Electroencephalography

Question 1

Define EEG (Electroencephalography) (4)

Answer 1

Electro = electrical
Encephelo = brain
Graphy = writing

• non-invasive measure of brain activity
• array of surface electrodes on the scalp
• record potential differences (micro-volts)
• electrical activity of brain provides a unique insight how the brain reacts to external stimuli

Question 2

pros of EEG (5)

Answer 2

Completely safe and painless procedure

Useful tool in the discipline of cognitive psychology

Reveals changes in brain state associated with differing levels of arousal or under different experimental conditions

Excellent temporal resolution

Reveals chronology of mental processes.

Question 3

EEG psychological research is based on measuring the (2)

Answer 3

Much psychological research is based on measuring the speed and accuracy of participants’ responses (i.e. reaction time tests).

EEG can show what happens in the brain before, during and after a response button is pressed.

Question 4

EEG and showing the brain’s consciousness

Answer 4

The brain is always active, whether awake or sleeping… or even while under anesthesia…

Question 5

purpose of EEG (2)

Answer 5

brain cells communicate by passing electrical impulses back and forth.

The purpose of EEG is to measure the electrical activity resulting from the interactions between neurons

Question 6

EEG Electrodes (3)

Answer 6

key to measuring these electrical charges

flat, metal discs which are attached to the scalp

EEG caps have multiple electrodes

Question 7

So how many electrodes should your EEG cap have? (2)

Answer 7

• EEG caps with 10-20 electrodes should be adequate for most academic needs
• also available with 32, 64, 128 electrodes for more advanced imaging studies.

Question 8

Additional features of a EEG cap (3)

Answer 8

Systems available with 4, 10 or 24 channels.

These include an additional ECG (heart activity), EMG (muscle activity) or EOG (eye movement) channel.

The number of electrodes an EEG cap has will strongly influence the price… as does sampling rate of system

Question 9

EEG: What about electrode placement/ fitting? (3)

Answer 9

The cap must allow electrodes to be positioned precisely… placed according to international standards.

Make sure to choose an EEG cap that is sized accurately.

EEG headsets should come with a variety of sizing options and be easily adjusted.

Question 10

EEG: What about the amplifier? (4)

Answer 10

Signal quality is of vital importance when it comes to EEG.

• The initial EEG signal is comprised of low frequency fluctuations which need to be amplified.

As a result, the amplifier is often the most expensive and defining feature of an EEG headset.

• If there is one thing to check, make sure it’s that the amplifier is of the highest quality.

Question 11

EEG: Should I use wet or dry electrodes? (3)

Answer 11

“Wet electrodes” use a conductive gel to improve signal quality

“Dry Electrodes” do not require such gel.

The gel used can be sticky and unpleasant for participants. For this reason many of those wishing to use EEG for market research purposes opt for dry electrodes in order to improve participation and compliance. This is understandable; the last thing you want is the participant having a negative experience!

Question 12

Comparing wet vs. dry electrodes: (2)

Answer 12

Although many modern dry electrodes provide excellent signal quality, wet electrodes provide better contact between electrode and scalp, improving data quality.

An excellent compromise comes in the form of headsets that use cleaner and more comfortable saline based wet sensors. These provide both comfort and quality. Newer, cleaner electrode creams are also available.

Question 13

EEG: Wired vs Wireless? (3)

Answer 13

In order for EEG technology to become more user-friendly, many EEG systems have moved from the bulky systems used in clinical settings to sleek, convenient wireless systems.

This is allowing them to be used in a range of new settings.

If your research question requires simulation of a real-world scenario, or you wish to use EEG with virtual reality software, then a wireless system is the way to go.

Question 14

Wireless EEG issues (3)

Answer 14

Wireless EEG has however experienced some issues when it comes to “electrical noise”.

This is due to many factors including interference from other hardware, movement from participants and inappropriate experimental conditions.

when good transmission quality is used for wireless data, there is little to no effect on signal quality.

