small vocabulary of wave forms with specific meanings,
including a "surprise wave," an "intention wave," and a "double-take
wave," which appears when the mind briefly tries to make sense of se-
mantic nonsense, as in the statement "She took a drink from the radio."
Taken together, the MEG research so far seems to be establishing that
every electrical evoked potential is accompanied by a magnetic evoked
potential. This would mean that the evoked potentials and the EEG of
which they're a part reflect true electrical activity, not some artifact of
nerve impulses being discharged in unison, as was earlier theorized.
Some of the MEG's components could come from such additive nerve
impulses, but other aspects of it clearly indicate direct currents in the
brain, particularly the central front-to-back flow. The MEG doesn't
show the EEG's higher-frequency components, however, suggesting that
some parts of the two arise from different sources.
Since every reaction and thought seems to produce an evoked poten-
tial, the DC system seems directly involved in every phase of mental
activity. At the very least, the electric sheath acts as a bias control, a
sort of background stabilizer that keeps the nerve impulses flowing in
the proper direction and regulates their speed and frequency. But the
analog structure probably plays a more active role in the life of the
mind. Variations in the current from one place to another in the peri-
neural system apparently form part of every decision, every interpreta-
tion, every command, every vacillation, every feeling, and every word of
interior monologue, conscious or unconscious, that we conduct in our
heads.
This part of the analog system's job is much less well understood,
however, than its integrative function throughout the rest of the body.
Perineural cells accompany every part of the nervous system. Even the
tiniest twiglets of sensory nerves in the skin, which don't have a myelin
covering, are surrounded by Schwann cells. The perineural