1. An introduction to MEG
Lecture 1
Matt Brookes

2. Cellular currents produce magnetic fields
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

3. Dewar filled with liquid helium
What is Magnetoencephalography?
Head is placed in a helmet surrounded by ~300 field detectors
Spatial topography of the magnetic field measured
Field Detectors
Dewar filled with liquid helium
Subject

4. 275 channel MEG scanner at the SPMMRC
What is Magnetoencephalography?
275 channel MEG scanner at the SPMMRC

5. Schematic Illustration of a neuron
Neural generation of magnetic fields
Schematic Illustration of a neuron

6. 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

7. Neural generation of magnetic fields
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

8. 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

9. Neural generation of magnetic fields
Dendritic current / post synaptic potential
Action potential
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…

10. The forward problem
Given a known current distribution in the brain, can we compute the magnetic field distribution outside the brain?

11. The inverse problem
Given a known magnetic field distribution outside the head, can we compute the current distribution in the brain?

12. The MEG forward and inverse problems
An introduction to MEG
Lecture 2
The MEG forward and inverse problems

13. 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.

14. Dipolar field patterns
Left – measured dipolar field pattern representing the neuromagnetic response to a somatosensory stimulus
Right – schematic showing dipolar magnetic field

15. Measured magnetic fields in response to an auditory stimulus
Dipolar field patterns
Measured magnetic fields in response to an auditory stimulus

16. Inverse Solution
fMRI
MEG

17. Inverse Solution

18. Detectable neuromagnetic effects
An introduction to MEG
Lecture 3
Detectable neuromagnetic effects

19. 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

20. 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.

21. Induced and evoked effects
Two types of MEG signal
Time-locked and Phase-locked evoked responses
REST
STIM
REST
STIM
REST
Time-locked and non-phase-locked induced oscillatory responses
REST
STIM
REST
STIM
REST

22. Neuromagnetic responses to visual stimulation
β-band ERS (15-30Hz) Ŧ>1.2
7T BOLD
T>6
β-band ERD (15-30Hz) Ŧ>1.2
3T BOLD
T>5.5
VEP Ŧ>5
γ-band ERS (60-80Hz) Ŧ>4

23. Neuromagnetic responses to visual stimulation