Electroencephalography, commonly known as EEG, is a technique for recording the electrical activity of the brain by placing sensors on the scalp. The brain's billions of neurons communicate using tiny electrical pulses, and EEG captures …
Every neuron in your brain maintains an electrical charge across its membrane, typically around -70 millivolts when at rest. When a neuron fires, charged ions rush through channels in the membrane, creating a tiny electrical current that flows through the neuron and into the surrounding tissue. A single neuron's signal is far too weak to detect from outside the skull, but when thousands of neurons fire together in synchronized waves, their individual electrical fields add up like ripples merging into larger waves on a pond.
The neurons most responsible for EEG signals are pyramidal cells in the brain's cortex, arranged perpendicular to the skull surface like trees in a forest. When these neurons activate together—triggered by sensory input, thoughts, or sleep cycles—they create electrical dipoles with positive charges at one end and negative at the other. These aligned dipoles generate electrical fields strong enough to conduct through brain tissue, cerebrospinal fluid, skull bone, and finally reach the scalp where electrodes wait to detect them.
The synchronization matters more than the raw number of neurons firing. When neurons fire randomly, their electrical fields cancel each other out like crowd noise in a stadium. But during specific brain activities—processing visual information, entering deep sleep, or having a seizure—large populations of neurons fire in rhythmic patterns, creating the coherent electrical waves that EEG can actually measure.
EEG electrodes are typically small metal discs made of silver, gold, or tin, placed at standardized positions across the scalp according to the International 10-20 system. This system divides the skull into proportional distances, ensuring that electrode placement remains consistent between different people and different testing sessions. Each electrode is filled with conductive gel that bridges the gap between the metal and skin, reducing electrical resistance that would otherwise block the weak brain signals.
The electrodes don't measure absolute electrical activity—they measure voltage differences between pairs of locations. One common approach compares each electrode's signal to a reference electrode placed on the ear or mastoid bone behind the ear. Another method compares neighboring electrodes to each other, revealing how electrical activity flows between adjacent brain regions. These voltage differences typically range from 10 to 100 microvolts—roughly one-millionth the voltage of a AA battery.
Modern EEG systems use anywhere from 19 to 256 electrodes distributed across the scalp. More electrodes provide better spatial resolution, helping pinpoint which brain region generates a particular signal. However, because the skull blurs electrical signals as they pass through, EEG's spatial precision remains limited to roughly 2-3 centimeters at best, no matter how many electrodes you add.
The electrical signals emerging from the brain arrive at the scalp incredibly weak—typically 10 to 100 microvolts, smaller than the electrical noise from nearby power lines, muscle twitches, or even heartbeats. Before these signals can be digitized and analyzed, they must be amplified by factors of 10,000 to 100,000 times, transforming microvolts into the millivolt range that analog-to-digital converters can accurately measure. This amplification must be done carefully to preserve the brain's genuine signals while avoiding distortion or introducing additional noise.
EEG amplifiers operate as differential amplifiers, meaning they amplify the voltage difference between two inputs while rejecting voltages that appear equally at both inputs. This design is crucial because external electrical noise—from fluorescent lights, power outlets, or radio signals—affects all electrodes similarly. By amplifying only the differences between electrode pairs, the amplifier automatically cancels out this common noise, much like noise-canceling headphones work.
The amplifiers also provide electrical isolation between the patient and the recording equipment, preventing any possibility of electrical shock. Modern EEG systems digitize the amplified signals at rates of 250 to 2,000 samples per second, capturing rapid brain wave fluctuations while creating manageable file sizes for storage and analysis.
Raw EEG signals contain a jumble of electrical activity from multiple sources: genuine brain waves, muscle tension in the scalp and face, eye movements, heartbeats, and environmental electromagnetic interference. Digital filters act like sieves, allowing brain signals within specific frequency ranges to pass through while blocking unwanted frequencies. A high-pass filter removes slow drifts caused by electrode movement or sweat, typically blocking frequencies below 0.5-1 Hz. A low-pass filter removes high-frequency noise from muscles and electrical equipment, typically blocking frequencies above 30-70 Hz.
The most troublesome interference comes from electrical power lines, which create a strong 50 Hz signal in most countries or 60 Hz in North America. A notch filter specifically targets and removes this narrow frequency band without affecting nearby frequencies. However, excessive filtering risks removing genuine brain signals, so technicians must balance noise reduction against signal preservation. Some researchers prefer minimal filtering during recording, saving aggressive cleanup for later analysis.
Beyond digital filters, proper technique prevents noise from contaminating recordings in the first place. Patients are asked to relax facial muscles, minimize eye movements, and sit still. Electrodes are carefully applied to reduce impedance—the resistance to electrical flow—between scalp and metal. High impedance allows more external noise to infiltrate the recording, so technicians aim for impedance values below 5,000 ohms for each electrode.
The filtered and amplified EEG signal displays as a series of squiggly lines, each representing voltage fluctuations at one electrode over time. The most fundamental analysis involves identifying different brain wave frequencies, each associated with distinct mental states. Delta waves (0.5-4 Hz) dominate during deep sleep. Theta waves (4-8 Hz) appear during drowsiness and meditation. Alpha waves (8-13 Hz) emerge when you're awake but relaxed with eyes closed. Beta waves (13-30 Hz) characterize active thinking and concentration. Gamma waves (above 30 Hz) correlate with intense cognitive processing.
Neurologists read EEG tracings like musicians read sheet music, recognizing normal patterns and spotting abnormal signatures. Epileptic seizures produce characteristic spikes and sharp waves—sudden high-amplitude deflections that stand out dramatically from background activity. Sleep disorders reveal themselves through disrupted progression of sleep stages. Brain injuries or tumors create localized slow-wave activity in affected regions. Encephalitis and other infections produce diffuse abnormal patterns across multiple channels.
Modern computer algorithms assist interpretation by automatically detecting specific patterns, calculating the power within different frequency bands, and comparing activity between brain hemispheres. Advanced techniques like event-related potentials examine brain responses to repeated stimuli—flashes of light, spoken words, or decision-making tasks—revealing cognitive processing that isn't visible in resting EEG. These averaged responses can measure attention, memory formation, and even detect awareness in patients who appear unresponsive.
EEG's temporal resolution—its ability to track changes in milliseconds—remains unmatched by other brain imaging techniques. While MRI and PET scans show detailed brain anatomy and metabolism, only EEG captures the rapid electrical rhythms underlying moment-to-moment brain function, making it indispensable for diagnosing epilepsy, monitoring surgical anesthesia, and researching how the brain processes information in real time.