Electroencephalography (EEG)

A non-invasive method for recording the electrical activity of the brain, characterized by high temporal resolution.

Key takeaway: EEG primarily measures the post-synaptic potentials of pyramidal neurons in the cerebral cortex. Because these cells are oriented perpendicular to the brain surface, their synchronized activity generates an electrical dipole measurable at the scalp.

Physiological Basis

Signal Origin Action potentials vs. Post-synaptic potentials
- Unlike extracellular microelectrodes, EEG does not record individual action potentials (spikes). Spikes are too fast and highly localized.
- EEG records the slower Excitatory and Inhibitory Post-Synaptic Potentials (EPSPs/IPSPs). These potentials last longer (tens of milliseconds), allowing them to summate over thousands of cells.
Volume Conduction The spatial blur problem.
- Electrical signals must travel through the cerebrospinal fluid, pia mater, arachnoid mater, dura mater, skull, and scalp.
- The skull acts as a strong low-pass spatial filter (it smears the electrical signal), dramatically reducing the spatial resolution to several centimeters.

Clinical & BCI Applications

Clinical Diagnostics Standard paradigms in neurology.
- Epilepsy: Identifying intervertebral spikes and localizing seizure foci.
- Sleep Studies (Polysomnography): Tracking transitions between sleep stages (e.g., assessing Delta waves).
Non-Invasive Brain-Computer Interfaces Translating thought to action.
- P300 Spellers: Rely on the P300 event-related potential (ERP), an oddball response occurring ~300ms after a target stimulus.
- Motor Imagery (SMR): Users imagine moving a hand or foot, modulating the mu (8-12 Hz) and beta (13-30 Hz) bands over the sensorimotor cortex to control cursors or robots.
- SSVEP: Steady-State Visually Evoked Potentials. Flickering visual stimuli drive the visual cortex to oscillate at exactly that flicker frequency.

Hardware & Electrodes

Electrode Types Wet vs. Dry systems.
- Wet / Ag-AgCl: The gold standard. Uses conductive gel or paste to reduce skin-electrode impedance.
- Dry Electrodes: Uses conductive pins or conductive polymers to bypass the need for gel, trading some signal-to-noise ratio for rapid setup time and comfort.
Spatial Resolution (10-20 System) Standardized placement.
- The International 10-20 system ensures reproducible placement of electrodes based on skull landmarks (Nasion, Inion). Modern high-density systems can utilize up to 256 channels.

Interactive EEG Trace Simulator

Simulate raw electrical potential differences from the scalp. Select a dominant brain state or inject common clinical artifacts.

Dominant Brain State (Frequency Band):

Deep Sleep (Delta 1-4 Hz) Drowsy (Theta 4-8 Hz) Relaxed, eyes closed (Alpha 8-13 Hz) Focused, active (Beta 13-30 Hz) High cognition (Gamma 30+ Hz)

― Fp1 (Frontal Pole - near eyes) ― Cz (Central - top of head) ― O1 (Occipital - visual cortex)