HC2 EEG: Physiological basis and recording techniques I

Question 1

Why use electroencephalogram (EEG)?

Answer 1

• Reaction time is the final outcome of sensory, decision and motor processes
• EEG can track the time course of these stages with millisecond precision
• EEG can inform us about cognitive processes when there is no behavioral response

Question 2

What is measured by EEG?

Answer 2

• Post synaptic potentials at apical dendritic trees of pyramidal cells
• For brain electrical activity to be detectable through skull, must be strong signal summed over many neurons
• Pyramidal Cells in the cortex have the right properties

Question 3

Post synaptic potentials at apical dendritic trees of pyramidal cells

Answer 3

o Action potentials can not be added up so not useful for EEG
o Post synaptic potentials can be added up so useful for EEG.

Question 4

For brain electrical activity to be detectable through skull,
must be strong signal summed over many neurons

Answer 4

o All behaving similarly at same time
o All oriented in same way
▪ This way the potentials can be added up without canceling each other out
▪ The sum is 0 in the amygdala because of the dendritic architecture that causes the PSPs to cancel each other out.
o So negative and positive don’t cancel each other out when summed

Question 5

Electromagnetic field

Answer 5

Is a physical field produced by electrically charged objects (e.g., a piece of brain tissue)
- It has properties of both electricity and magnetism
- Electric and magnetic field are oriented perpendicular

Question 6

How is EEG measured?

Answer 6

• Voltage difference between two electrodes
  o One electrode on the scalp
  o Other non-cortical (reference) electrode (e.g. earlobes)
  o Compare voltage from electrode at the brain to neutral voltage of reference electrodes (at place with no brain)
• Result in rhythmic fluctuations in voltage

Question 7

Reference and ground electrode locations

Answer 7

• These electrodes pick up noise from monitors and lights
  o Ground electrodes necessary for keeping (a lot of) the noise from the outside from the EEG signal
• Mastoid is the thickest point of skull, this makes it a good reference point

Question 8

Electrode placement - the international 10-20 system

Answer 8

• Odd to the left, even to the right
• The higher the number, the more lateral placed (more towards the ears)
• Regions:
  o FP= prefrontal region
  o F= frontal region
  o P= parietal region
  o O= occipital region
  o T = temporal region
  o C= central region (NOT an brain region)
• A1 and A2 are the electrodes at the mastoids
• Cz is the central electrode, located at the vertex
• Two lines used for the placement
  o A line connecting the nasion to the inion
  o A line connecting both preauricular to each other

Question 9

Electrooculography (EOG)

Answer 9

Eye movements mess up the EEG therefore EOG necessary to “clean up” the EEG afterwards
- Large vertical lines in EEG indicates blinking
o Eyes can be seen as high voltage batteries while the brain is a low voltage battery
- Both horizontal as vertical movements can be seen in EEG

Question 10

EOG - electrode locations

Answer 10

• Left of left eye (checks for vertical movement)
• Right of right eye (checks for vertical movement)
• Above the eye (checks for horizontal movement)
• Underneath the eye (checks for horizontal movement)

Question 11

How electrodes work

Answer 11

• A scalp electrode picks up electric signal from the brain. Contact point is often silver chloride (AgCl)
• Signals have to travel through the skull and scalp = high resistance: lower the resistance (impedance) with conductive gel
• A ground electrode reduces electrical environmental noise.
• A reference electrode provides a biological baseline.
• EOG (eyes), ECG (heart), and EMG (muscle) electrodes are attached to monitor the artifacts.

Question 12

Experimental EEG set up - you need..

