W3 - EEG

Question 1

What is EEG

Answer 1

Method of detecting neural activity via. electrodes on the scalp. Electrodes pick up small fluctuations of electrical signals from activity of (mostly cortical) neurons

Question 2

What are the properties of the raw signals picked up by EEG

Answer 2

• Very noisy and might not look like much, but they are systematically related to cognitive processes > Use these signals to learn something about cognition when people perform tasks

Question 3

What are 2 types of EEG

Answer 3

Scalp: Non-Invasive Intra-Cranial: Measure directly at exposed cortex

Question 4

Who invented EEG and how

Answer 4

Hans Berger. Detected first EEG with wife’s scalp in 1924.

Question 5

What is the Alpha Rhythm

Answer 5

Alpha rhythm – When people closed their eyes, - Inconsistent electrical signal varying between 8 - 13 Hz - Used two electrodes (silver wires and later foil), one attached to the front of the head and one to the rear, and recorded the potential/voltage difference.

Question 6

What are advantages/disadvantages of EEG

Answer 6

Cheap Temporal Resolution Poor Spatial Resolution

Question 7

How is EEG Recorded: What are the 4 tools

Answer 7

Electrode Cap > Amplifier > EEG Recording Experimental Stimulation > EEG Recording

Question 8

What are the channels in EEG

Answer 8

10 - 32 - 64 - 128 - 256

Question 9

How are numbers on the scalp displayed

Answer 9

Odd and Even. Each corresponds. F = frontal P = parietal C = central O = occipital T = temporal

Question 10

Neurophysiology: Where is the origin of EEG Signal

Answer 10

Post-synaptic potentials (Voltage when NT binds to post-synaptic membrane) > Causes ion channels to open/close, leading to graded changes in potential across membrane Not Action Potential

Question 11

What can the post-synaptic potential be considered as

Answer 11

• Understood as a small “dipole” (magnet) - Signals from single cells are not strong enough - Many neurons spatially align > summed potentials add up and create the signals we can record

Question 12

Many neurons spatially align > summed potentials add up and create the signals we can record: What is this called and where is the origin

Answer 12

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

Question 13

What is the functional unit of EEG (How many Neurons must be spatially aligned)

Answer 13

The functional unit is >10,000 simultaneously activated neurons

Question 14

What determines the sign of the recorded potentials

Answer 14

Orientation of the neurons determines the sign of the recorded potentials Some orientations lead to signals which cannot be recorded

Question 15

What are the limitations of EEG (Neurophysiology)

Answer 15

1.) Biased to Gyri (Sulci harder to detect / masked by gri signals) 2.) Meninges, CSF and skull “smear” the EEG signal > Localisation difficult - INVERSE PROBLEM: one given scalp configuration of signals can have multiple dipole solutions!

Question 16

What is EEG measured in relation to

Answer 16

In relation to a reference electrode - Reference either a neutral point or average of all scalp electrodes

Question 17

Typical amplitude of EEG and Steps to make it clear

Answer 17

1.) 10μV to 100μV (Tiny) 2.) Amplified by 1,000/100,000x 3.) Signal is digitalized. Typical sample frequency is 256-1024Hz, but can be >4000Hz 4.) Signal is band-pass filtered to remove the low (<0.5-1Hz) and high frequencies (typically >35-70Hz) because they cannot reflect brain activity.

Question 18

What is the most relevant step in EEG signal analysis

Answer 18

Finding all the artefacts that are not brain signals! - sweating - electrical noise (“notch filter”) - eye movements and blinks

Question 19

How does eye movement affect EEG.

Answer 19

• Eye can be regarded as a dipole - Signals originating from the eye will contaminate the signal of interest (And will be much larger)

Question 20

How do we remove eye movement in signal

Answer 20

• Record eye signal by placing electrodes next to and under the eye to capture horizontal and vertical eye movements - Remove by excluding contaminated trials, or mathematical algorithms, such as ICA

Question 21

What is wrong with single EEG-trial studies

Answer 21

Far too noisy. Too many variance between sessions from same participants and between participants

Question 22

What is P and N

Answer 22

Deflection in amplitude. P = Signal < 0 N = Signal > 0

Question 23

How can ERP be analysized

Answer 23

1.) Peak-amplitude (used in 70%)) 2.) Area-under-the-curve (used in 20%) 3.) peak-to-peak (used in 10%) 4.) on-set of component (ambiguous) No clear rule, and results might differ between measures

Question 24

Woodman & Luck (1999): What is the signal they used and what did it index.

Answer 24

N2pc (second negativity, posterior contralateral) > index attention Strongest over the posterior cortex contralateral to where the observer is attending

Question 25

Woodman & Luck (1999): Study Aims

Answer 25

Visual Search Task: Parallel / Serial Serial Fashion: Attention switch from one hemifield to the other, until the target is found . Parallel: Nothing

Question 26

Woodman & Luck (1999): Study Overview

Answer 26

People to attend one hemifield first by manipulating probability specific colour was target (C75 and C25)

Question 27

Woodman & Luck (1999): Study Results

Answer 27

Target Absent + C75 v C25: N2pc Target Absent + C75 same C25: No shift Target Present + C75 (Target) v C25: No shift Target Present C75 v C25 (target): N2pc All supported serial search

Question 28

Gehring et al., 1993: Study Aims

Answer 28

ERN: Whether a cognitive mechanism for the detection of and compensation for errors.

Question 29

What is the ERN

Answer 29

The ERN is a negative deflection of up to 10μV in amplitude observed at central electrodes ~80-100ms after an erroneous response.

Question 30

Gehring et al., 1993: Study Overview and 2 Hypotheses

Answer 30

Emphasise accuracy/speed in a Flanker-task H1.) Incongruent displays should lead to more errors H2.) Error detection should only matter in the accuracy condition

Question 31

Gehring et al., 1993: First Hypothesis supported?

Answer 31

Yes. ERN on incorrect trial in comparison to correct trials (Recorded with EMG)

Question 32

Gehring et al., 1993: Second Hypothesis supported?

Answer 32

Yes. The ERN was strongest when people emphasised accuracy, and weakest for speed

Question 33

Gehring et al., 1993: Further study of ERN compensation for errors

Answer 33

Investigated how ERNs of different sizes were related to response parameters, which might in turn be related to correcting/avoiding errors

Question 34

Gehring et al., 1993: Further study of ERN compensation results

Answer 34

1.) The greater the ERN, the lower the response force > trying to correct for the error 2.) The greater the ERN, the higher the probability to get it right on next trial > successful learning from errors 3.) The greater the ERN, the slower the response on next trial > successful learning from errors