The electroencephalogram (EEG) — the recording of electrical potentials across the scalp — is one of the oldest and most widely used tools in cognitive neuroscience. Since Hans Berger's first human EEG recording in 1929, this technique has provided invaluable insights into brain function by capturing the electrical activity of large populations of neurons with exquisite temporal resolution. While neuroimaging methods like fMRI offer superior spatial resolution, EEG's ability to track neural activity on a millisecond timescale makes it uniquely suited for studying the rapid, dynamic processes that characterize cognition — attention, perception, language processing, and decision-making.
Key Structures
- Cortex (surface) — The outer layer of the cerebrum composed of layered neural tissue supporting all higher cognitive functions, particularly in relation to surface.
- Memory Consolidation — The process by which newly formed, fragile memories are stabilized into durable long-term representations, involving molecular changes, sleep, and systems-level reorganization.
- Working Memory — A limited-capacity system for temporarily holding and manipulating information during complex cognitive tasks such as reasoning, comprehension, and learning.
- Event-Related Potentials — Voltage changes in the EEG time-locked to specific events, providing precise temporal markers of cognitive processes including attention, perception, language, and memory.
- fMRI — Functional magnetic resonance imaging, a neuroimaging technique that measures brain activity by detecting changes in blood oxygenation, providing detailed spatial maps of which brain regions are engag.
Key Functions
- Records electrical potentials generated by synchronized neuronal activity.
- used to study brain states, sleep, and cognition.
The Neural Basis of EEG
EEG electrodes placed on the scalp detect voltage fluctuations generated primarily by the synchronous postsynaptic potentials of large populations of cortical pyramidal neurons. Because individual neurons generate tiny electrical fields, EEG requires the coordinated activity of tens of thousands of neurons oriented in parallel (perpendicular to the cortical surface) to produce a measurable scalp signal. This means EEG is most sensitive to activity in cortical gyri (where neurons are oriented radially toward the scalp) and less sensitive to activity in sulci or deep brain structures.
Event-Related Potentials (ERPs)
By time-locking EEG recordings to the onset of stimuli or responses and averaging across many trials, researchers extract event-related potentials (ERPs) — voltage waveforms reflecting the brain's systematic processing of events. Specific ERP components have been associated with distinct cognitive processes. The P300 (a positive deflection ~300 ms after stimulus onset) reflects the updating of working memory and the evaluation of stimulus significance. The N400 (a negative deflection ~400 ms) indexes semantic processing difficulty — it is larger for semantically unexpected words. The N170 reflects face-specific processing. These components provide precise temporal markers for when specific cognitive operations occur.
EEG also reveals oscillatory brain rhythms — rhythmic patterns of neural activity at characteristic frequencies. Alpha waves (8–12 Hz) increase during relaxed wakefulness with eyes closed and decrease during active visual processing ("alpha suppression"). Theta waves (4–8 Hz) are prominent during memory encoding and retrieval. Gamma waves (30–100 Hz) may reflect the binding of features into unified perceptual objects. Beta waves (13–30 Hz) are associated with active thinking and motor planning. Delta waves (0.5–4 Hz) dominate slow-wave sleep, which is critical for memory consolidation. The study of these oscillations has become a major focus of cognitive neuroscience.
Advantages and Limitations
EEG's temporal resolution (milliseconds) far exceeds that of fMRI (seconds), making it indispensable for studying rapid cognitive processes. It is also relatively inexpensive, portable, non-invasive, and silent — making it suitable for studying infants, clinical populations, and naturalistic cognition. However, EEG's spatial resolution is limited (centimeters, compared to fMRI's millimeters) because electrical signals are blurred as they pass through the skull and scalp. Source localization — determining where in the brain an EEG signal originates — remains an ill-posed inverse problem, though modern methods (beamforming, dipole modeling) have improved spatial inference considerably.
Disorders
- Epilepsy (diagnostic gold standard) — A neurological condition characterized by recurrent unprovoked seizures due to abnormal excessive neural synchronization, particularly in relation to diagnostic gold standard.
- Sleep disorders — Conditions disrupting normal sleep patterns, impairing cognitive function, memory consolidation, and emotional regulation.
- Encephalopathy — A broad term for brain disease characterized by altered mental status, resulting from metabolic, toxic, or infectious causes.
- Used to detect brain death