6.2 Localization & Pattern Drills
Key Takeaways
- Drill phase-reversal localization until you can name the maximal-voltage electrode from a bipolar chain in seconds, and recognize that referential montages show the maximum as the largest-amplitude channel with no reversal.
- When the montage changes, the brain does not - a focal discharge that looks different on longitudinal bipolar, transverse bipolar, and referential montages is the same source viewed three ways.
- Calibration must be checked before interpretation: square-wave (DC) calibration verifies amplifier polarity, sensitivity, and time constant; biocalibration confirms all channels respond identically to the same input.
- An artifact is cerebral-versus-not by field and physiology: cerebral activity has an orderly voltage field across adjacent electrodes, while artifacts are often too sharp, too isolated, or time-locked to ECG, eye, or equipment events.
- Eye-movement artifact obeys the cornea-positive dipole rule - eyes up drives Fp1/Fp2 down, lateral gaze drives the contralateral and ipsilateral frontotemporal electrodes in opposite directions.
How to Use These Drills
Localization and artifact discrimination are the highest-yield reasoning skills on the Performing the Study domain (75% of the exam). Work each drill below before reading its answer rationale. Speed matters: on a 4-hour exam you cannot re-derive montage theory from scratch for every item.
Drill Set A: Phase-Reversal Localization
Remember the rule for a bipolar (longitudinal) montage: the electrode shared by the two channels whose deflections point toward each other (negative field) or away from each other (positive field) carries the maximal voltage. Walk a chain like Fp2-F8, F8-T4, T4-T6, T6-O2.
- If F8-T4 deflects up and T4-T6 deflects down, the reversal is at T4.
- If the same source is shown on a referential (e.g., ipsilateral ear) montage, you will see no reversal - just the largest-amplitude channel at T4-A2.
- A discharge with no phase reversal anywhere in a complete bipolar chain but maximal at the chain ends suggests the source is at an end-of-chain electrode or in the reference (referential contamination).
Drill Set B: Montage Translation
The brain source is invariant; the montage is just a viewpoint. Use this table to translate the same right-mid-temporal spike across montages.
| Montage | What the Same T4 Spike Looks Like |
|---|---|
| Longitudinal bipolar (LB) | Phase reversal at T4 between F8-T4 and T4-T6 |
| Transverse bipolar (TB) | Phase reversal at T4 between C4-T4 and T4-A2/T4-T6 |
| Referential (ear/Cz) | Largest single-channel amplitude at T4; no reversal |
| Average reference | T4 maximal; small opposite-sign deflections spread across other channels |
If two montages disagree about the side or region, suspect a labeling/lead-placement error, a reference problem, or an artifact - not two separate sources.
Drill Set C: Artifact vs. Cerebral Discrimination
Cerebral activity produces an orderly voltage field: amplitude rises and falls smoothly across physically adjacent electrodes. Artifacts break that rule. Classify each below:
- A spike-like transient appearing on one electrode only, identical morphology each time, no field to neighbors -> electrode pop / technical artifact.
- A repetitive sharp deflection time-locked to the ECG channel -> ECG (cardiac) artifact; confirm with a simultaneous ECG channel.
- Frontal-predominant slow deflections that reverse with eye-open/eye-closed instructions -> eye-movement artifact (cornea-positive dipole).
- Bilateral frontotemporal high-frequency 'fuzz' that disappears when the patient relaxes the jaw -> muscle (EMG) artifact.
- Rhythmic activity at exactly the flash frequency during photic stimulation confined to occipital leads -> photic driving (a normal response, not pathology).
The Eye-Movement Dipole
The eye is a dipole with the cornea positive relative to the retina. Practical rules the exam tests:
- Eyes up (or blink): cornea moves toward Fp electrodes, driving Fp1/Fp2 surface-positive (downward by negative-up convention).
- Lateral gaze right: the right cornea swings toward F8 (positive there) while the left retina swings toward F7 - producing opposite-direction deflections at F7 and F8. This is the classic lateral rectus / gaze artifact pattern.
Drill Set D: Calibration Problems
No record should be interpreted before calibration is verified.
- Square-wave (DC) calibration: a known voltage step tests amplifier polarity, sensitivity (gain), and time constant. A channel whose calibration pulse decays too fast has a shortened time constant (low-frequency filter set too high), which will distort slow waves.
- Biocalibration: all channels record the same patient signal simultaneously to confirm every channel responds identically. One channel that is flat or attenuated during biocalibration indicates a bad electrode, broken lead, or amplifier fault on that channel - not a cerebral finding.
- A calibration pulse that deflects the wrong way indicates a polarity/input reversal that will invert every interpretation on that channel.
Drill Set E: Polarity and Measurement
The outline lists measurement of frequency, voltage/sensitivity, and duration and localization techniques and polarity as analysis skills. Drill them:
- Frequency: count cycles per second. A wave completing one cycle every 250 ms is 4 Hz; one cycle every 100 ms is 10 Hz (alpha). Practice converting period to frequency instantly.
- Voltage/amplitude: read peak-to-peak height against the sensitivity. At 7 uV/mm, a 10 mm deflection equals 70 uV. At 2 uV/mm (the ECI setting), the same 10 mm equals only 20 uV - which is why high sensitivity is used to detect tiny activity.
- Duration: a spike is <70 ms; a sharp wave is 70-200 ms. At a 30 mm/s display (10 s/page), 70 ms occupies just over 2 mm - train your eye to estimate it.
- Polarity: negative-up convention. Input 1 more negative than input 2 -> upward. A wrong-way calibration pulse means an inverted channel.
Drill Set F: Montage Design Logic
Given a finding, choose the montage that best displays it. A broad, low-gradient field (e.g., generalized or a widespread frontal slow wave) can cancel in a bipolar montage and is best shown referentially. A sharply localized focal spike is best localized by a longitudinal bipolar montage's phase reversal, then confirmed referentially for true amplitude. A midline (Cz) finding may be invisible on a left-right longitudinal montage and is best seen on a transverse montage that crosses the vertex. The rule: bipolar localizes, referential quantifies, transverse catches the midline.
Localization Drills Self-Check
In a longitudinal bipolar chain Fp2-F8, F8-T4, T4-T6, T6-O2, the channels F8-T4 and T4-T6 deflect toward each other while the rest of the chain is quiet. Where is the source, and how would it appear on an ipsilateral-ear referential montage?
During biocalibration, every channel reproduces the same input waveform except one channel that is nearly flat. What is the most likely cause?
A sharp transient appears on the T6 channel only, with identical morphology each occurrence and no voltage field at neighboring electrodes. The best classification is:
On square-wave (DC) calibration, one channel's pulse decays back to baseline much faster than the others. This indicates:
During lateral gaze to the right, the technologist sees opposite-direction deflections at F7 and F8 that are time-locked to the patient's eye movements. This pattern is best explained by: