carriers involved, we observed the effect of freezing a segment of the nerve located between the two
measurement electrodes. Charge carriers of the type proposed by Szent-Gyorgyi (electrons in a
semiconducting lattice) would be enhanced in their movement by freezing, resulting in an increase in
the current, while movement of ions would be inhibited by the freezing, causing the current to decrease
and the potentials to decline. The experiment was carried out on the sciatic nerve of the bullfrog and
enhancement of the voltage was found each time the segment of the nerve was frozen. If the voltage
gradient was dropped to zero by section of the spinal cord or very deep anesthesia, freezing of the
segment between the electrodes did not produce any measurable voltage. The freezing effect therefore
seemed to be genuine and not the result of any artifact of the freezing process. However, the
experiment, while suggestive, was not conclusive proof of the existence of a longitudinal current
associated with the peripheral nerve fibers. A sensitive technique was required that could provide
unequivocal evidence of the existence of the current while minimizing exposure and manipulation of
the nerve. It seemed that the Hall effect could be such a technique.
The Hall effect consists of exposing a current-carrying conductor to a magnetic field oriented at
90° to the axis of the conductor. The field will produce some deviation of the charge carriers which can
then be sensed as a steady voltage at the second 90° axis to the conductor. This is called the Hall
voltage and its magnitude is very dependent upon the degree of mobility of the charge carriers (being
almost undetectable with ionic currents, only slightly more detectable with electron flow in metallic
conductors, but easily detected with semiconducting currents due to the high mobility of their charge
carriers). Since the nerve currents were obviously not metallic conduction and since the detection of
ionic Hall voltages was far beyond the capability of our equipment, we reasoned that if any Hall
voltages were observed, they would indicate not only the existence of the current but also that it was
probably analogous to semiconducting current.
The experiment was performed using the foreleg of the salamander as the current-carrying
conductor (the nerve was not exposed, the intact limb being contacted only by the two soft measuring
electrodes to obviate any injury effects). In 1961 we reported observing Hall voltages under these
circumstances (19) . The magnitude varied inversely with the state of anesthesia of the test animal,
indicating that the voltages were real, not artifactual, and directly related to the operational state of the
CNS. There was now strong evidence that some component of the CNS generated and transmitted
direct currents that produced the measurable voltages on the surface of animals, including humans.
It remained to determine if these voltages were related to the functions, as described by Libet
and Gerard for the nervous system and Burr and Lund for the total organism. In our measurements of
the surface potentials we had noted that they varied in a definite pattern with the level of consciousness
of the subject, particularly a midline, occipito-frontal voltage vector across the head, which seemed to
accurately reflect the level of anesthesia. In the conscious subject this vector was frontally negative,
diminishing in magnitude and going positive as anesthesia was induced and deepened. This voltage on
the skin surface exactly paralleled that observed by Libet and Gerard and by Caspers in the brain and
we postulated that this current vector represented current flow in median unpaired structures of the
brainstem area. If Libet and Gerard and Caspers were correct, this current should be the determiner of
the level of excitability (hence consciousness) of the subject, and electrically reversing the normal
frontally-negative potential should induce the same loss of consciousness as chemical anesthesia.
Again using the salamander, we observed that currents as small as 30 µamp administered in this
direction produced loss of consciousness and responsiveness to painful stimuli. In addition, they
produced in the subject animal electroencephalographic patterns typical of the anesthetized state (high
amplitude delta slow waves) (20). In the converse experiment, attempting to restore consciousness to a
chemically anesthetized animal, the EEG evidence was suggestive but much less convincing.
ELECTROMAGNETISM & LIFE - 28