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What are we measuring with
EEG and MEG
James Kilner
Outline
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Why use EEG or MEG
What are EEG and MEG
A little history of EEG and MEG
What are we measuring
Why use MEG or EEG?
The measurements
EEG
MEG
A little background - EEG
In 1875, English physician
Richard Caton became the first
to publish what are now known
as the Electroencephalogram
(EEG) and Event-Related
Potentials (ERPs).
A little background - EEG
"The electroencephalogram represents a
continuous curve with continuous
oscillations in which ... one can distinguish
larger first order waves with an average
duration of 90 milliseconds and smaller
second order waves of an average duration
of 35 milliseconds [Beta waves]. The larger
deflections measure at most 150 to 200
microvolts...."
Hans Berger 1929
“Caton has already published experiments on the brains
of dogs and apes in which bare unipolar electrodes were
placed either on the cereral cortex and the other on the
surface of the skull. The currents were measured by a
sensitive galvanometer. There were found distinct
variations in current, which increased during sleep and
with the onset of death strengthened, and after death
became weaker and then completely disappeared. “
Hans Berger 1929
A little background - MEG
In 1963 Gerhard Baule and
Richard McFee of the Department
of Electrical Engineering,Syracuse
University, Syracuse, NY detected
the biomagnetic field projected
from the human heart.
They used two coils, each with 2
million turns of wire, connected to
a sensitive amplifier. The
magnetic flux from the heart will
generate a current in the wire.
Very noisy signal.
Biomagnetic fields are very small compared to urban noise.
In the late 1960’s David Cohen, at
MIT, Boston recorded a clean MCG
in an urban environment. This was
possible due to:
1) Magnetically shielding the
recording room.
2) Improved recording sensitivity.
(The introduction of SQUIDS)
MCG
MEG
SQUIDS
The SQUID (Superconducting Quantum
Interface Device) was introduced by
James Zimmerman in the late 1960s.
It is an ultrasensitive detector of magnetic
flux.
It is made up of a superconducting ring
interrupted by one or two Josephson
Junctions.
It has been demonstrated that fields of
the order of fT are within the scope of a
SQUID.
Brian Josephson, winner of the Nobel
Prize for physics in 1972, demonstrated
in 1962 the existence and very
particular characteristics of the 'tunnel
effect' that can be produced between
two
superconducting
materials
separated by a thin insulating layer.
This is called a Josephson Junction.
A Josephson Junction enables the flow
of electrons – even in the absence of
any external voltage.
SQUIDS
The SQUID has two generators of
current flow.
One a ‘classical’ current flow caused by
the bias current across the SQUID
The second is caused by the Josephson
Junction and flows around the ring.
The magnetic field measured causes a
change in the current flow around the
ring which will effect the overall current
flow across the SQUID.
The sensitivity of the SQUID to magnetic fields
may be enhanced by coupling it to a
superconducting pickup coil having greater area
and number of turns than the SQUID inductor,
alone.
This pickup coil is termed a "flux" transformer".
The pickup coil is made of superconducting
wire and is sensitive to very small changes in
the magnitude of the impinging magnetic flux.
The magnetic fields from the brain causes a
supercurrent to flow.
Magnetometers
First Order
Gradiometer
Inside the MEG dewar
SQUIDs
Sensors
(Pick up coil)
What are we measuring with EEG
and MEG?
Mainly Noise!
Blinks, eye-movements, subject movement,
heartbeat, sweat, electrode movement,
swallowing, muscle activity, breathing,
mains electricity, environmental noise,
system noise …..
and a signal related to neuronal activity.
The main source of the extracranial magnetic
fields is current flow in the long apical dendrites
of the cortical pyramidal cells. NOT action
potentials.
Sink
A distal excitatory synapse will induce a dipolar
dendritic current towards the soma of the
pyramidal cell, meaning that the electricity is
flowing in one direction along the entire length
of the dendrite, which therefore may be
considered an electric dipole.
Pyramidal neurons constitute nearly 70% of
neocortical neurons, and the cells are oriented
with their long apical dendrites perpendicular to
the brain cortex. There are more than 100,000
of these cells per square millimeter of cortex.
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Source
The right hand rule
Unlike EEG, MEG is only sensitive to
tangential components.
However the brain is not a sphere so
all areas will have some tangential
components.
Summary
• MEG and EEG are related measures
• They mainly measure noise!
• The neuronal signal reflects post-synaptic
potentials of thousands of pyramidal cells.
• One is not a better measure than the other
– it will depend on what you are interested
in.