Question 15

Additional features of wireless EEG (2)

Answer 15

Some wireless headsets provide a transmission range of 10 meters.

iMotions’ EEG software takes EEG measurement up a notch. It allows researchers to combine EEG data with a range of other biosensors including eye tracking, ECG, respiration and surveys. This is an important feature as it opens the door to a range of novel research questions.

Question 16

Why is it useful to combine the eye tracking with EEG?

Answer 16

Example: when combined with eye tracking we have the means to understand where the person is looking when demonstrating a particular brain response. This gives us an incredible insight into their reaction to the stimulus provided – something which could not be achieved with EEG alone.

Question 17

Configuration/ scalp distances in positioning EEG (7)

Answer 17

Various electrode types exist

10-20 configuration is common

On the scalp, distances between two electrodes are given as 10% and 20% of the distance between specified points.

Nasion and Inion are the two reference points, as well as near the ear lobes
the distance between nasion and inion are divided into 5 points

placed at a distance of 10%, 20%, 20%, 20%, 20% and 10% of this length.

nasion – inion distance is measured along the temporal lobes and five electrode are placed as two in frontal, 2 in temporal and 1 in occipital lobes respectively.

remaining six electrodes are fixed as 2 in frontal, 2 in central and 2 in parietal (19 electrodes total) + one reference electrode (placed at the ear lobe)

Question 18

Event-Related Potentials (ERP)

Answer 18

Brain-wave activity timelocked to an event… typically higher cognitive functions

• Example: Brain-wave activity differs whether you recognize something as a target or not (classification)

P-300 Wave?
Between 200
and 500 msec

Question 19

How do we get a clear signal when reading EEG signals

Answer 19

Noise vs. signal (SNR)

ERP’s are obtained after averaging EEG signals obtained over multiple trials (trials are aligned by stimulus onset).

Question 20

Event-related potentials (ERPs) Steps to measure (4)

Answer 20

1. There is continuous and ongoing EEG activity as well as random noise completely unrelated to the onset of a stimulus continually occurring.

• This is your “default activity“ (your ongoing thoughts and mental states). When you present a stimulus, you trigger stimulus-related EEG activity.

2. To uncover the stimulus-related EEG data, stimulus is shown several times - 50 times or more

• 50 trials of data time- locked to stimulus-onset
• Each trial is a time-course of data at each electrode

3. exclusion of trials with artifacts (e.g. blinking)
   remaining trials are averaged creating average time-course of EEG data
   only the stimulus-related EEG activity survives while noise is attenuated (the more repetitions you complete, the cleaner the event-related EEG data will be)
4. remaining average EEG waveform is the event-related potential, which reflects the average stimulus-related EEG activity as triggered by a specific stimulus.

Question 21

EEG: What are we measuring?

Answer 21

primarily synchronized activity of pyramidal neurons

found in all cortical areas

they are always oriented perpendicular to the cortical surface

cell bodies heading away from the surface, dendrites heading towards the surface

Question 22

What do neurons need to do to be detected by EEG electrodes?

Answer 22

Many neurons need to sum their activity in order to be detected by EEG electrodes. The timing of their activity is crucial. Synchronized neural activity produces larger signals.

Question 23

EEG: What are we measuring? Orientation

Answer 23

orientation of the cells generates an electrical field with a very stable orientation

• cells in deeper brain structures don’t have specific orientation

the electrical fields are more likely to spread into various directions and cancel out instead of projecting towards the scalp surface

electrical signals are very small

sent to an amplifier and digitized

can be displayed as a time series of voltage values or averaged

Question 24

EEG: Measurement with many neurons firing

Answer 24

postsynaptic potential of a single neuron is too small to be detected

If hundreds of thousands of similarly oriented neurons fire in synchrony, they sum up and generate an electric field, which is rapidly propagated throughout brain tissue and skull

can be measured from the scalp.

neural oscillations are even visible in raw, unprocessed data

But… the signal is a mixture of several underlying base frequencies

Question 25

What do the EEG measurement mean? (2+5)

