Magnetoencephalography (MEG) is a noninvasive technique for investigating neuronal activity in the living human brain. The time resolution of the method is better than 1 ms and the spatial discrimination is, under favorable circumstances, 2-3 mm for sources in the cerebral cortex. In MEG studies, the weak 10 fT-1 pT magnetic fields produced by electric currents flowing in neurons are measured with multichannel SQUID (superconducting quantum interference device) gradiometers. The sites in the cerebral cortex that are activated by a stimulus can be found from the detected magnetic-field distribution, provided that appropriate assumptions about the source render the solution of the inverse problem unique. Many interesting properties of the working human brain can be studied, including spontaneous activity and signal processing following external stimuli. For clinical purposes, determination of the locations of epileptic foci is of interest. The authors begin with a general introduction and a short discussion of the neural basis of MEG. The mathematical theory of the method is then explained in detail, followed by a thorough description of MEG instrumentation, data analysis, and practical construction of multi-SQUID devices. Finally, several MEG experiments performed in the authors' laboratory are described, covering studies of evoked responses and of spontaneous activity in both healthy and diseased brains. Many MEG studies by other groups are discussed briefly as well.

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Magnetoencephalography—theory, instrumentation, and applications to noninvasive studies of the working human brain

Magnetochemistry: advances in theory and experimentation

A device for discharging fastening elements, and a method of preventing a device from discharging fastening devices into human flesh, are disclosed. The device includes a coil proximate a location of discharge, a capacitive element coupled in parallel with the conductive coil to form a resonant tank circuit, an oscillator that drives the tank circuit, a frequency detector, an amplitude control circuit and a processor. The detector detects a frequency of oscillation of the tank circuit as affected by a material proximate the coil. In response to an electrical signal from the oscillator, the control circuit generates a control signal that is provided back to the oscillator. Based upon the frequency and an additional signal functionally related to the control signal, the processor provides an output signal that prevents the device from discharging when the material proximate the coil is human flesh.

Apparatus and method for restricting the discharge of fasteners from a tool

The lipids of biological membranes and intact biomembranes display chain melting transitions close to temperatures of physiological interest. During this transition the heat capacity, volume and area compressibilities, and relaxation times all reach maxima. Compressibilities are thus nonlinear functions of temperature and pressure in the vicinity of the melting transition, and we show that this feature leads to the possibility of soliton propagation in such membranes. In particular, if the membrane state is above the melting transition solitons will involve changes in lipid state. We discuss solitons in the context of several striking properties of nerve membranes under the influence of the action potential, including mechanical dislocations and temperature changes.

https://www.pnas.org/content/pnas/102/28/9790.full.pdf

On soliton propagation in biomembranes and nerves.

Magnetic fields from neuronal action potentials (APs) pass largely unperturbed through biological tissue, allowing magnetic measurements of AP dynamics to be performed extracellularly or even outside intact organisms. To date, however, magnetic techniques for sensing neuronal activity have either operated at the macroscale with coarse spatial and/or temporal resolution-e.g., magnetic resonance imaging methods and magnetoencephalography-or been restricted to biophysics studies of excised neurons probed with cryogenic or bulky detectors that do not provide single-neuron spatial resolution and are not scalable to functional networks or intact organisms. Here, we show that AP magnetic sensing can be realized with both single-neuron sensitivity and intact organism applicability using optically probed nitrogen-vacancy (NV) quantum defects in diamond, operated under ambient conditions and with the NV diamond sensor in close proximity (∼10 µm) to the biological sample. We demonstrate this method for excised single neurons from marine worm and squid, and then exterior to intact, optically opaque marine worms for extended periods and with no observed adverse effect on the animal. NV diamond magnetometry is noninvasive and label-free and does not cause photodamage. The method provides precise measurement of AP waveforms from individual neurons, as well as magnetic field correlates of the AP conduction velocity, and directly determines the AP propagation direction through the inherent sensitivity of NVs to the associated AP magnetic field vector.

https://www.pnas.org/content/pnas/113/49/14133.full.pdf

Optical magnetic detection of single-neuron action potentials using quantum defects in diamond.

The magnetic field of the action potential from an isolated frog sciatic nerve was measured by a SQUID magnetometer with a novel room-temperature pickup coil. The 1.2 x 10(-10) tesla field was measured 1.3 millimeters from the nerve with a signal-to-noise ratio of 40 to 1.

http://science.sciencemag.org/content/sci/208/4439/53.full.pdf

Magnetic field of a nerve impulse: first measurements

The effects of steady magnetic fields upon the conduction of a nerve impulse are considered. Microscopic chemical effects as well as those due to induced fields are ignored. Arguing from an analysis of transport properties, we show that the minimum magnetic field required to produce observable effects is quite large. For the best case, the rield required to produce a 10 percent reduction in conductivity is roughly 24 T.

An Estimate of the Steady Magnetic Field Strength Required to Influence Nerve Conduction

Possible sources of new information in the magnetocardiogram.

A method and apparatus for measuring magnetic fields and electric current flow in biological systems and other systems employing a room temperature pick-up probe connected to a superconducting quantum interference device (SQUID).

Method and apparatus for measuring magnetic fields and electrical currents in biological and other systems

Records of the magnetic field associated with the propagating action potential in a frog sciatic nerve have been obtained. Peak fields of about 60 picotesla are measured by a system utilizing a SQUID magnetometer, signal averaging, and a novel ferrite‐core toroidal transformer. Design considerations for the latter are given and, for example, indicate that it need not have a large number of turns. A method is presented to obtain the net electric current threading the toroid by deconvolution of the magnetometer signal. This routine allows detailed magnetic or current profiles to be obtained of compound neural events.

J. P. Barach

Papers

Magnetic field of a nerve impulse: first measurements

An Estimate of the Steady Magnetic Field Strength Required to Influence Nerve Conduction

Possible sources of new information in the magnetocardiogram.

Method and apparatus for measuring magnetic fields and electrical currents in biological and other systems

Experiments on the magnetic field of nerve action potentials