1. Neuronal Firing and Electrical Potentials

Neurons communicate through electrical impulses called action potentials. When a neuron is activated, it generates a small electrical current, which affects the surrounding extracellular space. This electrical activity from a large group of neurons creates a cumulative extracellular potential, which can be detected by EEG electrodes placed on the scalp. These potentials are typically produced by synchronized post-synaptic potentials (EPSPs and IPSPs) in neurons.

2. Synaptic Activity and EPSPs/IPSPs

Excitatory Postsynaptic Potentials (EPSPs) and Inhibitory Postsynaptic Potentials (IPSPs) are the primary contributors to the signals seen on an EEG. When excitatory synaptic transmission occurs, positive ions (like sodium) flow into the neuron, causing a depolarization (EPSP), and when inhibitory synapses are activated, negative ions (like chloride) flow in, causing hyperpolarization (IPSP).

These electrical changes occur at the dendrites of neurons and, when they occur in large, synchronized groups, create dipoles that produce measurable electrical fields.

3. Dipoles and EEG Measurement

The summed electrical activity from multiple neurons creates a dipole, which consists of a positive charge in one area and a negative charge in a neighboring area. Dipoles aligned perpendicularly to the cortical surface create stronger EEG signals, while those parallel to the cortex (or tangential to the scalp) result in weaker signals. The distribution and orientation of these dipoles influence the strength, frequency, and pattern of the EEG signal, which can be detected by surface electrodes.

4. Brain Waves and EEG Frequencies

EEG signals are characterized by various frequency bands that correspond to different types of brain activity:

  • Delta waves (0.5–4 Hz): Linked to deep sleep and restorative brain functions.
  • Theta waves (4–8 Hz): Associated with light sleep, relaxation, and meditative states.
  • Alpha waves (8–12 Hz): Occur during quiet wakefulness and relaxation.
  • Beta waves (12–30 Hz): Linked to active thinking, problem-solving, and focus.
  • Gamma waves (30–100 Hz): Associated with higher cognitive functions like attention and perception.

5. Spontaneous and Evoked Activity

Spontaneous activity refers to the brain's ongoing electrical activity at rest, which can vary depending on the mental state (awake, asleep, relaxed, focused).
Evoked potentials or event-related potentials (ERPs) occur in response to specific stimuli or tasks and are used to study sensory processing, attention, and cognitive functions. These signals are typically small and require averaging over many trials to detect.

6. Clinical Relevance

Abnormalities in EEG patterns can indicate brain disorders. For instance:

  • Seizures: Marked by abnormal synchronization of neural activity, leading to distinct patterns like spikes or sharp waves.
  • Epileptic activity: Can be localized using EEG to pinpoint the regions responsible for abnormal discharges.
  • Sleep disorders and neurodegenerative conditions (like Alzheimer's) can be monitored using EEG by studying changes in brain wave patterns.