Answer 25

considered to reflect certain cognitive, affective or attentional states

these frequencies vary slightly dependent on individual factors, stimulus properties and internal states

classify these frequencies based on specific frequency ranges, or frequency bands:

• delta
• theta
• alpha
• beta
• gamma

Question 26

EEG: Delta band

Answer 26

(1 – 4 Hz), deepest sleep states

Question 27

EEG: Theta band

Answer 27

(4 – 8 Hz), generally associated with brain processes underlying mental workload or working memory

Question 28

EEG: Alpha band

Answer 28

(8 – 12 Hz), reflecting sensory, motor and memory functions. Increased alpha during mental and physical relaxation with eyes closed and reduced, or suppressed, with eyes open.

Question 29

EEG: Beta band

Answer 29

(13 – 25 Hz) , plan or execute movements, particularly when reaching or grasping requires fine finger movements and focused attention. Interestingly, also noticeable when we observe others’ bodily movements. (mirror neuron activity)

Question 30

EEG: Gamma band

Answer 30

(> 25 Hz). Some argue that gamma serves as a carrier frequency for binding various sensory impressions of an object together to a coherent form, therefore reflecting an attentional process.

Question 31

Seizure

Answer 31

the clinical manifestation of an abnormal and excessive excitation and synchronization of a population of cortical neurons

Question 32

Febrile seizure

Answer 32

seizure induced by very high heat

Question 33

Epilepsy

Answer 33

a tendency toward recurrent seizures unprovoked by any systemic or acute neurologic insults

Question 34

Epileptogenesis

Answer 34

sequence of events that converts a normal neuronal network into a hyperexcitable network

Question 35

Polysomnography

Answer 35

Sleep studies

monitors many body functions, includingbrain activity (EEG),eyemovements (EOG), muscle activity orskeletal muscleactivation (EMG), andheartrhythm (ECG), during sleep, breathing functions,respiratoryairflow, and respiratory effort indicators, peripheralpulse oximetry.

Question 36

Polysomnography Measures:

Answer 36

• sleep onset latency (SOL), the REM-sleep onset latency, the number of awakenings during the sleep-period, the total sleep duration, percentages and durations of every sleep stage, and the number ofarousals

Question 37

Theelectrooculogram(EOG) in Polysomnography

Answer 37

Theelectrooculogram(EOG) uses two electrodes; one that is placed 1cm above the outercanthusof the right eye and one that is placed 1cm below the outer canthus of the left eye. These electrodes pick up the activity of the eyes in virtue of the electropotential difference between the cornea and the retina (the cornea is positively charged relative to the retina). This helps to determine when REM sleep occurs, of which rapid eye movements are characteristic, and also essentially aids in determining when sleep occurs.

Question 38

Theelectromyogram(EMG) in Polysomnography

Answer 38

Theelectromyogram(EMG) typically uses four electrodes to measure muscle tension in the body as well as to monitor for an excessive amount of leg movements during sleep (which may be indicative ofperiodic limb movement disorder, PLMD).

Two leads are placed on the chin with one above the jawline and one below. This, like the EOG, helps determine when sleep occurs as well as REM sleep. Sleep generally includes relaxation and so a marked decrease in muscle tension occurs. A further decrease in skeletal muscle tension occurs in REM sleep.

REM-sleep Behaviour Disorder (RBD).

Question 39

(REM) sleep behavior disorder (RBD)

Answer 39

a parasomnia characterized by dream-enactment behaviors that emerge during a loss of REM sleep atonia.

Question 40

Polysomnography: snoring

Answer 40

• Snoring may be recorded with a sound probe over the neck, though more commonly the sleep technician will just note snoring as “mild”, “moderate” or “loud” or give a numerical estimate on a scale of 1 to 10.

Question 41

EEG : electrocardiogram/ legs (5)

Answer 41

Finally, two more leads are placed on theanterior tibialisof each leg to measure leg movements.

