What is Electroencephalography (EEG)?
Electroencephalography (EEG) is a non-invasive neurophysiological technique used to record the electrical activity of the brain. Since its first clinical application in the early 20th century by German psychiatrist Hans Berger, EEG has evolved into an essential tool in neurology, sleep medicine, research, and even psychiatry.
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Table of Contents
Definition of Electroencephalography (EEG)
Electroencephalography (EEG) is a non-invasive neurophysiological technique used to record, measure, and analyze the electrical activity generated by neurons in the brain. It captures the spontaneous fluctuations of voltage produced mainly by postsynaptic potentials in cortical neurons, particularly pyramidal cells, through electrodes placed on the scalp.
EEG provides a real-time representation of the brain’s electrical activity as waveforms, typically displayed as rhythmic patterns of different frequencies and amplitudes. It is widely used in clinical practice to diagnose and monitor conditions such as epilepsy, sleep disorders, encephalopathies, and brain death, as well as in research to study brain function, cognition, and neural networks.
Physiological Basis of Electroencephalography (EEG)
EEG relies on the fundamental principle that the human brain’s electrical activity, produced by neurons communicating through electrical impulses, can be detected and recorded from the scalp. However, the origin, nature, and detectability of these electrical signals are rooted in the unique structure and function of cortical neurons, synaptic activity, and the way large groups of neurons interact.
Origin of EEG Signals: Postsynaptic Potentials
Contrary to what might be intuitively assumed, EEG does not directly record action potentials (spikes) fired by individual neurons. Instead, it primarily records postsynaptic potentials—that is, voltage changes generated when neurotransmitters bind to receptors on the dendrites and cell bodies of neurons, leading to ionic currents across the cell membrane.
These postsynaptic potentials can be excitatory (EPSPs) or inhibitory (IPSPs):
✔ EPSPs depolarize the membrane, bringing it closer to firing an action potential.
✔ IPSPs hyperpolarize the membrane, making it less likely to fire.
While the voltage change in a single postsynaptic potential is extremely small (microvolts), when thousands or millions of cortical pyramidal neurons fire synchronously, their small electrical fields sum together (spatial summation) to create voltage fluctuations large enough to be recorded at the scalp.
Role of Cortical Pyramidal Neurons
The primary contributors to the EEG signal are the large pyramidal neurons found in cortical layers III and V. These neurons:
✔ Are aligned perpendicularly to the cortical surface.
✔ Have long, vertically oriented apical dendrites reaching up toward the cortical surface.
This uniform vertical orientation is crucial: it causes the extracellular current flow (known as a dipole) to summate rather than cancel out, creating an electrical field that extends to the surface of the scalp.
Dipoles and Volume Conduction
When a group of pyramidal neurons is synchronously activated:
✔ Synaptic currents flow into the dendrites (sink) and return via the cell body and axon hillock (source).
✔ This sets up an open-field dipole that produces a potential difference detectable on the scalp.
These currents propagate through the brain tissue, cerebrospinal fluid, skull, and scalp—a process known as volume conduction—before reaching the electrodes.
Because the skull has high electrical resistance, EEG signals become smoothed and attenuated; however, large-scale synchronous neuronal activity still produces measurable fluctuations in voltage.
Synchronization and Rhythmic Patterns
EEG waveforms emerge from synchronized rhythmic activity of neuronal populations:
1. Alpha rhythm (8–13 Hz): Generated when the occipital cortex is at rest (eyes closed); thought to be modulated by thalamic pacemaker activity.
2. Beta rhythm (13–30 Hz): Associated with active, alert states and cortical processing.
3. Delta (<4 Hz) and theta (4–8 Hz): Prominent during sleep or drowsiness.
The balance of excitatory and inhibitory postsynaptic potentials across large neural networks leads to the waxing and waning of these oscillations.
Thalamocortical Interactions
A central element in EEG generation is the interaction between the cortex and the thalamus:
✔ The thalamus acts as a pacemaker, coordinating widespread cortical rhythms.
✔ Thalamic relay neurons, via excitatory and inhibitory loops, help synchronize cortical pyramidal neurons, creating stable rhythmic patterns like the alpha rhythm.
This interplay explains why certain EEG patterns are modulated by attention, arousal, and sleep.
