6. MEG

Question 1

What does SQUID stand for?

Answer 1

Superconducting Quantum Interference Device

Question 2

Electrical current flow in a wire produces a magnetic field _____ to the current flow…

Answer 2

perpendicular

• Think of the right-hand rule where thumb points in the direction of the current flow and the fingers wrapping around the wire determine the direction of the magnetic field

Question 3

Which neurons does MEG focus on?

Answer 3

Pyramidal neurons (they are arranged in columns in the neocortex)

Question 4

What are radial fields and what is one issue with detecting them?

Answer 4

they are magnetic signals produced by neuronal activity at the top of the gyri

• they are less detectable by MEG

Question 5

What are tangential fields?

Answer 5

they are magnetic signals produced by neuronal activity in the sulci

• they are detectable by MEG

Question 6

What are the potentials we primarily measure with MEG and how do they differ to the ones we measure with EEG?

Answer 6

Post-synaptic potentials (these can be excitatory or inhibitory) DON’T CONFUSE WITH ACTION POTENTIALS

• When an impulse arrives at a synapse from an activated presynaptic neuron, a neurotransmitter is released causing channels in the membrane of the postsynaptic neuron to open creating a shift in the resting membrane polarization.

This polarization leads to activation or inhibition of voltage-gated channels responsible which cause action potentials

Question 7

In MEG we measure the strength of the ______ (___)

Answer 7

magnetic field (in Tesla)

• aka magnetic flux density

Question 8

What is magnetic permeability (μ0)?

Answer 8

How much a material magnetizes if we apply a magnetic field to it

Question 9

Ferromagnetic materials

Answer 9

Retain magnetism after being magnetised (really bad around fMRI scanners)

Question 10

Paramagnetic materials

Answer 10

Amplify magnetic fields but do not stay magnetized (make magnetic fields bigger when passing through them)

Question 11

Diamagnetic materials

Answer 11

Weaken applied fields (repulse them, will do anything to drive a magnetic field to 0)

Question 12

How is MEG cooled?

Answer 12

liquid helium

Question 13

What are superconductors?

Answer 13

Materials which lose all of their electrical resistance when cooled to a low enough temperature

• So, if we put a current into a superconducting loop, it will continue infinitely (as long as we maintain the low temperature)
• In reverse, applying a magnetic field to a superconducting loop will induce a current to cancel out the field

SQUIDS in MEG use such superconductors (~4K/ -269.15C)

Question 14

Magnetometers…

Answer 14

measure the magnetic field (a coil with wrapped one way in a circle - 1 loop)

• very prone to background noise
• more sensitive to deeper parts of the brain

Question 15

Gradiometers…

Answer 15

Measure the magnetic field (a coil wrapped into two loops going in opposite directions)

• more of a noise resistant measurement
• comes at a cost of being less sensitive to deeper parts of the brain

Question 16

How does MEG work?

Answer 16

Magnetic signal present during post-synaptic potentials induces a current in the pickup coil.

This current is transferred to another coil located just under the squids where it is turned back into a magnetic field which can be measured by the SQUID

• this mechanism is known as a flux transformer

A current is put into the SQUID and we measure how much current we have to inject to cancel a magnetic field

(From our POV what we’re measuring is the magnetic field next to the person’s head)

Question 17

What is the approx MEG sampling rate? (At York)

Answer 17

~1000Hz (1000 times per second)

measurements down to 1ms

Question 18

Physiological MEG artifacts

Answer 18

• Cardiac
• Breathing
• Eye movements
• Muscle movements

Question 19

Non-physiological MEG artifacts

Answer 19

• Magnetic material in the room
• Cars
• Electronics
• Other magnetic “noise”
• Poorly installed equipment

Question 20

What is a butterfly plot? (MEG)

Answer 20

Shows magnetic flux/field vs time for all of the channels

Question 21

What is a sample period? (MEG)

Answer 21

amount of time between samples (1 / sample frequency (Hz))

More generally:
Duration = 1/frequency

Question 22

What is a nyquist frequency?

Answer 22

Maximum theoretical frequency we can see

in theory: sample rate / 2

• in reality, we limit the signals to a maximum of 200Hz (this is know as our badwidth)

Question 23

What is frequency resolution?

