3.2 Recommended EEG Derivations & EOG Placements

3.2 Recommended EEG Derivations & EOG Placements

In clinical polysomnography, continuous electroencephalography (EEG) and electrooculography (EOG) provide the neurophysiological substrate required to score sleep stages, detect cortical arousals, and identify sleep-disordered breathing architecture. The American Academy of Sleep Medicine (AASM) Manual for the Scoring of Sleep and Associated Events establishes standardized electrode montages, derivation pairings, and backup configurations to ensure uniform recording quality across sleep laboratories.

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1. Principles of Differential Amplification

Polysomnographic recording channels do not measure absolute electrical potential at a single scalp site; instead, they employ differential amplifiers that register the difference in electrical potential between two distinct recording inputs:

$\text{Output Voltage} = \text{Gain} \times (\text{Input 1} - \text{Input 2})$

• Input 1 (Grid 1 / Exploring Electrode): Placed directly over the cortical area of interest (e.g., F4, C4, O2).
• Input 2 (Grid 2 / Reference Electrode): Placed over an electrically neutral, non-cortical site, conventionally the contralateral mastoid process (M1 or M2).
• Common Mode Rejection: Any electrical voltage that is identical in amplitude and phase at both Input 1 and Input 2 (such as ambient 60 Hz electromagnetic radiation or cardiac far-field artifact) is cancelled out by subtraction, leaving only the localized cerebral neurophysiological signal.

                  [DIFFERENTIAL AMPLIFIER SIGNAL PROCESSING]

   Input 1 (Exploring: C4) ─────┐
                                 ├───> [ DIFFERENTIAL AMPLIFIER ] ───> Recorded EEG Waveform
   Input 2 (Reference: M1) ─────┘         (Input 1 - Input 2)
                                                  ▲
   Ground (Fpz / Forehead) ───────────────────────┘ (Common Reference Point)

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2. AASM Standard Recommended & Backup EEG Derivations

The AASM scoring manual establishes a primary recommended three-channel EEG montage along with mandatory backup electrodes applied during initial setup.

+-----------------------------------------------------------------------------------------+
|                        AASM EEG DERIVATION RECORDING TOPOGRAPHY                         |
|                                                                                         |
|   RECOMMENDED MONTAGE (RIGHT EXPLORING / LEFT REFERENCE):                               |
|   ┌─────────────┐       ┌─────────────┐       ┌─────────────┐                           |
|   │ F4 (Input 1)│       │ C4 (Input 1)│       │ O2 (Input 1)│                           |
|   └──────┬──────┘       └──────┬──────┘       └──────┬──────┘                           |
|          │                     │                     │                                  |
|          └──────────────┬──────┴─────────────────────┘                                  |
|                         ▼                                                               |
|                  [ M1 (Input 2) - Left Mastoid Reference ]                              |
|                                                                                         |
|   BACKUP MONTAGE (LEFT EXPLORING / RIGHT REFERENCE):                                    |
|   ┌─────────────┐       ┌─────────────┐       ┌─────────────┐                           |
|   │ F3 (Input 1)│       │ C3 (Input 1)│       │ O1 (Input 1)│                           |
|   └──────┬──────┘       └──────┬──────┘       └──────┬──────┘                           |
|          │                     │                     │                                  |
|          └──────────────┬──────┴─────────────────────┘                                  |
|                         ▼                                                               |
|                  [ M2 (Input 2) - Right Mastoid Reference ]                             |
+-----------------------------------------------------------------------------------------+

Clinical Rationale for Specific Derivations:

1. Frontal Region (F4-M1 / F3-M2): The frontal cortex generates the highest amplitude slow wave activity during non-rapid eye movement (NREM) sleep. Recording from F4-M1 provides optimal sensitivity for identifying slow wave sleep (Stage N3), where delta waves must achieve peak-to-peak amplitudes of at least 75 µV.
2. Central Region (C4-M1 / C3-M2): The central sensorimotor region overlying the Rolandic fissure generates sleep spindles (11–16 Hz bursts lasting ≥0.5 seconds) and vertex sharp waves. C4-M1 is the primary derivation utilized to establish sleep onset and score Stage N2.
3. Occipital Region (O2-M1 / O1-M2): The visual cortex in the occipital region generates the posterior dominant alpha rhythm (8–13 Hz) during relaxed, eyes-closed wakefulness. Attenuation of occipital alpha activity is the hallmark physiological indicator of the transition from Stage W to Stage N1.

