Week 4

Question 1

What is needed for EEG recording?

Answer 1

1. At least one active electrode and the reference electrode
2. Typically, caps have 32-256 active electrodes or channels. EEG is recorded from its active electrode. It is the voltage difference between that electrode and the reference electrode in microvolts.
3. Since the signal is very weak - it needs to be amplified (for ex., by a factor of 20,000) and filtered and digested by a computer
4. Digitisation is a process of converting an analogue signal to a discrete set of samples that can be stored in the computer.

Question 2

What are EEG characteristics?

Answer 2

1. Mixture of rhythmic waves
2. Oscillating at various frequencies
3. Representing a mixture of activity from many neural sources that are active at the same time.

Question 3

What is the 10-20 system?

Answer 3

The international standard, which prescribes the location of each electrode in terms of distance from specific landmarks on the head. Letters underly region, numbers hemisphere, etc.

Question 4

What do the oscillations detected by Eeg reflect?

Answer 4

Neuronal oscillations. These are the changes in the excitability of populations of cortical pyramidal neurons that are synchronously active over time.

Question 5

What are the three scales of the neuronal organisation?

Answer 5

1. The microscopic scale: reflects brain dynamics and single neurons or columns of pyramidal neurons like synaptic events. EEG cannot detect it.
2. The mesoscopic scale: reflects brain dynamics at the level of populations of cortical neurons occupying several cubic millimetres of the cortex. It may be visible by EEG using special analysis techniques.
3. The macroscopic scale: reflects brain dynamics and the level of populations of neurons occupying the large patches of the cortex. Readily visible by EEG.

Question 6

Why can EEG not detect deep brain structure activities?

Answer 6

1. Strength of the field decreases exponentially as a function of distance from the sensor; therefore, even strong fields will be attenuated by the time they reach the electrodes of the scalp.
2. Neurons in subcortical structures are not usually arranged in parallel geometric orientation like pyramidal neurons in the cortex.

Question 7

How can oscillations be described (3)?

Answer 7

1. Frequency: the speed of oscillation, measured in Hertz (numbers of cycles per second).
2. Power of oscillation: the amount of energy in the frequency band, which is a squared amplitude of the oscillation.
3. Phase: position along the sine wave at a given time point.

Power and phase are independent.

Question 8

How can oscillations be separated into different bands?

Answer 8

By applying signal processing techniques.

Question 9

To what changes in rhythmic activity have been linked?

Answer 9

With various perception, cognitive, motor, and emotional processes.

Question 10

What are the advantages of EEG?

Answer 10

1. Best temporal resolution: excellent for neurocognitive processes as it can track changes in specific aspects of brain dynamics almost as they occur.
2. Voltage fluctuations are direct reflections of biophysiological phenomena at the level of populations of neurons.
3. EEG provides multidimensional signals: time, space, frequency, power and phase information. Each of these can provide information about different aspects of the brain.
4. EEG promising tool for translational research on non-humans.

Question 11

What are the disadvantages of EEG?

Answer 11

1. Scores low with respect to functional localisation. That is identifying a specific region for a specific process.
2. Not suited to investigate deep brain structures, that may play a role in cognitive functions.
3. It May not be appropriate to study slower cognitive processes, associated with emotions.
4. Cannot measure individual molecular or synaptic events produced by the specific neurotransmitter, action potentials, or neurons that are not in parallel geometric orientation.

Question 12

What is event-related potential (ERP)?

Answer 12

EEG reflects brain dynamics resulting from both background activity and activity evoked by processing a stimulus or a cognitive event.

Question 13

Two types of task-related activities

Answer 13

1. Phased-locked: phase of the evoked waves is similar in every trial and aligns with time 0.
2. Non-phase locked: phase is different in every trial.

Question 14

How are ERP waveforms typically described?

Answer 14

1. Letter refers to the polarity of the component (N-negative, P-positive).
2. Number refers to the relative order with which the component appears after stimulus onset. Sometimes - latency - how long does it take for the component to appear after an event is experienced.

