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?
Source at F8; it would show a phase reversal again on the referential montage
Source at T4; on the referential montage it would appear as the largest-amplitude channel with no phase reversal
Source at O2; the referential montage would show no activity at all
There is no localizable source; this pattern is always artifact
During biocalibration, every channel reproduces the same input waveform except one channel that is nearly flat. What is the most likely cause?
A focal cerebral abnormality under that electrode
A bad electrode, broken lead, or amplifier fault on that channel
A correctly functioning notch filter
Normal regional variation in EEG amplitude
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:
A focal epileptiform spike requiring physician notification
An electrode pop / technical artifact isolated to that electrode
A normal variant called RMTD
A photoparoxysmal response
On square-wave (DC) calibration, one channel's pulse decays back to baseline much faster than the others. This indicates:
The high-frequency (low-pass) filter is set too low on that channel
A shortened time constant (low-frequency filter set too high) that will distort slow waves on that channel
Normal channel behavior that needs no action
Aliasing due to an excessive sampling rate
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:
A bitemporal independent epileptiform focus
The cornea-positive eye dipole producing lateral-rectus (gaze) artifact at frontotemporal electrodes
A 60 Hz mains artifact
Vertex waves of N2 sleep
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