Electroencephalography

Electroencephalography, commonly known as EEG, is a method used to record the spontaneous electrical activity of the brain.

This process produces an electrogram, which is a visual representation of the brain's bio signals. These signals represent the postsynaptic potentials of pyramidal neurons located in the neocortex and allocortex. EEG is a vital tool in both clinical medicine and neuroscience research. It helps doctors diagnose various conditions and allows scientists to study how the brain functions in real time.

The mechanism of EEG involves detecting voltage fluctuations using electrodes. In a standard scalp EEG, these electrodes are placed along the scalp using the International 10–20 system. The electrical activity originates in neurons within the underlying brain tissue. As these signals travel toward the surface, they must pass through intermediary tissues and bones. These biological materials act similarly to resistors and capacitors in an electrical circuit. Consequently, the recorded value is often distorted by these layers. The signal strength also varies based on the orientation and distance of the neurons to the electrodes. This means the EEG primarily reflects the activity of cortical neurons near the scalp. Deep structures like the thalamus, hippocampus, and brain stem do not contribute directly to a scalp EEG.

Researchers categorize brain activity into specific frequency groups. A healthy human EEG typically shows frequencies between 1 and 30 Hz. The amplitudes, or the strength of the signal, usually vary between 20 and 100 μV. These frequencies are subdivided into four main types of waves. Alpha waves range from 8 to 13 Hz and appear during relaxed wakefulness.

Beta waves are faster, ranging from 13 to 30 Hz, and occur during intense mental activity.

Theta waves move between 4 and 7 Hz.

Delta waves are the slowest, ranging from 0.5 to 4 Hz.

If theta or delta waves appear while a person is awake, it may indicate brain dysfunction.

History shows that the ability to record these signals changed how we understand the mind. One of the first human EEG recordings was obtained by Hans Berger.

Since his early work, the technology has evolved into a sophisticated diagnostic standard. While EEG was once a primary method for diagnosing tumors or strokes, its role has shifted. High-resolution anatomical imaging, such as MRI and CT scans, now handles many of those tasks. However, EEG remains essential because of its unique temporal resolution. It can capture changes in the millisecond range, a speed that MRI, PET, or CT scans cannot match.

In clinical settings, EEG is the gold standard for confirming epilepsy. Doctors look for abnormal discharges like sharp waves, spikes, or spike-and-wave complexes.

A routine EEG usually lasts 20 to 30 minutes. While the sensitivity of a routine scan is between 29% and 55%, the specificity is very high. This means if an EEG shows these specific abnormal patterns, it almost certainly confirms epilepsy. For more detailed study, patients may stay in an Epilepsy Monitoring Unit (EMU). In an EMU, doctors perform ictal recordings to capture actual seizures. These hospital stays can last several days or even a week.

Beyond epilepsy, EEG is used to monitor many different states of health. It helps diagnose sleep disorders, the depth of anesthesia, and coma. It is also used to evaluate encephalopathies and cerebral hypoxia following cardiac arrest. In intensive care units, it can monitor for non-convulsive seizures. For patients needing higher precision, surgeons may use electrocorticography (ECoG). This involves the surgical placement of electrodes under the dura mater. ECoG provides better spatial resolution and can detect high-frequency components that scalp EEGs miss. This is especially important when determining if a patient needs surgery to treat epilepsy.

Finally, EEG serves as a powerful tool in cognitive science and psychology. Scientists use derivatives of the technique, such as event-related potentials (ERPs). ERPs involve averaging EEG activity that is time-locked to a specific stimulus, like a sound or a light. This allows researchers to see how the brain processes complex information. While some research continues into areas like ADHD and concussion, the EEG remains a fundamental pillar of neuroscience. Its ability to provide a mobile and fast window into the brain's electrical life is unmatched by most other technologies.