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What Is an EEG (Electroencephalogram)? Brain Waves, 10-20 Electrode System & Neurological Diagnosis

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An electroencephalogram, commonly abbreviated as EEG, is a non-invasive neurodiagnostic technique that records the spontaneous electrical activity generated by the brain. Developed in 1924 by German psychiatrist Hans Berger, who published the first human electroencephalographic recording in 1929, the technique transformed clinical neurology by demonstrating that cerebral cortex neurons emit measurable electrical oscillations. Electrodes placed across the scalp detect minute voltage fluctuations, typically measured in microvolts, that arise from the synchronized summation of excitatory and inhibitory postsynaptic potentials within large populations of cortical pyramidal neurons. Unlike action potentials, which are brief and localized, postsynaptic potentials endure long enough to propagate through brain tissue, skull bone, and scalp.

To standardize electrode placement across diverse skull shapes and clinical centers, the International Federation of Clinical Neurophysiology established the International 10-20 system. This protocol determines electrode sites at proportional intervals of ten percent or twenty percent along four primary skull landmarks: the nasion at the bridge of the nose, the inion at the occipital protuberance, and the two preauricular points adjacent to each ear. Letter codes designate specific anatomical cerebral lobes: F for frontal, T for temporal, P for parietal, O for occipital, and C for the central sulcus boundary. Odd numbers denote electrode placements over the left cerebral hemisphere, even numbers mark the right hemisphere, and the letter Z signifies midline positions.

Recorded brain oscillations are categorized into distinct frequency bands that correspond with specific states of consciousness and physiological vigilance. Delta waves, spanning zero point five to four Hertz, exhibit high amplitude and predominate during deep non-rapid eye movement sleep and in infants. Theta waves, ranging from four to eight Hertz, appear during drowsiness, meditation, and childhood. Alpha waves, oscillating between eight and thirteen Hertz, emerge when an adult is awake and relaxed with eyes closed; opening the eyes suppresses alpha activity, an event termed alpha blocking. Beta waves, from thirteen to thirty Hertz, reflect active cognitive focus, while gamma waves above thirty Hertz correlate with multi-sensory binding.

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#1
Electroencephalography (EEG) is a non-invasive electrophysiological method that records electrical voltage oscillations produced by brain neurons.
#2
German psychiatrist Hans Berger invented human electroencephalography in 1924 and recorded the rhythmic alpha frequency band.
#3
Scalp EEG electrodes record the summation of postsynaptic potentials from pyramidal cells oriented perpendicularly to the cerebral cortical surface.
#4
EEG signals are measured in microvolts (µV), requiring differential operational amplifiers to isolate brain signals from environmental electrical noise.
#5
The International 10-20 electrode system standardizes scalp electrode placement based on proportional cranial measurements of 10% and 20%.
#6
The four anatomical reference points used in the 10-20 system are the nasion, the inion, and the left and right preauricular points.
#7
Electrode letter labels indicate underlying brain regions: F (frontal), T (temporal), P (parietal), O (occipital), and C (central).
#8
In the 10-20 placement schema, odd-numbered electrodes correspond to the left hemisphere, even numbers to the right, and Z denotes midline electrodes.
#9
Delta waves have the lowest frequency (0.5 to 4 Hz) and highest amplitude, dominating during deep slow-wave sleep in healthy adults.
#10
The persistence of localized delta activity in an awake adult indicates focal brain lesions, ischemia, or structural cerebral damage.
#11
Theta waves (4 to 8 Hz) reflect light sleep, drowsiness, and deep meditative states, and represent normal baseline rhythms in young children.
#12
Alpha waves (8 to 13 Hz) predominate over posterior occipital head regions during quiet, wakeful rest with the eyes closed.
#13
Alpha blocking or the Berger effect occurs when opening the eyes or engaging in mental arithmetic promptly attenuates the alpha rhythm into beta waves.
#14
Beta waves (13 to 30 Hz) are low-amplitude, high-frequency oscillations that reflect active sensory processing, analytical thinking, and alert attention.
#15
Gamma waves (above 30 Hz) represent rapid cortical synchrony linked to working memory, feature binding, and conscious perceptual awareness.
#16
EEG is the definitive clinical diagnostic tool for detecting and classifying epilepsy, identifying abnormal paroxysmal spike-and-wave discharges.
#17
Absence seizures typically present on an EEG as generalized, synchronous three-Hertz spike-and-slow-wave complexes across all recording channels.
#18
Polysomnography combines EEG with electromyography (EMG) and electrooculography (EOG) to assess sleep stages and diagnose sleep apnea.
#19
An isoelectric or flatline EEG recording across all leads is a primary confirmatory criterion for determining clinical brain death.
#20
Unlike functional MRI or PET, which measure metabolic blood flow changes over seconds, EEG provides millisecond-level temporal resolution.

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Educator's Insight
An electroencephalogram captures the electrical pulses of billions of brain neurons communicating with each other. In 1924, Hans Berger discovered that metal sensors on the scalp could pick up these rhythmic signals. Today, technicians use the standard 10-20 electrode map based on skull measurements. Doctors examine these wave patterns to identify epilepsy seizures, monitor sleep disorders, and confirm brain activity in critical care.
In UPSC and State PSC exams, questions often confuse the diagnostic tools: EEG maps brain electricity, ECG measures heart rhythm, and EMG tracks skeletal muscles. Memorize the brain waves by frequency using the acronym "BAT-D" from fast to slow: Beta (alert focus), Alpha (relaxed with eyes closed), Theta (drowsy or light sleep), and Delta (deep sleep). Notice that opening your eyes instantly halts relaxing alpha rhythms.

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