Basics of EEG

Primary Category

Essential Neurology

P-Category

Secondary Category

Epilepsy

S-Category

Authors:

Shehzeen Fatima Memon MBBS
Faisal Jahangiri

Introduction

A non-invasive method to measure electrical activity in the cerebral cortex

Synchronized depolarization and repolarization of thalamic and cortical neurons leads to the epileptiform activity that is recorded

At least 6 of synchronous activity is needed in the cortex to make a recording

Routine EEG is around 20 minutes of artifact-free recording.

Prerequisites

Avoid low blood sugar and fasting

Sedative and any medicines alike should be avoided before the recording

The scalp should not be oily or hair-sprayed recently

Food and drinks are to be avoided for around 8 hours before an EEG

Electrode placement

Skin is cleaned and brushed to make space

Sixteen channels are used for the 20-electrode placement system

Electrode gel allows contact between skin and electrodes to record activity

Each electrode is represented by a number and a letter

The electrodes with an ‘odd number’ are located on the left while the ‘even-numbered’ electrodes are located on the right

These electrodes are named Frontal (F), Frontopolar (Fp), Occipital (O), Parietal (P), Temporal (T), Auricular (A) and Central (C)

The ‘z’ with the electrodes represents centrally placed electrodes while ‘A” represents auricular placements

Figure 1: The International 10-20 electrode placements. Showing a longitudinal bipolar montage

Common referential points include the vertex (Cz electrode), the mastoid process (in either ears, or a mathematical equal of both sites), or an average common reference.

Three kinds of montages (selected locations) were used namely referential, bipolar, and Laplacian.

Bipolar montage

The potential difference between two contiguous electrodes

For example, F7-T3 means that F7 is the active electrode while T3 is the reference. On the other hand, T3-T5 means T3 is the active electrode and T5 is the reference electrode

Referential montage

A potential difference between the recording electrode and a preselected reference

Preselected reference is another point in the body that is a collective average of different electrodes

Figure 2: Ipsilateral ear referential montage. EEG electrode placement using the International 10-20 electrode placement system

Laplacian montage

Combination or average of the potentials presents around a particular electrode or region of interest.

Beneficial for finding a focal activity

Neuronal generators

Scalp electrodes are located a few centimeters from the neurons

Focus on localization of neuronal activation in an oscillatory manner

Both spatial and temporal summation for the electrical activity done

Synchronized activity in patches of gray matter found in pyramidal cells of cortex in layers III, IV, VI are used as generators

Current flows between pyramidal layers producing evoked potentials

Slow field potentials less than 250 Hz are recorded as a basis of an EEG

After one location is covered, the source changes its location and a new voltage field is produced

Propagation occurs in tens of milliseconds

EEG Recordings

Four kinds of waves seen in an EEG recording namely Alpha, Delta, Beta, Theta

Beta wave frequency is above 30 Hz, Delta is 1-3.99 Hz, Alpha is 9-12.99 Hz, Theta is 4-7.99 Hz

Alpha-wave

Normal posterior dominant rhythm is an Alpha wave which is found in wakefulness with eyes closed

Ablated or decreased reactivity on opening eyes or mental alertness

Alpha wave generator presumably located inside the occipital lobe

Several variants of the alpha rhythm

First, is Temporal alpha seen in older patients over the temporal region

Second is Frontal alpha seen after taking drugs, anesthesia, or after arousal from sleep over the frontal regions

Third is Paradoxical alpha where alpha activity seen with an alerting stimulus or eye-opening

Mu rhythm

A rare frequency in the range of 8-12 Hz seen in the central region

Seen in around 20-40 % of people

Occurs due to movement, somatosensory stimulus, or even a mere thought of movement

Thought to form in parietal and frontal lobes due to the effect of sensorimotor cortices

Enticed by asking the patient to intend to move

Figure 3: The posterior dominant alpha rhythm. The normal background EEG during wakefulness contains posteriorly dominant, symmetrical, and reactive alpha rhythm

Figure 4: Mu rhythms. (a) A prominent Mu rhythm is seen over the right central region. Note the arci-form waves of approximate alpha-range frequency of 8 to 12 Hz. Mu is reactive to movement or the thought of movement, unlike alpha activity, which is reactive instead of eye-opening. Longitudinal bipolar montage. (b) Trains of asynchronous mu are seen over either central region during drowsiness

(a) Right central mu activity. Figure courtesy of Jeffrey W. Britton, MD.

(b) Asynchronous mu, bilateral central regions. Courtesy Dr. Jennifer L. Hopp, MD, University of Maryland.

Beta-wave

Low amplitude

Enhanced during decreased consciousness

Seen after Barbiturate or Benzodiazepine use too

Loss of beta activity corresponds to cortical injury

Figure 4: (a) Generalized beta activity in excess. (b) Frontally predominant excess beta activity.

