EEG

Question 1

What is EEG

Answer 1

Detecting neural activity by placing electrodes on the scalp

These electrodes pick up small fluctuations of electrical signals, originating from activity of (mostly cortical) neurons

Can also record intra-cranial EEG at the exposed cortex

Question 2

What is alpha rhythm

Answer 2

When people closed their eyes, the electrical signal was not constant, but it varied with a characteristic frequency of 8-13 Hz

Question 3

Pros and cons about EEG

Answer 3

The temporal resolution is great

But the spatial resolution is not so good:
EEG is biased to signals generated in superficial layers of cerebral cortex on the gyri (ridges) directly bordering the skull

Signals in the sulci are harder to detect than from gyri, and may additionally be masked by the signals from the gyri

The meninges, cerebrospinal fluid (CSF) and skull “smear” the EEG signal, making it difficult to localise the source
This is known as the inverse problem

Question 4

What neural activity does EEG record

Answer 4

The EEG activity does not reflect action potentials but originates mostly from post-synaptic potentials – voltages that arise when neurotransmitters bind to receptors on the membrane of the post-synaptic cell

This causes ion channels to open or close, leading to graded changes in the potential across the membrane

This pooled activity from groups of similarly oriented neurons mostly comes from large cortical pyramid cells

Question 5

What determines the sign of the recorded potential

Answer 5

The orientation of the neurons determines the sign of the recorded potential; Some orientations lead to signals which cannot be recorded

If many neurons spatially align, then their summed potentials add up and can be recorded

Question 6

How does analyzing EEG data generally work

Answer 6

EEG signals measured from the scalp in relation to a reference electrode
The reference should be a neutral point (e.g., tip of the nose, mastoids (behind the ear) as the baseline), but some reference to the average of all scalp electrodes

Need to be amplified

Then digitalized

Signal is band-pass filtered to remove the low and high frequencies

Signal is also notch-filtered to remove line noise

The EEG is the sum of signals originating from many neural units

Question 7

What are the Artefacts in EEG

Answer 7

The artefacts that are not brain signals
Eyeblink artefacts; muscle artefact; skin potential artefact

Signals originating from the eye will contaminate the signal of interest – be much larger

Placing electrodes next to and under the eye to capture horizontal and vertical eye movements

Excluding these contaminated trials, or mathematical algorithms (ICA)

Question 8

How to analyse Event-related potentials (ERPs)

Answer 8

Find the brain activity that is reliably related to cognitive processes of interest

For this, the single-trial EEG is too noisy
There are variance between sessions from the same participants and between participants

There are different aspects of the ERP component of interest that can be analysed:
Peak-amplitude (used in 70% of studies)
Area-under-the-curve (used in 20%)
Peak-to-peak (used in 10%)
Eetermine the onset of a component (tricky)

There is no clear rule, and results might differ between measures

Question 9

Reason for using ERPs to investigate cognition

Answer 9

Many components are very well studied – hence, finding that a specific component is modulated by the experimental task might shed light on what cognitive process is involved

ERPs require a lot of knowledge about the various components (careful: reverse inference)

Question 10

Research in ERP and ERN

Answer 10

Woodman and Luck (1999)
Investigated whether people searched in parallel or serial based on N2pc (attention)

Gehring et al. (1993): Whether there is mechanism for thedetection andcompensation for errors