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Psychology - CN > Lecture 6 - MEG > Flashcards

Lecture 6 - MEG Flashcards

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1
Q

Who used the first SQUID?

A

Cohen (1972)

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2
Q

What does SQUID stand for?

A

Superconducting Quantum Interference Device

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3
Q

MEG signal is ____ to EEG

A

perpendicular

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4
Q

What contributes to the MEG signal?

- PSPs, APs, or secondary volume currents?

A

post synaptic potentials

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5
Q

Which is faster, APs or PSPs?

A

APs

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6
Q

Where do radial fields originate? Do they contribute to the MEG signal?

A

From the top of gyri

Not really

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7
Q

Where do tangential fields originate? Do they contribute?

A

Folds in sulci, therefore magnetic field is perpendicular to sensors

Detectable by MEG

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8
Q

What unit of measurement is detected in MEG

A

B

Magnetic Flux Density / Induction (Tesla)

Tesla= [Wb/m2])

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9
Q

When trying to detect small magnetic fields, what two conditions are requires?

A

Sensitive detectors (SQUIDs)

Low noise measurement environment (MSR)

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10
Q

What are SQUIDS and how do they work?

A

Superconducting Quantum Interference Device (SQUIDs)

Very sensitive detector of magnetic flux

Cooled using liquid helium

Most modern MEG systems use DC-SQUIDS

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11
Q

Three configurations for signal (pickup) coils?

A

Magnetometer
Planer gradiometer
Axial Gradiometer

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12
Q

Magnetometers can see ____ in the brain

Gradiometers are regarded as ____

A

deep

shortsighted

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13
Q

Pros and cons of magnometers

A

can see deeper into the brain

However, they pickup more noise

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14
Q

How are SQUIDs and pickup coils connected?

A

Using a flux transformer

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15
Q

What are larger, SQUIDS or pickup coils?

A

pickup coils

SQUIDS are only 1mm3 ish

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16
Q

Examples of physiological artifacts?

A

Cardiac
Breathing
Eye movements
Muscle movements

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17
Q

Examples of non-physiological artifacts?

A

Magnetic material in the room
Cars
Electrics

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18
Q

Methods of artifact removal?

A

ICA
Epoch/Data rejection
EOG/ECG can provide information

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19
Q

MEG ___-___ data can be selected and averaged in a similar way to EEG data

A

sensor-space

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20
Q

Can choose which data points in sensor-space to include based on ____ information

A

trigger

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21
Q

What is the maximum frequency we can sample?

A

Sample rate/2
MEG systems often sample at 678.17Hz, so /2 = 339Hz
This is the Nyquist frequency

in reality, signals are limited to a maximum of 200Hz

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22
Q

Frequency resolution is determined by what?

A 
The amount (s) of data we have 
Calculated as 1/(data length in seconds)
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23
Q

Time-domain representation of a waveform can be turned into a frequency-domain representation by what tranformation?

A

Fourier transformation

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24
Q

Time and frequency-domain representation can be transformed into what combined view?

A

time-frequency representation

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25
Q

What is oscillatory activity thought to arise from?

A

Assembles of neurones acting together

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26
Q

What do the red and blue parts of an equivalent current dipole?

A

Red shows fields coming “out” of the brain, blue shows fields going back “into” the brain

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27
Q

The source of activity in equivelent current dipoles is most likely found at what point?

A

the 0-point between dipole peaks

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28
Q

The distance between “peaks” in a dipole pattern gives a clue as how ___ the dipole is

A

deep

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29
Q

Dipole strength drops off as a ___/___ with increasing depth

A

1/r^3

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30
Q

A focal deep source and distributed superficial source will produce ___ magnetic fields

A

similar

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31
Q

ECD is composed of…

A

A position in the head
An orientation
A strength

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32
Q

What is the unit of an ECD?

A

Am (ampere-meters i.e. current)

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33
Q

From an ECD, we can predict what the MEG sensors would show. This is called the ____ model.

A

forward

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34
Q

An example of phase-locked response is…

A

Evoked responses to stimuli

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35
Q

ERP in MEG is known as…

A

ERF - evoked response fields

36
Q

Analysis of phase-locked responses involves…

A

averaging the time representation

37
Q

An example of a non-phased-locked response is…

A

Induced responses

38
Q

Analysis of non-phase-locked responses involves…

A

Averaging the frequency domain representation

Changes in power are not time locked and may be jittered

39
Q

Primary sensory responses tend to have more interesting ___-___ components, where as higher level functions and inter-area communications tend to involve ______-___ activity.

A

phase-locked

non phase-locked activity

40
Q

What is an inverse problem?

A

When we attempt to make inferences about a system from a set of observations

41
Q

In regards to the inverse problem in MEG, the things we know are…?

A

MEG sensor measurments and their positions

Structural information about the brain

Knowledge of how electrical currents relate to magnetic fields (via approximations of Maxwell’s equations)

42
Q

Von Helmholtz (1853) showed there is ___ unique solution to explaining a set of magnetic fields outside a conducting sphere

A

NO

The problem is “ill-posed”

43
Q

To “solve” the inverse problem, we need to apply ___

A

constraints

44
Q

There are two main groups of constraint based inverse-problem solutions…

A

Fitting/Localization methods (Dipole Fitting)

Scanning or Imaging Methods (Minimum Norm Estimates; Beamforming)

45
Q

To solve the inverse problem, we can model the ___ problem

A

Forward problem

46
Q

The forward model describes…

A

Given a set of currents in the brain (position, orientation, strength), what would we expect to see on the sensors?