Other Contributors and Influences
While cortical pyramidal neurons are the main source of EEG signals:
✔ Other neurons (e.g., stellate cells) and subcortical structures (e.g., hippocampus) also contribute to specific rhythms.
✔ Neurotransmitter systems (e.g., GABAergic inhibition, glutamatergic excitation) shape the excitability of cortical networks.
✔ Physiological factors such as age, metabolic state, and alertness also influence EEG patterns.
To simplify, the physiological basis of EEG rests on:
✅ The large-scale summation of synchronous postsynaptic potentials from cortical pyramidal neurons,
✅ Their vertical alignment creates open dipoles,
✅ The role of thalamocortical networks in coordinating rhythms,
✅ And the transmission of these potentials through brain tissues to the scalp.
EEG thus provides a real-time window into the collective electrical activity of the brain’s cortical networks, offering insights into states of wakefulness, sleep, cognition, and pathological processes such as epilepsy.
Procedure of Electroencephalography (EEG)
EEG is a safe and painless procedure with no side effects. The electrodes only record signals; they do not send any electricity into the brain.
1. Preparation Before the Test
Before the EEG begins, the patient is usually asked to wash their hair and avoid using oils, conditioners, or hair sprays, as these can interfere with electrode adhesion. The patient is also advised to avoid caffeine or stimulant medications that might alter brain activity unless otherwise instructed by the doctor. In some cases, patients may be asked to come sleep-deprived if the EEG aims to capture sleep patterns or provoke certain brain activities.
The procedure is explained to the patient to reduce anxiety, and the patient is seated comfortably or asked to lie down in a reclined position in a quiet room with dim lighting to facilitate relaxation.
2. Placement of Electrodes
The core of the EEG procedure involves attaching small metal discs called electrodes to the patient’s scalp. These electrodes detect the electrical signals produced by neurons in the cerebral cortex.
Typically, between 16 and 25 electrodes are used, although some advanced systems can have many more (32, 64, or even 128 electrodes) to provide higher-resolution recordings.
The placement of electrodes follows the internationally standardized 10–20 system, which ensures consistent and reproducible positioning relative to skull landmarks like the nasion (bridge of the nose) and inion (back of the skull).
To ensure good contact, the technician gently abrades the scalp and applies a conductive paste or gel under each electrode. Sometimes, an elastic cap with built-in electrodes is used for convenience.
3. Recording Brain Activity
Once the electrodes are secured and connected to the EEG machine, the actual recording begins. The machine amplifies the very small electrical signals (in the range of microvolts) and records them digitally.
During the recording:
A. The patient is usually asked to relax, stay still, and keep their eyes closed to reduce movement artifacts.
B. At various times, the technician may ask the patient to open and close their eyes, breathe deeply and rapidly (hyperventilate), or look at flashing lights (photic stimulation). These activation procedures are used to provoke changes in brain activity that might help reveal abnormalities, such as epileptic discharges.
C. In sleep EEG studies, the patient may be encouraged to nap, or recordings might be performed overnight.
The EEG typically lasts from about 20 to 40 minutes for standard recordings. Longer recordings or video EEG monitoring (lasting hours or days) may be performed in hospitals, especially for diagnosing epilepsy.
Ending the Test and Post-Procedure
After the recording is complete:
✔ The electrodes and conductive paste are carefully removed.
✔ The scalp is cleaned to remove any remaining paste.
✔ The patient can return to normal activities immediately, as the procedure is non-invasive and painless.
The recorded waveforms are then reviewed by a neurologist or clinical neurophysiologist, who interprets the patterns of brain activity to help diagnose or monitor various neurological conditions.
EEG is a safe, non-invasive procedure involving careful preparation, electrode placement following the 10–20 system, recording of brain electrical activity under both resting and stimulated conditions, and detailed interpretation of the data.(alert-passed)
Electroencephalography (EEG) Leads and Electrodes
In electroencephalography (EEG), electrodes are small metal discs placed on the scalp to detect the brain's electrical activity. These electrodes are attached to the scalp using adhesive paste, conductive gel, or a specialized EEG cap to ensure good electrical contact. The electrical signals picked up by the electrodes are extremely small (in the microvolt range) and are transmitted to the EEG machine through cables or electrode wires. The EEG amplifier then magnifies these signals for recording and analysis.
