What are we measuring with EEG and MEG

James Kilner

Outline

• 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?

EEG MEG

The measurements

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).

"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

A little background - EEG

“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

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.

A little background - MEG

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

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.

SQUIDS

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.

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.

SQUIDS

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

Sensors (Pick up coil)

SQUIDs

Inside the MEG dewar

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.

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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+

Sink

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.