NeuroLogic: Fundamentals

When you observe an EEG trace, you are not reading the direct "all-or-nothing" firing of individual axonal action potentials. Due to their brief duration (1 millisecond) and random spatial orientations, action potentials cancel each other out. Instead, scalp EEG measures the collective, synchronized summation of extracellular voltage fluctuations driven by Postsynaptic Potentials (PSPs).

These PSPs are transient, graded potential shifts occurring at the dendrites and soma of cortical neurons. Individually, a single postsynaptic potential is microscopic (~0.1–2 mV). However, when thousands of aligned neurons undergo postsynaptic changes simultaneously, their individual electrical fields summate, projecting a detectable signal through the meninges, cerebrospinal fluid, skull, and scalp.

Scalp detection generally requires synchronous activation across several square centimeters of cortex; often-cited experimental estimates begin around 6 cm², but detectability varies with source depth, orientation, synchrony, amplitude, and background activity. Volume conduction through CSF, skull, and scalp attenuates and spatially blurs cortical potentials.

Routine scalp EEG mainly reflects synchronized postsynaptic transmembrane currents in populations of similarly oriented cortical pyramidal neurons. Visibility at the scalp depends on synchrony, spatial extent, depth, orientation, tissue conductivity, montage, and background activity.

1. The "Open Field" Geometry

Pyramidal cells are often aligned approximately perpendicular to the local cortical surface, allowing synchronous currents to sum as an open field. More symmetric cellular arrangements tend to cancel at a distance and contribute less to scalp EEG; “open” and “closed” are useful models, not an absolute visible/invisible divide.

2. Extracellular Sinks and Sources

In a simplified example, inward positive current at an apical dendrite creates a local extracellular sink, while distributed return current forms extracellular sources elsewhere. Their spatial separation produces a current dipole. Real cortical generators combine excitatory and inhibitory currents at different cellular locations, so scalp polarity cannot be translated directly into “excitation” or “inhibition.”

The orientation of the dipole relative to the scalp surface determines whether and where a signal will be detected by scalp electrodes:

Consistency is key. The International 10-20 System uses percentages (10% and 20%) of measured scalp distances to place electrodes reproducibly across different head sizes. C3 and C4 lie near the left and right sensorimotor regions, but a scalp coordinate does not guarantee an exact cortical structure in every individual.

• Letters: F (Frontal), T (Temporal), C (Central), P (Parietal), O (Occipital).
• Numbers: Odd (Left), Even (Right), z (Midline).
• Impedance Check: Ordinarily kept at ≤ 5 kΩ and reasonably balanced, while following the equipment manufacturer and local laboratory protocol.

EEG electrode placement is not arbitrary. The International 10-20 System uses anatomical head measurements to ensure reproducibility. Electrodes are placed at 10% and 20% intervals along perpendicular cranial arcs between four key bony landmarks:

• Nasion: The deep depression at the bridge of the nose, directly between the eyes.
• Inion: The prominent bony ridge at the base of the occipital skull (external occipital protuberance).
• Left & Right Preauricular Points: The depressions just anterior to the tragus of each ear.

Understanding the Electrode Codes:

• Letters represent underlying cerebral zones: Fp (Frontopolar), F (Frontal), T (Temporal), P (Posterior), O (Occipital).
• Central (C) is a virtual zone: Note that Central (C) is not an anatomical lobe; it represents the central sulcus and motor-sensory strip territory.
• Midline "Zero" (z) Code: The "z" in Fz, Cz, Pz, and Oz stands for Zero (representing the midline), not "zone."
• Hemisphere Codes: Odd numbers (Fp1, F3, T7) designate the Left hemisphere. Even numbers (Fp2, F4, T8) designate the Right hemisphere.
• Reference & Ground: Historically, A1/A2 represent earlobe (Auricular) targets. Modern systems often use M1/M2 representing the bony mastoid processes behind the ears.

A single electrode on the scalp cannot record an electrical potential. It has no value until it is compared to another point. This comparison (Input 1 minus Input 2) is executed by a differential amplifier.

An EEG montage is a logical display of differential channels. The longitudinal bipolar ("double banana") montage is an essential standard display, but no single montage is sufficient for every pattern. Clinical review uses complementary bipolar and referential displays and preserves the ability to reformat digitally.

In a Bipolar Montage, adjacent electrodes are linked sequentially in a chain (e.g., Channel 1: Fp1-F3, Channel 2: F3-C3). Each channel subtracts its second input from its first input.

