Electroencephalography (EEG): Recording Techniques and Montages
Domain 3 - EEG: Recording Techniques and Montages
This domain tests electrode placement, machine settings, montage logic, activation procedures, and whether you can localize a discharge from the tracing.
Core rules
- Placement. The 10–20 system uses nasion, inion, and preauricular points as landmarks; measure and mark Fp1/Fp2, F3/F4, C3/C4, P3/P4, O1/O2, F7/F8, T3/T4 (now T7/T8), T5/T6 (P7/P8), Fz/Cz/Pz, A1/A2. Keep impedances below 5 kΩ and reasonably equal.
- Montages. Referential (each active electrode to a common reference) is good for amplitude comparisons; bipolar (chain of adjacent electrodes) is good for localization. Longitudinal bipolar ('double banana') and transverse montages are standard; include ECG and, when indicated, eye channels.
- Phase reversal. In a bipolar chain, a focal negative discharge points toward the electrode common to both channels. This is the key localizing sign.
- Settings. Sensitivity near 7 µV/mm, 30 mm/s display (10 s/page), LFF 1 Hz, HFF 70 Hz, notch off unless needed. Calibrate at the start and document settings.
- Activation. Hyperventilation for 3–5 minutes, photic stimulation at 1–30 Hz, sleep recording, and eye opening and closing. Label each maneuver and any clinical event on the record.
Worked example
A spike appears at F8. In Fp2–F8, F8 is G2; a negative F8 makes G1 − G2 positive, so the trace deflects downward. In F8–T4, F8 is G1 and negative, so the trace deflects upward. The two channels point toward each other: phase reversal at F8. In a referential montage to A2, the same spike would simply show high amplitude at F8, with no phase reversal.
Traps
Looking for phase reversal in a referential montage; mislabeling T3/T4 versus T7/T8; failing to document activation; accepting high or unequal impedances; confusing eye-blink artifact with frontal slowing.