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3. Reference Electrodes (M1, M2) & Ground Placement

• Mastoid Electrodes (M1 & M2): Located on the bony mastoid processes directly posterior to the ear pinnae. Mastoid locations are chosen because they overlie dense, non-neural petrous bone, providing an electrically inactive reference that minimizes baseline artifact.
• Contralateral Referencing: Standard polysomnography pairs right-sided exploring electrodes with the left mastoid (F4-M1, C4-M1, O2-M1) and left-sided exploring electrodes with the right mastoid (F3-M2, C3-M2, O1-M2). Maximizing the physical inter-electrode distance across the skull enhances the amplitude of recorded cortical dipole voltages.
• Ground Electrode: Placed at Fpz or the high forehead midline. The ground electrode serves as the common zero-voltage reference for the differential amplifier circuit and is critical for safe patient grounding and common-mode noise suppression.

In-Study Electrode Failure & Re-Referencing Protocol:

If an exploring electrode (e.g., C4) develops high impedance, unresolvable 60 Hz artifact, or pops off during the night, the technologist must:

1. Immediately switch the montage display channel to the contralateral backup derivation (C3-M2).
2. Document the derivation switch, epoch timestamp, and reason in the acquisition log.
3. If reference electrode M1 fails, all right-sided channels must be re-referenced to M2 (F4-M2, C4-M2, O2-M2) or switched entirely to the left-sided backup set (F3-M2, C3-M2, O1-M2).

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4. Electrooculography (EOG) Placement & Dipole Physics

Electrooculography records eye movements by tracking the electrical field generated by the corneo-retinal potential.

                      [THE CORNEO-RETINAL ELECTRICAL DIPOLE]

                                  (+) CORNEA (Front of Eye)
                                       ┌─────┐
                                      │  ●  │
                                       └─────┘
                                  (-) RETINA (Back of Eye)

The Corneo-Retinal Dipole:

• The human eye acts as an electrical battery or dipole: the cornea is electropositive (+) relative to the retina, which is electronegative (-).
• When the eye rotates toward an electrode, the positive cornea approaches that electrode, inducing a positive electrical potential at Input 1 (G1).
• Standard polysomnographic polarity conventions dictate that Input 1 negativity causes an upward trace deflection, whereas Input 1 positivity causes a downward trace deflection.

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5. AASM Standard EOG Electrode Placement & Derivations

The AASM recommends a two-channel EOG recording configuration:

                         [AASM STANDARD EOG ELECTRODE PLACEMENT]

                                                    E2 (Right Eye)
                                              [ 1 cm above & lateral ]
                                                       ▲
                                                       │
                   Left Eye                         Right Eye
                   ┌─────┐                           ┌─────┐
                   │  ●  │                           │  ●  │
                   └─────┘                           └─────┘
                      │
                      ▼
                E1 (Left Eye)
           [ 1 cm below & lateral ]

Precise Placement Rules:

• E1 (Left Eye): Placed 1 cm below the outer canthus of the left eye.
• E2 (Right Eye): Placed 1 cm above the outer canthus of the right eye.
• Standard Referencing: Both electrodes are referenced to the contralateral mastoid or single mastoid:
  • Channel 1: E1-M2 (or E1-M1)
  • Channel 2: E2-M2 (or E2-M1)
  • AASM Recommended Standard Montage: E1-M2 and E2-M2.

Out-of-Phase vs. In-Phase Deflections:

Because E1 is positioned below the left eye and E2 is positioned above the right eye, conjugate (coordinated) vertical and horizontal eye movements generate out-of-phase (opposite direction) deflections across the two recording channels.

+-----------------------------------------------------------------------------------------+
|                    EOG DEFLECTION MECHANICS: EYE MOVEMENTS VS. ARTIFACT                 |
|                                                                                         |
|   1. CONJUGATE VERTICAL EYE MOVEMENT (PATIENT LOOKS UP):                                |
|      - Cornea (+) moves toward E2 (above right eye) ──> E2 becomes POSITIVE (Downward)  |
|      - Cornea (+) moves away from E1 (below left eye) ──> E1 becomes NEGATIVE (Upward)  |
|      - RESULT: Channel 1 (E1) deflects UP, Channel 2 (E2) deflects DOWN [OUT-OF-PHASE]  |
|                                                                                         |
|   2. CORTICAL EEG ARTIFACT / EYE BLINK (BELL'S PHENOMENON / FRONTAL SLOWING):           |
|      - Frontal cortical slowing conducts equally to both periorbital electrodes.        |
|      - Both E1 and E2 experience identical positive or negative voltage shifts.         |
|      - RESULT: Channel 1 (E1) and Channel 2 (E2) deflect in SAME DIRECTION [IN-PHASE]  |
+-----------------------------------------------------------------------------------------+

[!IMPORTANT] Clinical Rule of Thumb for Sleep Scoring:

• Out-of-Phase Deflections = True physiological eye movements (Slow Rolling Eye Movements in N1, Rapid Eye Movements in REM, voluntary saccades in Wake).
• In-Phase Deflections = Frontal EEG artifact, slow wave transmission from cortex, or high-amplitude blinks.