Answer 23

depends on how much data we have, we can tell our signal granularity

Calculated as: 1/(data length in s)

e.g. 1/10 = 0.1Hz (we can tell differences that are 0.1Hz apart)

the more data the better the resolution

Question 24

“high” Gamma

Answer 24

70-150Hz:

• cog processing

Question 25

very high freq oscillations

Answer 25

>150Hz: - low level perceptual processing / epilepsy

Question 26

Distance between the "peaks" of the dipole pattern (in MEG) gives a clue as to...

Answer 26

How deep the dipole is - Dipole strength drops off as 1/r (cubed) with increasing depth meaning that deeper dipoles will have a weaker magnetic signal HOWEVER - deep focused source will give the same/v similar signal as a more distributed shallow source

Question 27

What is an Equivalent Current dipole (ECD)

Answer 27

A summation of currents of many neurons with the same positive-negative direction can be mimicked as one strong dipole. They are comprised of an: - orientation - location - strength (Am)

Question 28

What is the unit of an ECD (equivalent current dipole)?

Answer 28

Am (Ampere-metres; i.e. current) Strength of ECD tells us how much current is flowing

Question 29

From an ECD (equivalent current dipole), we can predict...

Answer 29

what the MEG sensors would show This is the FORWARD model: - going from a description of an electrical source (orientation, location, strength) in the head and we predict what it would look like on the MEG sensors

Question 30

(MEG) evoked (think Event) responses are also known as...

Answer 30

phase-locked responses (or ERF - evoked response fields) They are the same every time, you can get a few responses and average them starting from the trigger (Like ERPs in EEG)

Question 31

(MEG) induced responses are also known as...

Answer 31

Non-phase-locked responses Unlike ERFs (Evoked response fields - equivalent of ERPs from EEG), these can be jittery and misaligned, thus, cannot be averaged Analysis involves averaging the frequency domain representation: - create freq(y) time(x) plots for each trial and average them out

Question 32

Phase-locked (____) responses are a ____ part of the overall MEG signal

Answer 32

(Evoked: think Event), small Total response = phase locked (evoked) + non-phase-locked (induced)

Question 33

What does sensor space analysis mean? (MEG)

Answer 33

Looking at the data on the sensors

Question 34

What is source space analysis? (MEG)

Answer 34

Trying to work out what is happening at a particular location in the brain based on what we observed outside - Where is something happening - when is it happening - what exactly is happening This is known as the "INVERSE PROBLEM" - opposite of the forward model (I.e. inferring from a magnetic field what is happening in the neurons)

Question 35

Dipole modelling is...

Answer 35

the simplest form of inverse problem solution which involves fitting a single dipolar source - most suitable for simple responses hypothesised to come from a single neuronal source (single location of grouped neurons)

Question 36

Minimum Norm estimates is a form of ... solution

Answer 36

inverse problem - Like dipole modelling, attempts to explain all of the data on the MEG sensors (at selected times) with a single solution

Question 37

What is Beamforming

Answer 37

MEG scanning technique - Takes each source (scans brain) location in turn and estimates activity at that point - for a given point in space a beamformer is a SPATIAL FILTER which attempts to focus on particular spatial location and attenuate (reduce) responses from other locations !Simply a weighted sum of sensor activity we see - once we have the weights for each "location" in the brain, we can produce a "virtual electrode"

Question 38

What is a "virtual electrode" in MEG?

Answer 38

An estimate of the current (in Am) flow at a point in the brain

Question 39

What is good about MEG?

Answer 39

- It is a measure directly coupled to brain activity (not via haemodynamics) - excellent temporal resolution - Doesn't suffer from spatial "smearing" of EEG, signals are much more focal (magnetic signal doesn't smear because of various tissues it travels through) - Can ask interesting questions about connectivity and networks beyond just "where is this happening". How do different areas work together(induced and evoked responses)?

Question 40

Individual MEG sensors can see ___ but not ____ radial sources

Answer 40

tangential, radial

Question 41

Whether an MEG channel is a magnetometer or gradiometer setup is determined by:

Answer 41

The configuration of the pickup (sensor) coils

Question 42

Minimum norm is a _____ analysis method

Answer 42

source-space

Question 43

(EEG) Evoked activity is analysed by...

Answer 43

taking a direct average of the time-series MEG data.