Most models use an ECD model for this.

47
Q

When forward modelling, we need to model the brain. What are some brain models used?

A

Single Sphere
Multiple spheres

Some models can tesselate:

Boundary Element Models (BEM)
Finite Element Models (FEM)

48
Q

After modelling the brain and ECDs in the forward model, we get a ____. This shows what the ___ ___ would ‘see’ for a given ____ at each ___ and ___ in the brain.

A 
Leadfield 
MEG Sensors 
dipole 
location 
orientation
49
Q

Simplest form of inverse solution is to fit a ___ ___ ___

A

single dipolar source

50
Q

Single dipole source (inverse solution) minimises the ____ ___ between ____ pattern and ____ pattern on the sensors. This is known as ____-____ minimisation.

A

Squared errors
predicted
observed
least-squares minimisation

51
Q

Although single dipole fitting is the simplest form of the technique, what are two alternative and more complex methods?

A

Multiple dipole fitting

Spatio-temporal dipole fitting

52
Q

What does the algorithm RAP-MUSIC do?

A

Estimates how many dipoles should be fitted

53
Q

Do you need to decide the number of dipoles to fit a-priori ?

A

yes

54
Q

Minimum norm estimate?

A

Similar to dipole fitting, however, finds the most likely distributed source rather than a single point.

55
Q

Beamforming

A

Does not try and minimise the error with respect to the MEG data for all source location at one

Takes each source location in turn and estimates activity at that point

Closely linked to radar

56
Q

In relation to beamforming, what does the spatial filter do?

A

Attempts to focus on a particular spatial location. The filter tries to attenuate responses from other locations.

57
Q

In beamforming, once we have the weights for each location in the brain, what can we produce?

A

A virtual electrode

58
Q

What is a virtual electrode time series? What are the units?

A

An estimate of current moment at each “voxel” in the brain

VE units are the same as ECDs: Am

59
Q

Methods to combat multiple comparisons?

A

Bonferroni Correction
Non-parametric
Random Field Theory

60
Q

Advantages of MEG

A

Doesn’t rely on indirect measure of brain activity (e.g. haemodynamics)

Good temporal resolution

Doesn’t suffer from same spatial smearing of EEG

Good for network connectivity

61
Q

Disadvantages of MEG

A

Complex analysis

Poorer resolution than fMRI

Expensive

62
Q

Working in source space involves solving the ____ problem

A

inverse

63
Q

What neural activity is thought to be the primary origin of the responses recorded in MEG?

A

Postsynaptic electrical activity from Pyramidal Cells

64
Q

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

A

The configuration of the pickup (sensor) coils

65
Q

What is the full name for a SQUID

A

superconducting quantum interference device

66
Q

Individual MEG sensors can see ___ but not ___ sources

A

tangential

radial

67
Q

What is the Nyquist freq?

A

sample rate / 2

68
Q

Minimum norm is a ____ analysis method

A

Soure-space

69
Q

Evoked activity is analysed by _____ of the time-series MEG data

A

taking a direct average

70
Q

What did Josephson do?

A

Describes Josephson Junctions

71
Q

Who recorded alpha rhythms using an induction coil?

A

Cohen (1968)

72
Q

Electrical current flow (I) in a wire
produces a magnetic field (B)
_____ to the current flow

A

perpendicular

73
Q

Why do secondary volume currents not really contribute to MEG signal?

A

Tend to cancel out

74
Q

Ferromagnetic?

A

Retain magnetism after being exposed

75
Q

Paramagnetic?

A

Amplify magnetic fields but do not stay magnetized

76
Q

Diamagnetic?

A

Weaken applied fields

77
Q

What is magnetic permeability?

A

How much does a material magnetize

when exposed to an external magnetic field?

78
Q

Superconductors are perfect ___

A

diamagnets

79
Q

MEG signal acquisition overview?

A
  1. Magnetic fields pass through the pickup coil
    • Either magnetometer or gradiometer configuration
  2. A current is induced in the pickup coil, and via the flux
    transformer, it is coupled to the coil next to the SQUID
  3. The measurement of this current tells us about the
    magnetic field
80
Q

Reference channels are automatically used to help ___ the data and we normally ___ ___ for data analysis purposes

A

‘de-noise’

ignore them

81
Q

What is a butterfly plot?

A

Shows magnetic flux/field VS time for all the channels

82
Q

‘high’ gamma?

A

70-150Hz - cognitive processing

83
Q

Very high frequency oscillations?

A

150Hz+

Low level perceptual processing/epilepsy

84
Q

Need ABOUT ____ neurons to detect signal (estimation ranges)

A

50,000

85
Q

How much space do 50,000 neurons take up? ABOUT

A

50,000 neurons would need a patch about 0.63mm2 in area

86
Q

The spatial resolution of MEG is _ ____ ____

A

a complex question

87
Q

Evoked activity is analysed by taking a ____ of the ____ MEG data.

A

direct average of the time-series data

Psychology - CN (9 decks)

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