The term “lead” in EEG refers to the difference in electrical potential measured between two electrodes. For example, a lead might represent the voltage difference between electrode Fp1 and electrode F3. The EEG machine records multiple such leads, each corresponding to different pairs of electrodes, providing a comprehensive map of the brain’s electrical activity.
Electrode Placement and the International 10–20 System
Standard EEG recordings typically use 21 electrodes arranged on the scalp according to the International 10–20 system, a globally recognized method for electrode placement. This system uses a combination of letters and numbers to designate electrode sites, ensuring consistency across EEG tests. The letters indicate the brain region underneath:
F = Frontal
C = Central
P = Parietal
O = Occipital
T = Temporal
Fp = Frontopolar (very front of the scalp)
Z = Midline (central line of the scalp)
Numbers differentiate the left (odd numbers) and right (even numbers) hemispheres. For example, Fp1 is the frontopolar electrode on the left side, while Fp2 is on the right.
Common EEG Electrode Sites and Their Functions
Fp1 and Fp2: Positioned on the forehead above the eyes, these electrodes measure activity in the frontal lobes, which govern functions like decision-making, planning, and emotional regulation.
F3 and F4: Located on the left and right sides of the frontal scalp, these electrodes reflect prefrontal cortex activity, involved in attention and working memory.
C3 and C4: Placed near the vertex (top center) of the scalp, they record from the central cortex, associated with motor control.
P3 and P4: Positioned over the parietal lobes at the back sides of the scalp, these electrodes monitor sensory processing and spatial awareness.
O1 and O2: Located at the rear of the head, these electrodes pick up activity in the occipital lobes, which are responsible for visual processing.
T3 and T4: Positioned above the ears on the temporal lobes, they reflect auditory processing and memory functions.
F7 and F8: Situated slightly in front of T3 and T4, these electrodes also cover portions of the frontal and temporal lobes involved in language and emotional processing.
Specialized EEG Electrodes
Besides the standard scalp electrodes, intracranial electrodes may be used in invasive EEG procedures. These include:
🔹 Subdural electrodes: Placed beneath the dura mater (the brain’s outer membrane) for detailed cortical recordings.
🔹 Depth electrodes: Inserted deeper into brain tissue to localize abnormal electrical activity, often used in epilepsy surgery evaluations.
These invasive electrodes provide higher spatial resolution but are used only when clinically necessary.
Each EEG electrode captures the summed electrical activity of underlying brain regions, and the leads formed between electrodes provide detailed information on the brain’s electrical function. The standardized 10–20 system ensures consistent placement, allowing clinicians to interpret EEG recordings reliably across patients and time.(alert-passed)
Electroencephalography (EEG) Waveforms
Electroencephalography (EEG) captures the brain’s electrical activity in the form of rhythmic oscillations known as brain waves or EEG waveforms. These waveforms vary in frequency, amplitude, and spatial distribution, and each pattern provides valuable insight into brain function, level of consciousness, and potential neurological disorders. EEG waveforms are broadly classified into several frequency bands: delta, theta, alpha, beta, and gamma. Each has characteristic features and clinical significance.
Delta Waves (0.5–4 Hz)
Delta waves are the slowest brain waves, with frequencies below 4 Hz and usually the highest amplitudes (up to several hundred microvolts). They are most prominent during deep sleep (slow-wave sleep or NREM stages 3–4) in healthy adults. In awake individuals, the presence of prominent delta waves over localized brain regions may indicate pathology, such as brain lesions, tumors, or areas of cerebral dysfunction. In infants and young children, delta activity is more common and considered normal.
Theta Waves (4–8 Hz)
Theta waves are slower waves typically seen in children and adolescents and during drowsiness, light sleep, and certain meditative states in adults. They often appear in the temporal and parietal regions. In awake adults, an excess of theta waves can sometimes suggest emotional stress, fatigue, or certain neurological conditions. Theta activity is also associated with creativity, memory encoding, and exploratory behaviors.