Epileptogenic Phase Reversals:

When a focal cortical discharge (e.g., a negative spike) occurs directly under an electrode (e.g., F3), it becomes the most negative point in the surrounding chain:

• Channel 1 (Fp1-F3): F3 is Input 2. Since Input 2 is negative, the subtraction formula outputs a positive value (Fp1 − F3 = 0 − (−100) = +100 μV). This projects Down (Divergence).
• Channel 2 (F3-C3): F3 is Input 1. Since Input 1 is negative, the formula outputs a negative value (F3 − C3 = (−100) − 0 = −100 μV). This projects Up (Convergence).

This creates a Converging Phase Reversal where the channel waveforms point toward each other, "kissing" at the point of maximum negativity (the spike focus). Conversely, a positive discharge (e.g., positive spikes or sharp transients of sleep) creates a Diverging Phase Reversal where waveforms point away from each other.

While Bipolar montages are excellent for localizing sharp transients via phase reversals, they can falsely depress amplitude or completely miss broad, generalized discharges that span across multiple channels (since Input 1 and Input 2 would have identical voltage, subtracting to zero).

In a referential montage, each active scalp electrode is compared with a shared reference. A common average reference (CAR) is one useful computed option, not a universally superior standard:

• Mathematical Average: CAR sums the potentials of all active scalp electrodes (e.g., 21 electrodes) and divides by the total number, creating a computed baseline reference.
• Amplitude and field distribution: The largest deflection helps identify the scalp maximum of a field, but source orientation, depth, reference composition, and volume conduction prevent a simple one-to-one equation between amplitude and generator proximity.
• Reference Contamination Warning: If a single electrode suffers a massive artifact (e.g., a loose electrode or localized muscle spike), that high voltage is mathematically distributed into the common average, causing a "paradoxical" ghost artifact to appear across every other channel.

In clinical practice, we interpret the raw trace by reading its structured "vocabulary." Two foundational descriptors are frequency (cycles per second, measured in hertz) and amplitude (the displayed voltage difference, measured in microvolts).

Frequency classification is the baseline step of EEG analysis. Normal adult waking activity represents a harmonic balance across these four primary physiological frequency bands:

Sleep is a dynamic neurophysiological process. Differentiating sleep stages is critical, as many sleep waveforms mimic epilepsy spikes, and certain epilepsy syndromes manifest exclusively during NREM sleep transitions.

Stage N1 (Light Sleep)

Drowsiness. The occipital PDR disappears and is replaced by low-voltage, mixed-frequency theta activity. Differentiated by:
• Vertex Sharp Waves (V-waves): Brief, sharp, negative transients maximal at the midline central region (Cz).
• Slow Rolling Eye Movements (SREMs): Gentle, slow horizontal oscillations.

Stage N2 (Stable Sleep)

Differentiated by two distinct, diagnostic waveforms:
• Sleep Spindles: Rhythmic sinusoidal runs of 11–16 Hz (classically 12–14 Hz) activity lasting >0.5 seconds, maximal in central/frontal channels.
• K-Complexes: High-voltage, biphasic sharp waves (negative then positive) lasting >0.5 seconds, maximal at the midline, often elicited by sound stimuli.

Stage N3 (Slow Wave Sleep)

Deep NREM sleep. In adults, N3 is scored when slow-wave activity at 0.5–2 Hz and at least 75 μV measured over the frontal derivations occupies at least 20% of a 30-second epoch. Other activity may still be present; spindles can occur and are not scored separately from the N3 epoch.

Stage REM (Dreaming State)

Low-voltage, mixed-frequency background resembling an active, awake state but with complete muscle atonia:
• Sawtooth Waves: 2–6 Hz triangular, jagged waveforms maximal in central channels, preceding rapid eye movements.
• Rapid Eye Movements (REMs): Sharp, irregular horizontal deflections.

Amplitude is the physical height of the wave in millimeters, which directly reflects the intrinsic voltage generated by the cortical dipoles, measured in microvolts (μV). Scalp recordings typically range from 10 to 100 μV.

Master these core diagnostic signatures of severe cerebral dysfunction and disease states:

Focal Slowing (PDA): Continuous, polymorphic delta activity localized to a single hemisphere or lobe. Strong indicator of focal structural injury (e.g., tumor, stroke, abscess).

Generalized Periodic Discharges (GPDs): Standard periodic complexes occurring in a synchronized, generalized fashion. Associated with severe hypoxic-ischemic encephalopathy, drug toxicity, or Creutzfeldt-Jakob disease.

Burst Suppression: Alternating intervals of high-voltage mixed discharges (bursts) and periods of flat-line inactivity (suppression) under <10μV. Indicates profound coma, severe anoxia, or deep anesthesia.

More than half of the transients recorded on an EEG are non-biological or extra-cerebral noise. Recognizing and filtering out these artifacts is crucial: