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SRI conducted an experiment to explore possible correlations between CNS activity and remote
stimuli.
This experiment
is the subject of this report.*
B.
Technological Background
Magnetoencephalography is a noninvasive technique used to measure, in three-dimensional
space, magnetic fields produced by neuronal electric currents in the cortex of the brain.
A
magnetoencephalography device (MEG) can determine the spatial distributions of specific groups
of neurons participating in a given activity and their patterns of activity over time.
This technology
has been used in research ranging from evaluating how normal brains process information to
diagnosing clinical conditions such as epilepsy and dementias. 7
Neurons that participate in a given functional activity communicate between themselves and
ultimately other parts of the body by electrical signals.
These signals are produced by a flow of
sodium, chlorine, potassium, and calcium ions traveling from the dendrites down the axon and to
the synaptic buttons of each neuron.
Such neurons may act as a magnetic dipole that produces a
magnetic
field.
The sensing device of a MEG is a cryogenic superconducting quantum interference device
(SQUID) coupled with a gradiometer. SQUIDs currently being used are cooled by liquid helium.
At a few degrees above absolute zero, an electrical current can flow through a superconductor with
no applied voltage.
“The material of the SQUID
consists of superconducting loops with two
sections of thin insulating material connecting them (Jospheson Junctions)
.
This configuration is
referred to as a DC SQUID. Some electrons can tunnel through this insulation. The presence of a
weak magnetic field produces a phase difference for the wave function of the magnetic field [and]
produces a phase difference for the wave function of the electrons across this barrier. The resulting
interference pattern produced by the two different wave functions on each side of the barrier can
be used to indicate the strength of these extremely weak magnetic fields. ”t
The neuronal magnetic fields from the human brain are only about 10 _13
tesla, while the
earth’s magnetic field
is 10'4
tesla and normal urban noise
is about 10’7
tesla.
Care must be
taken, therefore, to assure that the signal-to-noise ratio
is favorable.
This has been taken into
consideration by the manufacturer of MEG equipment (BTi of San Diego, California)
, who has
designed highly shielded sensors that use a second-order coupled gradiometer to reduce the
environmental noise by about 10®.
The use of an aluminum and p.-metal magnetically shielded
room can further reduce the noise by a factor of 10 3
.
If used together, these two precautionary
*
This report constitutes the deliverable for fiscal year 1989 Statement of Work, item 6.0.2.
f
We thank Dr. Edward Flynn, Neuromagnetism Laboratory, Life Science and Physics Division,
Los Alamos National Laboratory, Los Alamos, NewMexico, for providing this explanation.
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Approved For Release 2000/08/08
: CIA-RDP96-00789R002200600001-1
measures can reduce the ambient noise by a factor of about 10 9—equivalent to the internal SQUID
noise.
Because a MEG responds best to neuronal currents that are parallel to the
skull
(i.e.,
currents
producing
magnetic
fields
oriented
tangentially
to
the
skull),
neuronal
currents
perpendicular to the skull may be missed.
In reality, however, few neuronal electrical currents are
exactly perpendicular to the skull, so some tangential component is almost always available to the
SQUID.
Searching for a closely packed group of neurons can be a slow and tedious process. Due to
technological restraints, a maximum of seven sensors can be used simultaneously to gather MEG
measurements. Sensors on a seven-channel MEG are located on a 2-cm equilateral triangular grid
forming the center and vertices of a regular hexagon. A subject wears a spandex cap with grid
marks lined up with his nasion, inion, and earlobes to serve as a head-centered coordinate system.
To identify the location of a neuronal-equivalent current dipole, many measurements have to be
taken.
Isocontour maps of
field
strength
are used to represent the amplitude and
polarity
distribution of the magnetic fields.
A least-squares procedure is applied to the observed fields to
estimate the location of neuronal sources and orientation of the equivalent current dipole. 8
The
estimated location of the neuronal source can then be identified anatomically with a magnetic
resonance image scan of the head.
Developments in technology may soon allow for enough
channels to cover the whole head at once, thereby reducing data collection time and increasing
precision.
MEG technology is based on a cryogenic SQUID operating in liquid helium.
Because the
Dewar flask cannot exceed a 45-degree angle, subjects must
lie prone beneath the apparatus.
MEG sensors are not attached to the head, but are lowered into position over the skull; the subject
cannot move his head during monitoring without disturbing the measurement.
For these two
reasons, MEG equipment is not suited for long-term monitoring of a subject. These problems may
be solved in the near future as new technology, such as high-temperature SQUIDs, develops.
A response from the MEG
is a complex waveform consisting of a series of negative and
positive peaks or components.
Specific components of this waveform can be correlated with
perceptual and cognitive processes. The most commonly observed response to a visual or auditory
stimulus, for example, is a large component occurring approximately 100 ms after the onset of the
stimulus.
One hundred milliseconds appears to be the average latency period between stimulus
and the
first correlated neuronal activation in the brain. 8
The earlier EEG technology measures electric potential, or event-related potentials (ERPs)
produced by the
electrical
activity of the
brain.
A MEG measures the magnetic
fields,
or
event-related fields (ERFs) produced by the electrical activity of specific groups of active neurons
in the cortex. An EEG and a MEG, therefore, reveal different aspects of the electrical activity of
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