Basics of EEG

20 minutes of artifact-free recording of epileptiform activity that consists of synchronized depolarization and repolarization of thalamic and cortical neurons. 20-electrode placement system along with bipolar and referential montages allows comparison of epileptiform activity on both sides of the brain as well as localization of seizure. Many provocative techniques such as hyperventilation, intermittent photic stimulation, and sleep are used to produce epileptiform waves.

Primary Category
Essential Neurology
Secondary Category


  • 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.


  • 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

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Copyright 2013. Mayo Foundation for Medical Education and Research. All rights reserved. Courtesy of Dr. Jeffrey W. Britton, MD.
  • 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

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Copyright 2013. Mayo Foundation for Medical Education and Research. All rights reserved. Figure courtesy of Jeffrey W. Britton, MD.

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


  • 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

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Copyright 2013. Mayo Foundation for Medical Education and Research. All rights reserved. Figure courtesy of Erik K. St. Louis, MD.

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

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Copyright 2013. Mayo Foundation for Medical Education and Research. All rights reserved.
(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.


  • 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.

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(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.

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Copyright 2013. Mayo Foundation for Medical Education and Research. All rights reserved. Figure courtesy of Jeffrey W. Britton, MD.

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


  • 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

Further reading

  • Im, C. H. (2018). Basics of EEG: Generation, Acquisition, and Applications of EEG. In Computational EEG Analysis (pp. 3-11). Springer, Singapore.
  • Farnsworth, B. (2018). What is EEG (Electroencephalography) and How Does it Work?. imotions. https://imotions. com/blog/what-is-eeg8.


  • 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 neurophysiology23(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 Disorders23(1), 123-132.
  • Teplan, M. (2002). Fundamentals of EEG measurement. Measurement science review2(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 Society25(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 Practice39(2), 169.
Shehzeen Fatima Memon MBBS

PMC certified | General Practitioner at Shifaam Healthcare | Research Associate at NeuroCare.AI Academy

Faisal R. Jahangiri MD, CNIM, D.ABNM, FASNM, FASET

President & CEO, Global Innervation LLC

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