An introduction to MEG Lecture 1 Matt Brookes

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

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

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

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

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

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

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 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…

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?

The MEG forward and inverse problems An introduction to MEG Lecture 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

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

Inverse Solution fMRI MEG

Inverse Solution

Detectable neuromagnetic effects An introduction to MEG Lecture 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.

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

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

Neuromagnetic responses to visual stimulation