Alpha Waves (8–13 Hz)
Alpha waves are among the most recognized EEG rhythms, prominent over the occipital and parietal regions, especially when a person is awake, relaxed, and has their eyes closed. These waves typically attenuate or "block" when the eyes open or during mental effort—a phenomenon known as alpha blocking or desynchronization. Alpha rhythms are considered a marker of relaxed wakefulness, and their absence or asymmetry may point to brain injury, tumors, or other focal brain disorders.
Beta Waves (13–30 Hz)
Beta waves are faster, low-amplitude rhythms seen predominantly over the frontal and central areas when a person is awake, alert, and engaged in mental activity or problem-solving. They often increase with concentration, stress, or anxiety. Beta activity may also become prominent during pharmacological sedation or in individuals using certain medications like benzodiazepines and barbiturates. Excessive beta can sometimes indicate cortical irritation.
Gamma Waves (>30 Hz, often 30–100 Hz)
Gamma waves are the fastest EEG frequencies and have relatively low amplitude. They are less consistently recorded on standard EEGs but are believed to play roles in higher cognitive functions, such as perception, attention, memory integration, and consciousness. Research suggests gamma activity may help synchronize information across different brain regions.
Significance in Clinical and Research Settings
Each EEG waveform and its pattern help neurologists assess brain function and identify abnormalities. For example:
✔ Persistent focal delta activity can indicate structural brain lesions.
✔ Abnormal theta or delta activity during wakefulness might suggest encephalopathy or diffuse brain dysfunction.
✔ Altered alpha rhythms may signal stroke, tumor, or degenerative disease.
✔ Excessive fast activity (beta) might relate to medication effects or cortical hyperexcitability.
In epilepsy, specific waveforms such as spikes, sharp waves, or spike-and-wave complexes are diagnostic markers for seizure activity and epilepsy syndromes.
EEG waveforms are not just random patterns—they reflect the brain’s functional state and organization. By analyzing their frequency, amplitude, distribution, and reactivity to stimuli, clinicians can gain critical insights into neurological health, diagnose disorders, and monitor treatment responses.(alert-passed)
Interpretation for Electroencephalography (EEG) Results
The results of the EEG test are interpreted by a specialist, such as a neurologist. The specialist evaluates the patterns of electrical activity in the brain, looking for abnormalities such as seizures, brain injuries, or sleep disorders.
Clinical Applications of Electroencephalography (EEG)
EEG is used to evaluate brain activity and diagnose a wide range of neurological conditions, including epilepsy, sleep disorders, and brain injuries. The test can also be used to evaluate the effectiveness of treatments for these conditions.
A. Diagnosis and Management of Epilepsy
One of the most widely recognized clinical uses of EEG is in the diagnosis and management of epilepsy. EEG helps detect abnormal, excessive, or synchronous neuronal activity, which appears as characteristic epileptiform discharges, such as spikes, sharp waves, or spike-and-wave complexes. These patterns guide neurologists in confirming the diagnosis of epilepsy, classifying seizure types, and determining the underlying epilepsy syndrome. Long-term video EEG monitoring is particularly useful for pre-surgical evaluation, helping to localize the seizure focus when medications fail to control seizures.
B. Assessment of Altered Consciousness and Encephalopathies
EEG is invaluable in evaluating patients with altered mental status, such as coma, delirium, or stupor. Specific EEG patterns help identify causes like metabolic encephalopathy, anoxic injury, or toxic encephalopathy. For instance, generalized slowing of background rhythms often points to diffuse cerebral dysfunction, while triphasic waves are associated with hepatic encephalopathy. EEG also assists in detecting non-convulsive status epilepticus, a condition in which ongoing seizure activity causes unconsciousness without visible convulsions.
C. Brain Death Confirmation
EEG plays a role in confirming brain death, particularly when clinical criteria cannot be fully applied. An isoelectric or flat EEG, showing no electrical brain activity over a defined period, supports the diagnosis of irreversible cessation of cerebral function. This application is guided by strict medical protocols and complements other confirmatory tests like cerebral angiography or transcranial Doppler ultrasound.
D. Sleep Disorders and Sleep Medicine
EEG is central to sleep studies, especially polysomnography, where it monitors the electrical activity of the brain alongside other physiological parameters. EEG helps differentiate the stages of sleep, identify abnormal sleep architecture, and detect sleep-related disorders like narcolepsy, obstructive sleep apnea, and parasomnias. It also reveals nocturnal seizures that may go unnoticed during wakefulness.
