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last update: 2004 Dec 22

Brain Magnetic Fields

In the human brain, there are more than several hundreds millions of neurons connecting each other and working for information processing. In these neurons ion currents flow while these neurons are involved in the information processing. The ion current goes out of neurons, and flows in the conductive brain. The extracellular current is called volume current or secondary current while the intracellular current, converted from the chemical energy stored in the cell, is called impressed current or primary current. The extracellular current leaks out through much less conductive skull to the scalp, producing the potential difference between two points on the scalp as known to be brain electric potential (electroencephalogram or EEG). Thus, the electroencephalogram is based on the leaked extracellular current rather than the intracellular current.

The ion current produces the magnetic field according to Amp_re's cork screw rule. This magnetic field emerges out of the head through the brain, the skull and the scalp without receiving any distortion because the permeability shows almost no difference between these tissues and the air outside the head. Suppose that the brain is the sphere core, that the skull is the sphere crust covering the brain core, and that the scalp is the second sphere crust covering the skull crust. Moreover, suppose that the conductivity in each portion (brain, skull and scalp) is uniform. The following facts are easily derived;

1. Outside the sphere or crust, the radial impressed current in the sphere (core or crust) does not produce any magnetic field normal to the sphere surface.

2. Outside the sphere or crust, the tangential impressed current in the sphere produces magnetic fields normal to the sphere surface.

3. Outside the sphere or crust, the volume current in the sphere does not contribute to the magnetic field normal to the sphere surface.

The above facts implicate that the magnetic fields normal to the scalp is fundamentally based on the intracellular impressed current tangential to the sphere surface. In the analysis, however, the volume current is also taken into account because measuring the strictly normal component of the brain magnetic field is quite difficult. In any case, the brain magnetic fields strongly reflect the intracellular currents.

The brain magnetic fields are so weak, one hundred millionth to one billionth the strength of the earth's magnetic field, that they can be detected only by very sensitive sensor called SQUID (superconducting quantum interference device) at minus 270 degrees C. Outside the head, even this device is not able to detect the magnetic field produced by activity in one neuron. As a matter of fact, activities usually occur not in only one neuron but in a group of neurons, making it possible to record the brain magnetic fields noninvasively outside the head by a sensitive SQUID system.

The method of measuring and analyzing brain magnetic fields is called magnetoencephalography (MEG) and the recorded data are called magnetoencephalogram (MEG). By analyzing MEG data directly evoked by visual and/or auditory stimuli given to the human subjects and/or by an adequate cognitive task, one can see where in the brain is activated and/or how the brain works as a function of time with millisecond resolution.

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Figure: Revealing Brain Mechanisms by MEG

The cortex of the human brain (A) occupies the surface of the brain as shown in (B). In the cortex, a number of dendrites are uniformly arranged, directing normal to the cortex surface, as shown in (C) and (D). The intracellular currents in these dendrites produce magnetic fields detectable outside the head by SQUID; an example of contour map produced by the pinpoint neuron current (dipole current) is described in (E). Nowadays, a whole-cortex neuromagnetometer or a SQUID system covering a whole-brain is available as shown in (F).

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Figure: Analysis of Auditory Evoked Brain Magnetic Fields

A coil configuration example of the neuromagnetometer (Neuromag-122) is shown in (A), where a blue square indicates the coil location. Magnetic waveforms responding to a 1000Hz brief pure tone are depicted in (B), showing a large deflection about 100ms after the stimulus tone onset. The contour maps at 130ms are shown in (C) over the left and right hemispheres. The equivalent current dipoles are localized in the temporal cortex known as the auditory area. The estimation accuracy is several millimeters in location.

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Figure: Analysis of Visual Evoked Brain Magnetic Fields

Contour maps over the occipital cortex responding to the random line pattern are described in (A), where the magnetic flux comes out of the head in the red-line area and it goes into the head in the green-line area. The equivalent current dipole is localized in the occipital cortex as shown in (B), where the red circle indicates the dipole location and the short line the current direction. The change in equivalent current dipoles are able to be localized in every millisecond.