EEG and MEG

Question 1

What is EEG Based on

Answer 1

Small changes of voltage in the scalp that are caused by electrical currents by neurons in the brain.

Question 2

How does EEG detect electrical signals

Answer 2

in order of micro volts
detectable by placing electrodes in electrical contact with scalp

Question 3

What is Magnetoenceplalography based in

Answer 3

Detetction of small magnetic firlds which are induced by electrical current in the brain

Question 4

How are the magnetic fields detectable and how are they feasable

Answer 4

require extremely sesnitive magnetic fields dectors

only feasible with Superconducting Quantum Interference Devices (squid)

Question 5

How many electrodes are placed for EEG
How many coils are placed for MEG

Answer 5

32-128 ELECTRODES
300 radially oriented oick up coils

Question 6

Which has a higher spatial resolution and why

Answer 6

MEG but both have poor spatial resolution due to field cancellations.
But Electric potentials are distorted by inhomogeneous conductivity whereas magnetic is not

Question 7

What effects can be recorded by MEG and EEG

Answer 7

• Spontaneous rhythms
• Phase locked evoked effect
• Time-locked but not phase-locked induced effect

Question 8

Spontaneous rhythms

Answer 8

Spontaneous rhythms (sometimes called brain oscillations or brain waves) occur naturally in the brain and do not require any sensory stimulation of the subject.

Question 9

When is Delta frequenct most prominent?

Answer 9

Slowest of all spontaneous brain activity, the delta rhythm is most prominent in deep sleep.

Question 10

When is Theta frequenct most prominent?

Answer 10

spontaneous activity in the theta band is associated with sleep

spontaneous activity in the theta band is associated with sleep

Question 11

When is Alpha frequenct most prominent?

Answer 11

Most prominent in awake and relaxed subjects, alpha waves are blocked by sensory stimulation

Question 12

When is Beta frequenct most prominent?

Answer 12

motor and visual cortexes

Question 13

When is Gamma frequenct most prominent?

Answer 13

active processing of information in the cortex

Question 14

What is Evoked Effects

Answer 14

measured in response to the presentation of some external stimulus to the subject.

Question 15

How is the evoked effect in an experiment

Answer 15

present the same stimulus to subject a large number of times
Recorded MEG AND EEG THEN AVERAGED

Question 16

What is the Induced Effect

Answer 16

to stimulus-induced changes in spontaneous rhythms.

Question 17

Difference between induced and evoked effect

Answer 17

Induced are not phase-locked to stimulus so averaging would cause loss of induced effect.

Question 18

How must induced effect be calculated

Answer 18

EM signal misy be squared to obtain oscillatory power in frequency band

Averaging power across trials will result in observable induced effects.

Question 19

What are dendrites

Answer 19

Extention of soma which receive stimuli from other cells

Question 20

What are Pyramidal neurons

Answer 20

Dendrites orientated parallel to each other and perpendicular to the cortical surface.

Question 21

What are Stellate neurons

Answer 21

Dendrites are symmetrically distributed around the soma.

Question 22

How does current flow in neurons in the braon

Answer 22

In pyramidal neurons - Nirmal to cortical surface

In stellate neurons - isotropics - produces little electromagnetic field at a distance

Question 23

What is post synaptic currents

Answer 23

In the resting state, there are imbalances in ion concentrations across neuron

Cells communicate via chemical interaction

Termination of action potential at pre-synaptic cell causes neurotransmitter release across synaptic cell.

Change in local membrane permeability enables sodium ions to rush into the cell and give rise to primary current directed along dendrite of a neuron

Voltage change is post synaptic potentials.

Question 24

What is action potential

Answer 24

the leading edge of depolarisation followed by the trailing edge of repolarisation

Question 25

How is the electrical potential from EEG induced

Answer 25

potentials associated with postsynaptic events and is a measure of volume currents

Question 26

Why is it important for signals to be more sensitive ti signals which are different at different electrodes than those which are the same.

Answer 26

Electrical signals from brain vary from electrode to electrode but electrical signals from noise is generally uniform.

Question 27

Whar is the principle of common mode rejection

Answer 27

signals which are the same at two electrodes are largely attenuated than signals which are different at two electrodes

Question 28

What are EEG recording strategies

Answer 28

EEG measures potential differences between two electrodes connected to opposite ends of differential amplifier Recirding depends on position of individual electrodes and how they're paired

Question 29

What are the two types of EEG recording sstrategies

Answer 29

- Referential / unipolar strategy - Bipolar Strategy

Question 30

What is Unipolar strategy Advantages and Disadvantages

Answer 30

- p.d is determined with resoect to single common electrode ADVANTAGES - comparisons between signal amplitudes recorded at different electrodes can be made - inferences about source of electrical signal can be made as electrode recording is presumed to be nearest sources DISADBANTAHES - Source location is not related to the electrode with max signal amplitude as ref cannot be placed in truly inactive part of brain

Question 31

What is Bipolar strategy Advantages and Disadvantages

Answer 31

- p.d measured between adjancent electrode which are normally paired in a chain - electrodes common to two pairing are connected to possite sides of differential amplifier in each pairing ADVANTAGES - . If the source of the current vector causing the cortical activity is normal to the skull surface then this is an excellent way to locate the source of brain activity. DISADVANTAGES - , if the source is at an oblique angle to the skull then the voltage maxima and the source may be far from one another making source localisation with this information alone impossible.

Question 32

What is the MEG Forward problem

Answer 32

Given a known current distribution in the brain, can we compute the magnetic field distribution outside the head? Solved by Maxwells equations don't do this for eeg due to inhomogeneous conductivity

Question 33

MEG Forward problem; neuronal current approximations

Answer 33

1 - Spherically symmetric conductor approximation - head is modelled as single conducting sphere with homogenous conductivity 2 - Current dipole approximation - at a distance B -field results from large number of synchronised post synaptic current resulting from single current dipole Assumption 1 is reasonable, though more advanced models based on an MRI of a participant are available. Assumption 2 is reasonable provided the region of brain activated is small.

Question 34

Whatis lead fields

Answer 34

N-dimenstional vector containing B-fields measured at each N MEG sensor in response to single current dipole source if unit amplitude ar predetermined locations and orientations in brain