Basics of EEG

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

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.

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

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-eeg, 8.

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