EEG

Question 1

EEG

Answer 1

• a method of detecting neural activity by placing electrodes on the scalp
• these electrodes pick up small fluctuations of electrical signals originating from activity of neurons

Question 2

Hans Berger

Answer 2

• detected the first EEG signal in 1924 with electrodes attached to the scalp of the patient reported the results in 1929.
• Burger initially used two electrodes- silver wires placed under the scalp. One attached to the front of the head and one to the rear, and recorded the potential differences between them
• he first described the alpha rhythm- when people closed their eyes, the frequency 8-13 Hz.

Question 3

EEG process

Answer 3

• electrode gel is applied to the gap between the scalp and electrode to decrease the impedance (get a better signal.
• typical systems have 32, 64, 128 or even 256 head channels.
• we need a ground and reference electrode to use other electrodes to measure eye movement and blinks.

Question 4

signals

Answer 4

the typical sample frequency is between 256-1024 Hz
- the signal is band-pass filtered to remove the low and high frequencies because they cannot reflect brain activity
- the signal is also notch filtered to remove the line noise, which is also not brain activity.

Question 5

artefacts

Answer 5

• different kinds of artefacts can contaminate the signal and need to be removed
• some can be detected automatically, and some need to be identified manually

Question 6

eye movements and blinks

Answer 6

• create strong artefacts, much stronger than the brain signals we are interested in
• since we record directly from electrodes next to and under the eye
• we can either exclude contaminated trails, or use math algorithms such as ICA to remove just the eye component

Question 7

neurophysiology of the EEG signals

Answer 7

• the EEG activity originates mostly from post-synaptic potentials, voltages that arise when neurotransmitters bind to receptors on the membrane of the post-synaptic cell
• ## neurotransmitters cause ion channels to open or close, leading to graded changes in the potential across the membrane.

Question 8

the process of the neurophysiology

Answer 8

1. with these electrical changes, the neuron will act as a ‘dipole’
2. signals from single neurons are not strong enough to be recorded outside of the head, but if many neurons are spatially align, then they are summed potentials add up and create the signals we record.
3. the pooled activity from groups of similarly orientated neurons mostly comes from large cortical pyramid

Question 9

neurophysiology pt.3

Answer 9

• the functional unit is >10,000 simultaneously activated neurons
• the orientation of the neuron determines the sign of the recorded potentials
• some orientations result in signals which cannot be recorded

Question 10

Neurophysiology of the EEG (pt. 3)

Answer 10

• EEG is biased to signals generated in superficial layers to the cerebral cortex (ie. the gyri)
• signals in the sucli are harder to detect and additionally masked by the signals from the gyri.
• the meanings, cerebrospinal fluid and skull ‘smear’ the EEG signal.

Question 11

locating sources of the signal

Answer 11

• we cannot easily locate the sources of the signal, because it is a mathematical inverse problem- if the sources are known then the scalp configuration can be reconstructed
• however, one given scalp configuration of signals can have multiple dipole solutions

Question 12

analyzing the EEG signal

Answer 12

• when looking at frequency information, the raw signal can show systematic variations ( ie. which specific frequency is dominant)
• the EEG signal we measure is a mixture of many frequencies (plus noise) originating from different locations in the brain.

Question 13

analysing the EEG pt. 2

Answer 13

• the signal can be looked at in the time domain or in the frequency domain
• we can ask which frequencies in the signal are dominant and when they are dominant
• we can also measure the amplitude of the signal at specific moments in time when people engage with cognitive tasks- this is the basis of the event related potential.