Answer 12

o Brain with sensors (electrodes) on it
▪ The signal (voltage difference) is picked up at the electrodes attached to the scalp…..
o Converter
▪ …… Augmented by an amplifier and digitized by an analogue/digital (A/D) converter…..
▪ Amplifier increase the amplitude of the EEG signal
o Computer
▪ ……. Stored, displayed and analyzed on a computer

Question 13

Analogue to digital (AD) conversion

Answer 13

Analogue EEG signal (micro volts) is digitized (numbers) to have time series that represent the voltage values

Question 14

Sampling frequency

Answer 14

The rate of digitization in Hertz (Hz)
o Typical to use a sample frequency of 512 Hz

Question 15

Sample rate of 500 Hz:

Answer 15

500 x per second = each 2 ms, take one sample (per electrode)

Question 16

Nyquist-Shannon sampling theorem:

Answer 16

: Sample rate should be at least 2x the fastest frequencies in the signal
o If highest frequency of EEG is 100 –> use a frequency of at least 200

Question 17

AD level

Answer 17

o Amount of information in each sample
o Bit depth → 1 bit = 2^1 (0 1) , 2 bits = 2^2 (00 01 10 11), 3 bits (000 001 010 011 100 101 110 111) etc.

Question 18

Properties of the field signal

Answer 18

• Amplitude
• Frequency
• Phase
• The field activity oscillates in time. Thus, the signal is
  represented as a time wave. The wave can also be represented as a rotation. Properties of oscillation, such as frequency, phase and amplitude, are used to describe and analyze the signals.

Question 19

From EEG to ERP (event-related potentials)

Answer 19

ERPs are EEG changes that are time locked to sensory, motor or cognitive events that provide safe and noninvasive approach to study psychophysiological correlates of mental processes

Question 20

Design of ERP experiment: Trigger (Marker)

Answer 20

• Two computers necessary (Stimulation computer + digitalization computer)
• Stimulation computer sends marker codes/event codes to the digitalization computer.
  o For example, in Stroop task
  ▪ Color name and color congruent → marker code/event code 1
  ▪ Color name and color incongruent → marker code/event code 2
• The EEG signal (originating from the ground, active and reference electrodes) is send to the converter (filters + amplifiers) and then to the digitalization computer
• We look at tiny windows of the EEG signal we are interested in: the epoch’s
  o We gather the x’s and o’s and average them → ERP

Question 21

Importance of clean data

Answer 21

• ERPs are tiny
  o Many experimental effects are less than a few millionth of a volt
• ERPs are embedded in noise that is 20-100 µV
• Averaging is a key method to reduce noise
  o S/N (signal/noise) ratio is a function of sqrt(# of trials)
  o Doubling # of trials increases S/N ratio by 41% [sqrt(2)=1.41]
  o Quadrupling # of trials doubles S/N ratio [sqrt(4)=2]

Question 22

EEG measures:

Answer 22

• EEG oscillating signal across several electrodes
• 2 dimensions: time and location

Question 23

Multiple ways to score EEG

Answer 23

o Peak analysis
o Topography
o Source analysis
o Time-Frequency
o Etc

Question 24

Topography ≠ Neural source

Answer 24

Auditory N1 fronto-central peak but generated mainly in the auditory cortex

Question 25

Comparison between EEG and MEG

Answer 25

• The common origin
• Spatio-temporal resolution is similar
• MEG: Magnetic field permeates biological tissue, fluid, and air.
  o Therefore: less distortion and smearing out of the signal
  o MEG is better for localization of neural sources
  ▪ Peak in auditory cortex found, not in Auditory N1
  (as in EEG)
  o MEG costs more than EEG
• Butterfly plots: all sensor signals placed over each other

Question 26

Examples of Event-Related Potentials

Answer 26

• Mismatch Negativity (MMN)
  o MMN is evoked automatically by a change in a sequence of sounds
• ERP components related to language

Question 27

ERP component related to language

Answer 27

o N400 = ERP “component” related to meaning
▪ Bigger when word’s meaning doesn’t fit context
▪ Bigger for unfamiliar words
▪ May reflect amount of work required to integrate with context
o P600 = ERP “component” related to form
▪ Bigger when word not of expected type for a position in a sentence
▪ May be a type of P300 - Sometimes called Syntactic Positive Shift (SPS)
o N400 and P600 can be evoked both at once