An introduction to MEGLecture 1

Matt Brookes

What is Magnetoencephalography?

Cellular currents produce magnetic fields

Aim of MEG:

To detect these magnetic fields and use them to reconstruct the electrical neuronal activity in the brain

What is Magnetoencephalography?

Head is placed in a helmet surrounded by ~300 field detectors

Spatial topography of the magnetic field measured

Subject

Field Detectors

Dewar filled with liquid

helium

What is Magnetoencephalography?

275 channel MEG scanner at the SPMMRC

Schematic Illustration of a neuron

Neural generation of magnetic fields

Neural generation of magnetic fields

Pyramidal (left) and stellate (right) neurons

Symmetric distribution of dendrites in stellate cells means that the magnetic fields cancel out

Fields in MEG therefore due to pyramidal cells, not stellate cells

Post synaptic currents

• Caused by chemical interaction at a synapse

• Termination of an action potential from pre-synaptic cell causes neurotransmitter release

• Neurotransmitter causes opening of ion channels on post synaptic cell wall

• Ions rush into the cell and pass down the dendrites towards the cell body

• Result – Dendritic current

• Whole process lasts a few tens of milliseconds

Neural generation of magnetic fields

Axonal currents

• Dendritic currents from excitatory synapses increase electrical potential at the cell body

• When potential at the axon hillock reaches a threshold value (~ -40mV), an action potential is sent down the axon

• Axon is insulated with a myelin sheath

• Action potential mediated by leading edge of depolarisation

• Time scale of an action potential is ~1ms

Neural generation of magnetic fields

Dendritic current / post synaptic potential Action potential

31

cetanDis 2tan1

ceDis

Acts as a current dipole

Dipole moment ~25fAm

Magnetic fields falls off as…

Acts as two back to back current dipoles each with magnitude ~100fAm

But magnetic fields falls off as…

Neural generation of magnetic fields

The forward problem

Given a known current distribution in the brain, can we compute the magnetic field distribution outside the brain?

The inverse problem

Given a known magnetic field distribution outside the head, can we compute the current distribution in the brain?

An introduction to MEGLecture 2

The MEG forward and inverse problems

Radial Dipoles

Actual detection probability for a whole head (151 channel) MEG scanner. Notice that radial dipoles cannot be detected, however a large percentage of the cortex is detectable.

Dipolar field patterns

Left – measured dipolar field pattern representing the neuromagnetic response to a somatosensory stimulus

Right – schematic showing dipolar magnetic field

Dipolar field patterns

Measured magnetic fields in response to an auditory stimulus

Inverse Solution

fMRI MEG

Inverse Solution

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An introduction to MEGLecture 3

Detectable neuromagnetic effects

Brain rhythms

Hans Berger – 1929 – Discovered that electrical potentials can be recorded from the scalp surface.

These potentials are directly reflective of current flow in neurons in the cerebral cortex

Discovered the alpha rhythm

Brain rhythms

Name Frequency range

Description

Delta < 4 Hz Slowest of all spontaneous brain activity, the delta rhythm is most prominent in deep sleep.

Theta 4 – 8 Hz As with the delta rhythm, spontaneous activity in the theta band is also associated with sleep.

Alpha 8 – 13 Hz Most prominent in awake and relaxed subjects, alpha waves are blocked by visual or somatosensory stimulation.

Beta 13 – 30 Hz Beta activity is often associated with the motor cortex and is thought to reflect active cortical processing.

Gamma 30 – 100 Hz Gamma activity is often associated with the visual cortex and is thought to represent active cortical processing.

Two types of MEG signal

•Time-locked and Phase-locked evoked responses

•Time-locked and non-phase-locked induced oscillatory responses

STIM REST STIMREST REST

STIM REST STIMREST REST

Induced and evoked effects

7T BOLD

T>6

3T BOLD

T>5.5

β-band ERS (15-30Hz) Ŧ>1.2

VEP Ŧ>5

γ-band ERS (60-80Hz) Ŧ>4

β-band ERD (15-30Hz) Ŧ>1.2

Neuromagnetic responses to visual stimulation

Neuromagnetic responses to visual stimulation

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Hilbert Transform of VE timecourse from peak of gamma 60-80Hz Subj2