Though a typicalelectrocardiogram(ECG or EKG) would use ten electrodes, only two or three are used for a polysomnogram.

They can either be placed under the collarbone on each side of the chest or one under the collarbone and the other six inches above the waist on either side of the body.

These electrodes measure the electrical activity of the heart as it contracts and expands, recording such features as the “P” wave, “QRS” complex, and “T” wave.

These can be analyzed for any abnormalities that might be indicative of an underlying heart pathology.

Question 42

EEG: Respiration (4)

Answer 42

Nasal and oral airflow can be measured using pressure transducers, and/or a thermocouple, fitted in or near the nostrils

• measure the rate of respiration and identify interruptions in breathing. Respiratory effort is also measured
• Looking for hypopnea, or a period of reduced airflow, or an obstructive apnea.

Pulse oximetry determines changes in blood oxygen levels that often occur with sleep apnea and other respiratory problems. The pulse oximeter fits over a fingertip or an earlobe.

Question 43

Now what is the best place for the reference electrode?

Answer 43

In fact, there is none – the choice of the reference only affects the absolute electrode voltages across all electrodes while the relative voltages remain completely unchanged.

• This means that changing the site of the reference may make the scalp voltages look quite different, even though the relative distribution is completely identical.

Imagine a landscape with mountains and valleys.

Changing the reference electrode is similar to flooding the landscape with water. While the sea level changes, the absolute shape of the landscape is completely unchanged

Question 44

EEG: Sampling limits?

Answer 44

Something to bear in mind when considering this question is the Nyquist Theorem.

• It states that “all of the information in an analog signal (like EEG voltages) can be captured digitally as long as the sampling rate is more than twice as great as the highest frequency of interest in the signal.”

In more simple terms, the highest frequency that you can analyze in an EEG signal is half the size of the sampling rate.

• For example, if you sampled your data at 256 Hz, you should only analyze frequencies up to 256 / 2 = 128 Hz. Some researchers are even stricter and recommend to use only frequencies up to one third of the sampling rate (e.g., 256 / 3 = 85.3 Hz). Remember that the brain primarily generates lower frequencies (for example, from delta [1 - 4 Hz] to gamma [25 - 80 Hz]), so even with low EEG sampling rates around 100 Hz you can be quite certain to obtain interpretable data.

Question 45

EEG: Noise/Artifacts: Muscle activity (6)

Answer 45

generates electric currents that are picked up by electrodes.

• The closer the muscles are to the electrodes, the stronger their impact on the recording will be.

Particularly the activity of facial muscles (forehead, cheek, mouth), neck muscles and jaw musculature has severe effects on EEG recordings.

• Clenching should be avoided at all costs – instruct respondents to avoid chewing or tensing their jaw.

As the heart is muscle, it also affects EEG data quality. Hearts cannot be simply instructed to stop, you have to rely on signal decontamination procedures to remove ECG noise from EEG recordings.

Ideally, you can monitor heart rate with an optical sensor (e.g. photoplethysmography) or an ECG device.

Question 46

EEG: Noise/Artifacts: Muscle activity, Eye movements (6)

Answer 46

Eye movements: affect the electrical fields picked up by the electrodes.

• Vertical eye movements (up-down) look more sinusoidal,
• while horizontal eye movements (right-left) look more box-shaped.

The eye has a strong electromagnetic field that is established by the millions of neurons in the retina.

Moving your eyes also shifts the electrical field generated by the eye ball.

It’s recommended to record eye movements using eye trackers or by placing additional EEG electrodes surrounding the eyes.

Question 47

EEG: Noise/Artifacts: Movement and line noise

Answer 47

Movement of an electrode or headset movements can cause severe artifacts that are visible in the affected channel or in all channels

• an electrode loses contact with the socket
• electrodes are securely attached to the skin

Line noise (60 Hz in the US, 50 Hz in the EU)

• Filters
• cognitive frequencies of the brain are often below the 50 or 60 Hz range