EEG and MEG

Question 1

Define EEG (encetoencephalography)

Answer 1

A way of measuring small voltage changes at the scalp caused by electrical currents from neurons. There can be between 32 and 128 electrodes placed on the scalp to measure these (sometimes less in clinical settings).

Question 2

What is the typical size of an EEG signal?

Answer 2

A few µV

Question 3

Define MEG (magnetoencephalography)

Answer 3

A way of detecting small magnetic fields induced by the same electric currents as EEG (from neurons). Superconducting Quantum Interference Devices (SQUIDs) are used to measure these. An MEG helmet has ~300 sensors that are radially oriented.

Question 4

What is the typical size of an MEG signal?

Answer 4

~ 10⁻¹⁵ T

Question 5

Both EEG and MEG are based on ________ activity and are a direct measure of ______ ________.

Answer 5

Electrical
Brain activity

Question 6

Describe the temporal resolution of EEG and MEG

Answer 6

Both have excellent temporal resolution: < 1ms

Question 7

Describe the temporal resolution of fMRI

Answer 7

Very poor temporal resolution: ~5 s

Question 8

Describe the spatial resolution of EEG and MEG

Answer 8

Both have poor spatial resolution.
EEG: ~20-30 mm
MEG: ~5 mm

Question 9

Describe the spatial resolution of fMRI

Answer 9

Very good spatial resolution: ~1 mm

Question 10

Compare the cost of EEG, MEG, and fMRI

Answer 10

EEG is the cheapest at £20,000.
fMRI and MEG are more expensive at £1,000,000.

Question 11

What are the 3 main categories of neuro-electrical effects recorded by EEG and MEG?

Answer 11

1. Spontaneous rhythms
2. Evoked effects
3. Induced effects

Question 12

What are spontaneous rhythms?

Answer 12

Naturally occurring rhythms that don’t require external stimulation.

Question 13

What is the alpha rhythm?

Answer 13

The most easily detected rhythm at ~10 Hz in frequency. It is thought to be a measure of the level of cortical inhibition and is most prominent for resting, awake subjects with their eyes closed.

Question 14

What is the frequency range of the alpha rhythm?

Answer 14

8-13 Hz

Question 15

What is the frequency range of the delta rhythm?

Answer 15

< 4 Hz

Question 16

What is the frequency range of the theta rhythm?

Answer 16

4-8 Hz

Question 17

What is the frequency range of the beta rhythm?

Answer 17

13-30 Hz

Question 18

What is the frequency range of the gamma rhythm?

Answer 18

30-100 Hz

Question 19

What are evoked effects?

Answer 19

A response to an external stimulus. It is averaged across all stimulus repeats because these effects are time-locked and phase-locked to the stimulus and occur shortly after the stimulus onset.

Question 20

What are induced effects?

Answer 20

Stimulus-induced changes in spontaneous rhythms. These are NOT phase-locked to the stimulus onset or offset.

Question 21

How are induced effects recorded so that the result is observable?

Answer 21

The signal is squared to get the power average over repeats because the effects are not phase-locked so averaging across trials without any moderation loses these effects from the signal.

Question 22

Describe the composition of a neuron

Answer 22

A single neuron consists of the cell body, the dendrites, and an axon.

Question 23

What are the two types of neurons?

Answer 23

1. Pyramidal neurons
2. Stellate neurons

Question 24

Describe pyramidal neurons

Answer 24

Neurons whose dendrites are parallel to each other and are perpendicular to the cortical surface, causing current flow to be normal to the cortical surface.

Question 25

Describe stellate neurons

Answer 25

Neurones whose dendrites are symmetrically distributed so there is isotropic current flow and little EM field at a distance.

Question 26

Which type of neuron generates fields that are detectable in EEG and MEG?

Answer 26

Pyramidal neurons

Question 27

Describe the process of generating a post-synaptic current in a neuron

Answer 27

1. Initially (at resting state) there is an ion imbalance across the neuron membrane that gives a resting potential of -70 mV. 2. When an action potential arrives at a neuron from a pre-synaptic cell, it causes neurotransmitters to be released across the synaptic cleft. 3. Ion channels open in the post-synaptic cell, allowing sodium ions (Na+) to rush into the cell and travel down the dendrites. 4. This generates an intracellular current inside the neuron and an equal volume current in the surrounding tissue. The intracellular current is the post-synaptic current.

Question 28

Does EEG detect intracellular (primary) or volume current?

Answer 28

Volume current

Question 29

Does MEG detect intracellular (primary) or volume current?

Answer 29

Primary current

Question 30

The post-synaptic current falls off as ____.

Answer 30

1/r²

Question 31

Post-synaptic potentials change the _______ of the soma.

Answer 31

Voltage

Question 32

How high does the post-synaptic potential have to be to generate an action potential that is sent down the axon?

Answer 32

It must reach -40 mV from -70 mV

Question 33

How fast is an action potential?

Answer 33

~1 ms

Question 34

What are the 2 parts of an action potential?

Answer 34

- Leading edge of depolarisation - Trailing edge of repolarisation

Question 35

An action potential falls off as ____

Answer 35

1/r³

Question 36

Why do MEG and EEG measure the post-synaptic currents from pyramidal neurons rather than action potentials?

Answer 36

Action potentials fall off faster with distance so aren't picked up.

Question 37

Give the equation for the current dipole strength from a dendrite

Answer 37

|Q| = Iλ |Q| = curent dipole strength I = primary current λ = current decay distance

Question 38

What is the typical current decay distance along a dendrite?

Answer 38

~ 0.15 nm

Question 39

Give the equation for neuron resistance per unit length

Answer 39

r_s = resistance per unit length σ = conductivity of neuron d = neuron diameter

Question 40

State one cause of cancelling fields from brain signals

Answer 40

Pyramidal cells are organised radial to the cortical surface so folds in the cortex can cause cancelling fields.

Question 41

What is a scalp topography (field) map?

Answer 41

A map of the EM fields at several points over the scalp.

Question 42

Field maps produced by MEG and EEG are _______ to each other (right-hand rule).

Answer 42

Orthogonal

Question 43

State 3 applications of EEG

Answer 43

- Clinical uses (e.g. epilepsy, sleep staging, and sleep disorders) as cheap and readily available - Research - Brain-computer interfaces

Question 44

State the current application of MEG

Answer 44

- Research into epilepsy and schizophrenia (still on patients but not in the NHS)