Electroencephalography (EEG) records the brain's electrical activity through electrodes placed on the scalp. Because neural electrical signals propagate at near-instantaneous speed, EEG provides millisecond temporal resolution — far superior to fMRI — enabling researchers to track the time course of cognitive processes in real time. The trade-off is poorer spatial resolution (centimeters) due to the smearing of electrical signals through the skull and scalp.
Oscillations and Frequency Bands
EEG signals contain oscillations at characteristic frequency bands: delta (0.5-4 Hz, deep sleep), theta (4-8 Hz, memory, navigation), alpha (8-13 Hz, idle visual cortex, attention), beta (13-30 Hz, motor planning, active thinking), and gamma (30-100 Hz, binding, consciousness). These oscillations are thought to coordinate neural activity across brain regions, and changes in their power and coherence index different cognitive states.
By averaging EEG signals time-locked to repeated events (stimuli, responses), researchers extract event-related potentials (ERPs) — voltage changes reflecting specific stages of cognitive processing. The N400 indexes semantic processing, the P300 indexes attention and working memory updating, the N170 indexes face processing, and the error-related negativity (ERN) indexes error detection. ERPs provide precise temporal information about when different cognitive operations occur.