(a) Figure courtesy of Jeffrey W. Britton, MD. (b) Figure courtesy of Jennifer L. Hopp, MD, University of Maryland.

Slow frequencies (Delta and Theta)

Delta wave is the normal sleep rhythm

Can be induced by increasing respiration

Theta waves found in young individuals during sleep or drowsiness

Theta or delta activity in temporal regions in the elderly is abnormal

Figure 5: Image is of an encephalogram of an 8-year old boy that depicts slowing of theta and delta frequency and a burst of frontally dominant theta activity during drowsiness in the third and fourth seconds.

Provocation techniques

Used to produce epileptiform abnormalities

Various methods used include hyperventilation, intermittent photic stimulation, sleep are the most common methods used to induce seizures

Other methods used involve solving Rubik's cube, reading out foreign and native languages

Responses are considered abnormal when new epileptiform changes are noticed

After making a recording while the person is awake, photic stimulation is introduced for 4 minutes followed by hyperventilation

Hyperventilation is avoided if the patient went through a recent stroke attack or has coronary artery disease

Sleep deprivation is then carried out if there are hints of abnormal electrical activity

Artifacts

Signal distortions during EEG tracing

Categorized as biological, mechanical, or instrumental

Biological artifacts

Can be due to eye movements, blinking, muscle tension, ECG, increased respiration, sweating, tremors, or activities such as chewing, talking, extraocular movements, teeth brushing. etc.

Mechanical artifacts

These artifacts are found due to alteration in electrodes, intermittent lead wire disconnection due to body movement, rubbing or scratching of the scalp, or a 50Hz/60Hz noise,. etc.

Instrumental artifacts

Can be due to faulty electrodes, wires, amplifiers, electrode popping, faulty connectors.

EEG rhythm

Spike waves

Vertex waves

Seen in occipital region transiently during sleep

Abnormal in focal or generalized epilepsy

Seen up to 70 m/s

Sharp waves

Transient sleep epileptiform waves

6 per second phantom spikes

14 and 16 sharp spike waves

Seen up to 70-200 m/s

Clinical relevance - why is it needed?

A small and portable device, easy-to-use

Use for evaluating epileptic seizures

Used for evaluating any encephalopathy

Evaluate tumors and other brain masses

Tells whether the activity is focal or generalized as treatment differs between the two

Used for comatose patients to see continuous EEG (CEEG) patterns

Used to check drug-induced electrical activity e.g. Benzodiazepines

Helps exclude psychiatric diseases

Monitor brain development

Investigate sleep disorders

Bibliography

Files, B. (2011). An introduction to EEG. Perception.

Rana, A. Q., Ghouse, A. T., & Govindarajan, R. (2017). Basics of electroencephalography (EEG). In Neurophysiology in Clinical Practice (pp. 3-9). Springer, Cham.

Olejniczak, P. (2006). Neurophysiologic basis of EEG. Journal of clinical neurophysiology, 23(3), 186-189.

Billones, R. K. C., Bedruz, R. A. R., Caguicla, S. M. D., Ilagan, K. M. S., Monsale, K. R. C., Santos, A. G. G., ... & Dadios, E. P. (2018). Cardiac and brain activity correlation analysis using electrocardiogram and electroencephalogram signals. In 2018 IEEE 10th International Conference on Humanoid, Nanotechnology, Information Technology, Communication and Control, Environment and Management (HNICEM) (pp. 1-6). IEEE.

Louis, E. K. S., Frey, L. C., Britton, J. W., Hopp, J. L., Korb, P., Koubeissi, M. Z., ... & Pestana-Knight, E. M. (2016). The normal eeg. Electroencephalography (EEG): An Introductory Text and Atlas of Normal and Abnormal Findings in Adults, Children, and Infants [Internet].

Rayi, A., & Murr, N. (2021). Electroencephalogram. StatPearls [Internet].

Braga, P., Mameniskiené, R., Guaranha, M., Zeissig, E. V., Samaitienė, R., Özcelik, E. U., ... & Wolf, P. (2021). Cognitive tasks as provocation methods in routine EEG: a multicentre field study. Epileptic Disorders, 23(1), 123-132.

Teplan, M. (2002). Fundamentals of EEG measurement. Measurement science review, 2(2), 1-11.

American Clinical Neurophysiology Society. (2008). Guideline twelve: guidelines for long-term monitoring for epilepsy. Journal of clinical neurophysiology: official publication of the American Electroencephalographic Society, 25(3), 170-180.

Schoenberg, M. R., Ruwe, W. D., Dawson, K., McDonald, N. B., Houston, B., & Forducey, P. G. (2008). Comparison of functional outcomes and treatment cost between a computer-based cognitive rehabilitation teletherapy program and a face-to-face rehabilitation program. Professional Psychology: Research and Practice, 39(2), 169.

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