1. Neuronal Activity
Neurons in the brain communicate via electrical signals that cause changes in the voltage across their membranes.
Postsynaptic Potentials (PSPs) occur when neurotransmitters bind to receptors, leading to:
- EPSPs (Excitatory): Depolarization (more positive inside the neuron).
- IPSPs (Inhibitory): Hyperpolarization (more negative inside the neuron).
2. Extracellular Voltage Changes
EPSPs and IPSPs create localized changes in extracellular voltage due to ion flow (mainly Na⁺, K⁺, Cl⁻). These changes create a dipole (a region of positive charge and a region of negative charge).
3. Summation
EEG electrodes do not detect a single neuron but the summation of many PSPs over a large area of the cortex.
These signals are strongest from synchronously active pyramidal neurons, which are organized in columns perpendicular to the cortex surface.
4. Cortical Depth Matters
Neuronal activity at different depths creates different extracellular voltage patterns:
- Surface EPSPs → Negative extracellular voltage near the electrode.
- Deep EPSPs → Positive extracellular voltage relative to the surface.
- The inverse applies to IPSPs.
5. Electrode Detection
The extracellular voltage differences create an electric field that the EEG electrode detects.
These detected signals reflect the spatial and temporal summation of PSPs.
6. Signal Processing
The raw voltage is amplified and filtered by the EEG machine.
Negative potentials are displayed as upgoing waves, and positive potentials as downgoing waves on the EEG tracing.
In essence, the EEG signal represents the collective, organized electrical activity of the brain's neurons, shaped by their spatial arrangement and synchronized firing.