As mentioned earlier, the surest sign of actual electrical current flow in any biological structure
would be the detection of the resulting magnetic field in space around the structure. Technology had
long been quite inadequate to detect the extremely weak fields predicted. The first detection of a
"biomagnetic" field was in 1963 by Baule and McFee who used classical techniques (a coil of million
turns of wire and a ferrite core) to detect the field associated with the action of the human heart (21).
The field intensity was five orders of magnitude lower then the earth's norma! field and it was
necessary to conduct the experiment in a rural area as free of extraneous fields as possible. Two years
later, Cohen, using the same technique coupled with a signal averaging computer, found evidence of a
magnetic field around the human head of even weaker strength (about eight orders of magnitude lower
than earth's normal field).
The invention of the SQUID (a superconducting quantum interference device based upon the
Josephson junction) permitted detection of these and even lower intensity fields with relative ease. It
was first applied by Cohen in 1970 for a more complete detection of the human magnetocardiogram,
and in 1972 he reported that the magnetic field in space around the human head demonstrated a wave
form pattern, similar to, but not identical with, the EEG as measured by skin electrodes (22). Cohen
called this the magnetoencephalogram or MEG. Since then improvements in the technology have
permitted detection of the magnetic fields associated with evoked responses, and correlations between
the EEG and the magnetoencephalogram have shown that the low-frequency components of the EEG
are well represented in the MEG, but that usually the high frequency components (i.e., sleep spindles)
are missing (23, 25) . Most recently it has been possible to record the magnetic field associated with the
nerve impulse itself, again using the SQUID and certain specialized techniques necessary because of
the extremely small field strength (26).
Originally, it was considered likely that the MEG arose from the simultaneous action potential
activity in large numbers of neurones arranged in parallel. However, this same concept has been
advanced as the explanation of the EEG and as yet it remains unsubstantiated. While some aspects of
the MEG are compatible with this concept-the magnetic evoked response for example-others have
provided evidence supporting the existence of DC currents in the brain. The orientation and structure of
the total magnetic field around the head is most compatible, according to Reite and Zimmerman (24),
with a "longitudinally oriented current dipole within the head"-a concept identical to that proposed by
Libet, Gerard and Caspers on the basis of their DC measurements in the brain and by us, based on our
surface measurements and the results of low level direct currents injected along the same vector.
Furthermore, the retina of the eye is actually a direct extension of the brain and it demonstrates a
number of DC electrical phenomena as previously mentioned. The electroretinogram seems to be
generated by a steady dipole field extending from the retina to the cornea. A DC magnetic field has
been detected associated with it, indicating that an actual current flow is occurring along the vector.
Most recently, DC magnetic fields have been detected from the brain itself. The evidence provided by
the MEG has not only supported the much older concepts of DC activity in the brain outside of the
neurones, but it has stimulated renewed interest in the entire area.
Taken all together, the evidence seems to be quite conclusive that there are steady DC electrical
currents flowing outside of the neurones proper in the entire nervous system. The currents appear to be
nonionic in nature and there is some evidence that they may be similar to semiconducting type currents.
At present the perineural cells appear to be the most likely site in which the currents are generated and
transmitted. One apparent function of the currents is that of governing the level of activity of the
neurons proper; that is, the currents, via their polarity and magnitude, exert a biasing effect upon the
neurone's ability to receive, generate and transmit action potentials.
ELECTROMAGNETISM & LIFE - 30