2.3 Montages & Derivations
Key Takeaways
- A derivation is the recording from one pair of electrodes; a montage is the organized, logical set of derivations displayed together.
- In a bipolar montage each channel links two active scalp electrodes in a chain, so a focal negativity produces a phase reversal that pinpoints the location.
- In a referential montage every active electrode is compared to a single common reference, which shows amplitude well but can contaminate or 'spread' an active reference.
- EEG polarity convention is negative-up: when input 1 is more negative than input 2, the trace deflects upward.
- Ground and reference electrodes are different roles: the ground sets a common circuit reference for safety and noise, while the reference is the comparison electrode in referential recording.
Once the signal is amplified, filtered, and digitized, the technologist must decide how to combine electrodes into channels so that abnormalities can be seen and localized. That choice is the montage, and it is one of the highest-yield topics in the Performing the Study domain.
Derivation vs. Montage
- A derivation is the recording obtained from a single pair of electrodes connected to one differential amplifier (input 1 and input 2).
- A montage is the organized, logical arrangement of multiple derivations displayed together as a recording, named and ordered so a reader can interpret it systematically.
The American Clinical Neurophysiology Society (ACNS) recommends standardized montages so EEGs are comparable between labs. A routine recording typically reviews the EEG in several different montages because each one makes different abnormalities easier to see.
Polarity Convention (Negative-Up)
Before montages make sense you must fix the polarity convention in your mind. By long-standing EEG convention the differential amplifier is wired so that:
- When input 1 is more negative than input 2, the trace deflects UPWARD.
- When input 1 is more positive than input 2, the trace deflects DOWNWARD.
This is summarized as negative-up. Most epileptiform activity at the scalp is surface-negative, so spikes typically point upward. Holding this convention firmly is what makes phase-reversal localization (below) work.
Bipolar vs. Referential Montages
The two fundamental montage families differ in what each channel's input 2 is connected to.
| Feature | Bipolar Montage | Referential Montage |
|---|---|---|
| Channel inputs | Two active scalp electrodes, linked in a chain | One active electrode vs. a single common reference |
| Strength | Excellent localization of focal activity (phase reversal) | Accurate amplitude and good for widespread/diffuse activity |
| Weakness | Underestimates broad or low-gradient fields; cancels in-phase signals | A contaminated/active reference smears artifact into many channels ('reference contamination') |
| Common examples | Longitudinal bipolar ('double banana'); transverse bipolar | Common reference; average reference; ipsilateral ear/mastoid |
In a bipolar montage, electrodes are connected in chains (for example Fp1-F3, F3-C3, C3-P3, P3-O1). Because adjacent channels share an electrode, a focal abnormality at the shared site changes two channels in opposite directions.
In a referential montage, every channel is electrode-vs-reference (for example Fp1-Ref, F3-Ref, C3-Ref). This preserves the true amplitude relationship across the head but means any activity or artifact picked up by the reference electrode appears in every channel, which can mimic widespread cerebral activity if the technologist does not recognize it.
Localization and Phase Reversal
Phase reversal is the cornerstone of bipolar localization. In a bipolar chain, the electrode showing the maximum potential (for a negative spike, the most negative electrode) sits between two channels whose deflections point toward each other — the pens 'reverse phase' at that electrode.
Worked example for a longitudinal bipolar chain Fp1-F3, F3-C3, C3-P3 with a maximally negative focus at F3:
- Fp1-F3: F3 (input 2) is most negative, so input 1 is relatively positive vs. input 2 -> downward, then the channel deflects to show F3 negativity.
- F3-C3: F3 (input 1) is most negative -> upward (negative-up).
- The two channels surrounding F3 point toward each other: a phase reversal pinpointing the focus at F3.
A spike that appears with the same polarity and no phase reversal across a bipolar chain, with no clear maximum, usually points to an issue at the reference or to a field maximal at the end of the chain — a cue to switch to a referential montage to confirm. The general rule: bipolar montages localize via phase reversal; referential montages confirm amplitude and true maximum.
Ground and Reference Electrodes
Students frequently confuse these two electrodes; the exam tests the distinction directly.
| Electrode | Role | Key Points |
|---|---|---|
| Ground | Provides a common zero/return point for the amplifier circuit and improves common-mode rejection | Exactly ONE patient ground for the whole machine; multiple grounds create dangerous ground loops; it is not a clinical signal site |
| Reference | The comparison electrode (input 2) for every channel in a referential montage | Should be relatively electrically quiet; if it picks up EEG or artifact, it contaminates all channels |
Safety point tested on the exam: there must be only one ground electrode connected to the patient at a time. Connecting a patient to two grounded devices can create a ground loop and an electrical-safety hazard. The reference, by contrast, is chosen for being quiet — common choices include linked ears, an average reference (the averaged signal of many electrodes), or a vertex/Cz reference — and a poorly chosen 'active' reference (for example one over temporal cortex) will inject its own activity into every derivation.
Montage Design Principles
Well-designed montages follow ACNS recommendations: cover the entire head with adequate spatial sampling, run electrode chains in anatomically logical, consistent directions (front-to-back for longitudinal, left-to-right for transverse), keep interelectrode distances roughly equal so amplitudes are comparable, display channels in a consistent left-over-right order, and always include both a bipolar montage (for localization) and a referential montage (for amplitude and confirmation). The technologist should switch montages whenever a finding's location or field is uncertain.
In a longitudinal bipolar montage, a negative sharp wave shows a phase reversal at electrode T7 (the two surrounding channels deflect toward each other at T7). What does this indicate?
Which statement correctly contrasts referential and bipolar montages?
By standard EEG polarity convention, when input 1 of a channel is MORE NEGATIVE than input 2, the trace deflects:
How many ground electrodes should be connected to the patient during a routine EEG, and why?