EEG: principles; ERPs & time-domain

Question 1

Pioneer of EEG

Answer 1

Hans Berger

Question 2

Neural basis of EEG (2)

Answer 2

1. Post-synaptic potentials (PSPs) in apical dendrites create a dipole towards the soma of the cell
2. Modulations in extracellular currents due to PSPs are captured by EEG from the scalp

Question 3

Which EEG rhythms are visible with the naked eye?

Answer 3

Alpha and Beta

Question 4

What does EEG signal depend on?

Answer 4

1. Orientation of dipole
   –> EEG is especially sensitive to perpendicularly oriented dipoles generated in pyramidal neurons in the gyri
   2. Location of synaptic activity
   –> Excitatory input near the soma creates positive EEG deflections
   –> Excitatory input at apical dendrites creates negative EEG deflections
   3. Neural synchrony
   –> Amplitude of the EEG is modulated by the amount of synchrony in a local population of neurons

Question 5

What does the EEG signal amplitude depend on?

Answer 5

Amplitude is modulated by the amount of synchrony in a local population of neurons

Question 6

Advantages of EEG

Answer 6

○ High temporal resolution (ms range)
○ Good for studying dynamic processes
○ Non-invasive
○ Relatively cheap (compared to MRI)
○ Light-weight, can even be portable

Question 7

Disadvantages of EEG

Answer 7

○ Poor spatial resolution
○ Limited depth sensitivity (only outer layers of brain measures)
○ Susceptibility to artifacts
○ Difficulty in measuring activity from small populations of neurons
○ Long set-up time

Question 8

General EEG working principle:

Answer 8

EEG measures the summed up activity of thousands of parallel-oriented, neighbouring and synchronously active pyramidal neurons (cortex layer 5 and 6)

Question 9

EEG artifacts:

Answer 9

1. Eye movement artifacts, e.g., saccades, blinks
2. Muscle artifacts, e.g., clenching of jaw
3. Heartbeat
4. Slow drift, caused by e.g., sweating

Question 10

Pre-processing filters:

Answer 10

1. High-pass filter (0.01 Hz) - removes low frequencies, like slow drifts, breathing
2. Low-pass filter (e.g., 40 Hz), removes muscle (EMG) artefact, or electrical interference from nearby electronic devices.

Question 11

What do F, P, T, O, Z and C mark in EEG electrode placement?

Answer 11

■ F - frontal lobe
■ T - temporal lobe
■ Z - midline
■ P - parietal lobe
■ O - occipital lobe
■ C - Central sulcus

Question 12

Types of ERP components?

Answer 12

1. Early components (‘exogenous’)
2. Later components (‘endogenous’)

Question 13

What are early EEG components?

Answer 13

• Automatic sensory responses of the brain, usually in first few ms after event/stimulus
• Key: Short duration, high amplitude, and specific topographical distribution over the scalp
• Highly influenced by physical stimulus properties
• Clinical use (e.g., test integrity of sensory pathway)
• Can also be modulated by emotion, mood, attention, reward etc.
• e.g., P1, P2, N1

Question 14

What are later EEG components?

Answer 14

• Reflect internal, high-order processing, such as attention, memory, and decision-making
• Occur after the early components have peaked
• More distributed and diffuse scalp distribution
• e.g., P3 (or P300), which is often observed in response to an infrequent or unexpected stimulus

Question 15

Explain the power spectrum of EEG

Answer 15

It follows the 1/f Law, meaning that the power of the EEG signal is relatively high at low frequencies (e.g., delta and theta bands) and gradually decreases as frequency increases (e.g., alpha, beta, and gamma bands).

Question 16

Name the EEG rhythms:

Answer 16

Delta, Theta, Alpha, Beta, Gamma

Question 17

Main characteristics and Hz of the EEG rhythms

Answer 17

Delta (0.5-4 Hz) - infants, associated with some sleep stages
Theta (4-8 Hz) - children, light sleep and drowsiness in adults
Alpha (8-12 Hz) - peak at 10 Hz, idling, eyes closed, relaxation, observed occipitally,
Beta (13-30 Hz) - linked to waking consciousness, active concentration, motor processing, observed frontally and parietally
Gamma (above 30 Hz) - lnvolved in conscious perception, WM and attention

Question 18

Two main signals of interest in cognitive neuroscience:

Answer 18

1. Spontaneous oscillations
2. Event-related potentials (ERPs)

Question 19

Spontaneous oscillations

Answer 19

▪ Occur in specific frequency bands (’brain rhythms’)
▪ Modulated by e.g. sleep-wake-cycle, mental and physical activity

Question 20

Event-related potentials (ERPs)

Answer 20

▪ Short in duration (100 - 700 ms)
▪ Time-locked to an (internal/external) event
Modulated by many internal and external factors