Electroencephalography

Electroencephalography

Electroencephalography (EEG) involves recording electrical activity from the scalp. It measures voltage changes caused by ionic currents flowing within the brain's neurons. In clinical settings, EEG records the brain's spontaneous electrical activity for a short duration, typically 20–40 minutes, using multiple scalp electrodes.

Diagnostic applications mainly focus on the spectral content of EEG signals, indicating the type of neural oscillations present. In neurology, EEG is primarily used to diagnose epilepsy, as epileptic activity can cause distinct abnormalities in EEG readings.

Another clinical application of EEG is in diagnosing coma, encephalopathies, and brain death. While EEG was once a primary tool for diagnosing tumors, strokes, and other focal brain disorders, its use has declined with the introduction of high-resolution anatomical imaging techniques such as MRI and CT.

However, despite its limited spatial resolution, EEG remains valuable for research and diagnosis, particularly when precise temporal resolution in the millisecond range is required, which is not achievable with CT or MRI.

EEG derivatives include evoked potentials (EP), which involve averaging EEG activity time-locked to stimulus presentation (visual, somatosensory, or auditory). Event-related potentials (ERPs) refer to averaged EEG responses time-locked to more complex stimulus processing, commonly used in cognitive science, cognitive psychology, and psychophysiological research.

The brain's electrical charge is maintained by billions of neurons, polarized by membrane transport proteins that pump ions across their membranes. Neurons constantly exchange ions with the extracellular environment to maintain resting potential and propagate action potentials. When ions are pushed out of neurons simultaneously, they create a wave, known as volume conduction.

This wave reaches scalp electrodes, inducing voltage changes that can be measured. EEG reflects the synchronous activity of thousands or millions of neurons with similar spatial orientation, mainly pyramidal neurons of the cortex. Deep sources of activity are more challenging to detect due to the attenuation of voltage fields with distance.

Scalp EEG shows oscillations at various frequencies, each with characteristic features and associated with different brain states (e.g., wakefulness and sleep stages). These oscillations represent synchronized activity across neuronal networks. While some oscillations have well-understood underlying mechanisms (e.g., thalamocortical resonance for sleep spindles), many others remain poorly understood.

Studies correlating EEG with neuron spiking activity reveal a complex relationship, with surface EEG power primarily reflecting activity in gamma and delta bands.