Question 15

What are the advantages of ERP?

Answer 15

1. Suited to investigate differences in brain processing between two or more conditions.
2. High temporal resolution and accuracy.
3. Relatively easy to compare.
4. Extensive literature on ERPs and associated cognitive processes, facilitating the interpretation of results.

Question 16

What are the disadvantages of ERP?

Answer 16

Averaged ERP waveforms are weighted mixtures of time-phase locked event-related activities from many cortical resources. Therefore:

• activity recorded at each electrode does not necessarily reflect activity at the cortical patch under the particular electrode, but a weighted mixture
• ERPs only represent specific aspects of the brain dynamics, they do not reflect non-phase-locked but task-related brain activity

Question 17

What needs to be done for the EEG signal to be analysed?

Answer 17

1. Preprocess the signal to remove electrical signals which do not originate from the brain.
2. Reference and look at how we can extract neural oscillations using spectral analysis.

Question 18

What are the two sources of electrical noise?

Answer 18

1. Electrical equipment

2. The activity of participants’ eyes and muscles

Question 19

How to deal with noise in EEG?

Answer 19

Minimise contamination at acquisition by:

• Filters to reduce power line noise
• Resistance: keeping the impedances between the electrodes and the scalp low reduces this artefact.

Question 20

Two types of noise produced by eyes?

Answer 20

1. Low-frequency signals. They are caused by the fact that there is a dipole between the cornea and the retina. When eyes are turned, a large signal is produced in frontal EEG electrodes.
2. High-frequency signals

Question 21

How can scalp and muscle tension contaminate the signal?

Answer 21

Unless the participant is fully relaxed - there is tension in the scalp muscles. Activity increases with stress and cognitive tasks. Clenching teeth, neck muscle tensions, frowning.

Question 22

How the signal can be affected by sweating?

Answer 22

Sweat droplets cause sudden dots and changes in impedance, resulting in a large low-frequency swing in the EEG.

Question 23

How can we deal with and clean EEG signal artefacts?

Answer 23

1. Digital filter:
   - Block certain frequencies (50Hz)
   - Mainly used for ERP
   - Never use this when analysing high-frequency signals
2. Regression
   - Record artefacts, then use regression to remove noise.
   - Rg. Place an electrode above and below the eye, then subtract the signal from the recording
3. Independent component analysis (ICA)
   - ICA demixes the EEG into independent components
   - The artefacts are then identified and rejected
   - Be careful about the results reported, in the gamma range using ICA correction with less than 128 channels
4. Mathematical modelling
   - Designed to process many sources of noise in the high frequency
   - it’s a novel method.

Question 24

What is amplitude?

Answer 24

The amplitude of a sine wave is the magnitude of the deflection from zero (mean value).

Question 25

How is the power measured?

Answer 25

The power of a sine wave is the square of the amplitude (P).

Question 26

What is Spectral analysis?

Answer 26

A method that splits EEG signals into a series of sine waves of different frequencies. 2 main methods are Fourier transformation FFTs and Wavelet transforms.

Question 27

How is a spectral analysis used?

Answer 27

1. In the resting state - quantitative EEG (qEEG)
2. Event-related spectral analysis:
   - Induced (not time-locked to stimulus)
   - Evoked (time-locked to stimulus)

Question 28

What does the Fourier transform say?

Answer 28

Any function can be broken down into a series of sine and cosine waves.

Question 29

Advantages of FFTs?

Answer 29

Computationally efficient

Question 30

Disadvantages of FFTs?

Answer 30

• Frequency bands are fixed. They must be the same across all frequencies
• Frequency band is fixed by the length of the segment cut for the FT. the number of the data points must be squared2.

Question 31

What is a Wavelet analysis?

Answer 31

Involves convolution between the EEG signal and wavelets of different scales. Convolution means that you multiply together point by point the wavelet and the signal of interest and then sum up all resulting numbers - the result is a measure of the similarity of the wavelet and the EEG.