E. Monitoring in Critical Care and Anesthesia
In intensive care units (ICUs), continuous EEG monitoring is increasingly used to detect subclinical seizures, guide sedation depth, and assess cerebral function in critically ill patients. EEG also assists anesthesiologists by monitoring brain activity during surgery to maintain adequate but safe levels of anesthesia, especially in high-risk neurosurgical procedures.
F. Evaluation of Cognitive and Developmental Disorders
In pediatrics and psychiatry, EEG contributes to assessing children with developmental delay, autism spectrum disorder, or attention-deficit/hyperactivity disorder (ADHD). Although not diagnostic on its own, EEG may reveal abnormalities like diffuse slowing or epileptiform discharges that inform further evaluation and management.
G. Localization of Brain Lesions
EEG can help localize structural brain lesions, such as tumors, vascular malformations, or areas of prior infarction. Focal slowing of background rhythms, asymmetric amplitude, or focal epileptiform discharges may correspond to the lesion’s anatomical location, complementing neuroimaging techniques like MRI or CT.
H. Research and Cognitive Neuroscience
Beyond clinical medicine, EEG has applications in research, where it is used to study normal brain function, sensory processing, language, attention, and emotion. Its excellent temporal resolution makes it ideal for investigating rapid neural dynamics, such as event-related potentials (ERPs) that reflect brain responses to specific stimuli.
I. Evaluation of Head Injuries and Post-Concussive States
EEG can be part of the assessment of traumatic brain injury (TBI), especially when patients present with unexplained changes in consciousness, behavior, or new seizures. While imaging provides structural details, EEG can detect functional disturbances caused by injury.
J. Functional Brain Mapping
In surgical planning, particularly for epilepsy surgery or tumor resection, invasive EEG techniques (such as electrocorticography or depth electrodes) help identify eloquent cortical areas critical for speech, movement, or sensation, reducing the risk of postoperative deficits.
Recent Advances in EEG Technology
Electroencephalography (EEG) has long been valued for its ability to record brain activity in real time. In recent years, significant technological and analytical advances have transformed EEG from a traditional diagnostic tool into a dynamic platform for clinical and research innovation.
1. High-Density and Wearable EEG
One major advancement is high-density EEG, which uses up to 256 electrodes, providing more detailed spatial resolution and a clearer picture of cortical activity. At the same time, wearable EEG devices have become lighter and wireless, making it possible to monitor brain activity during daily life and outside clinical settings. These wearable systems open new possibilities for long-term monitoring of conditions like epilepsy, sleep disorders, and mood changes.
2. Advanced Signal Processing and AI
The integration of machine learning and artificial intelligence (AI) into EEG analysis has revolutionized how data is interpreted. AI algorithms can detect subtle patterns and predict seizures, diagnose neurological conditions faster, and even personalize treatment plans. Sophisticated signal processing techniques also help remove artifacts and improve the accuracy of EEG recordings.
3. Functional Connectivity and Brain Networks
New methods for analyzing functional connectivity allow researchers to study how different regions of the brain interact over time. This approach goes beyond looking at isolated electrical activity and helps reveal complex brain network dynamics involved in cognition, memory, and psychiatric disorders.
4. EEG in Brain-Computer Interfaces (BCIs)
EEG has become central to brain-computer interface (BCI) technology, which translates brain signals into commands to control external devices. This development holds promise for people with severe paralysis, allowing them to communicate or control prosthetic limbs using only their brain activity.
5. Multimodal Imaging
Finally, combining EEG with other imaging methods like fMRI, PET, or MEG has improved understanding of brain function. This multimodal approach provides both high temporal resolution from EEG and high spatial resolution from other imaging, offering a more complete view of brain activity.
These recent advances highlight the ongoing evolution of EEG, making it more accurate, accessible, and powerful than ever before, benefiting both clinical practice and neuroscience research.(alert-passed)
Electroencephalography remains an indispensable tool in clinical neurology and neuroscience research. Despite newer imaging technologies, EEG’s ability to non-invasively record real-time brain activity makes it uniquely valuable. With ongoing technological advances, its applications are expanding beyond diagnosis to include therapy monitoring, neurorehabilitation, and even direct brain-computer interfacing.
