6.1 High-Yield Recap
Key Takeaways
The International 10-20 System spaces electrodes at 10% and 20% of the nasion-inion and preauricular distances; odd-numbered electrodes are left, even-numbered are right, and 'z' electrodes (Fz, Cz, Pz) sit on the midline.
Common Mode Rejection Ratio (CMRR) of at least 100:1 (40 dB) and the Nyquist rule (sample rate at least twice the highest frequency of interest) are core instrumentation facts the R.EEG.T. exam tests directly.
In a bipolar (longitudinal) montage, a phase reversal points to the channel pair where the maximal voltage is located, localizing the source between the two reversing electrodes.
Hyperventilation runs 3-5 minutes and photic stimulation sweeps roughly 1-30 Hz; normal variants such as wicket spikes, rhythmic mid-temporal theta of drowsiness (RMTD), and 14-and-6 positive spikes are benign and must not be over-read as epileptiform.
Physiologic artifacts (eye, muscle, ECG, pulse) follow the patient; technical artifacts (electrode pop, 60 Hz mains, salt bridge) follow the equipment - resolving impedance and connections fixes most technical artifacts.
Why a Cross-Domain Recap Matters
The ABRET (American Board of Registration of Electroencephalographic and Evoked Potential Technologists) R.EEG.T. exam is a 4-hour computer-based test weighted 25% Pre-Study/Patient Preparation and 75% Performing the Study. Most missed questions are not from a single weak topic - they come from blending concepts: reading a montage without first confirming the calibration, or calling a benign normal variant epileptiform. This recap stitches the domains back together so you can answer integrative items quickly.
Electrode Measurement & Nomenclature
The International 10-20 System places electrodes at distances of 10% and 20% along the nasion-to-inion (front-back) and preauricular (ear-to-ear) measurements. Memorize the rules so a single labeled electrode tells you its hemisphere and region.
- Odd numbers = left hemisphere; even numbers = right hemisphere.
- The letter z (Fz, Cz, Pz, Oz) marks the midline.
- Letters encode region: Fp = frontopolar, F = frontal, C = central, T = temporal, P = parietal, O = occipital, A = auricular (reference).
- Lower numbers sit closer to the midline; higher numbers sit more laterally.
Topic to Must-Know Table
| Topic | Must-Know Fact |
|---|---|
| 10-20 measurement | Cz is the true vertex - 50% of both nasion-inion and preauricular distances |
| Electrode count | A standard adult routine montage uses 21 electrodes (19 active + 2 references) |
| Impedance | Keep electrode impedance balanced and generally below 5 kilohm; mismatched impedance degrades CMRR |
| CMRR | At least 100:1 (40 dB); higher CMRR rejects more common-mode noise like 60 Hz |
| Nyquist | Sampling rate must be at least 2x the highest frequency of interest to avoid aliasing |
| Filters | Low-frequency (high-pass) filter ~1 Hz; high-frequency (low-pass) filter ~70 Hz; notch only as a last resort |
| Phase reversal | In a bipolar chain, pen deflections pointing toward each other localize the maximal-voltage electrode |
| Sensitivity | Routine adult EEG ~7 microvolt/mm; lower sensitivity number = larger displayed amplitude |
| Paper/sweep speed | Standard display equals 30 mm/sec (10 sec per page) |
| Hyperventilation | 3-5 minutes; build-up (slowing) is normal, especially in children; contraindicated in recent stroke, sickle cell, severe cardiopulmonary disease |
| Photic stimulation | Flash frequencies roughly 1-30 Hz; a true photoparoxysmal response is generalized and outlasts the stimulus |
| Normal variants | Wicket spikes, RMTD, 14-and-6, SREDA, benign sporadic sleep spikes, mu rhythm - benign, not epileptiform |
| Epileptiform | Spikes (under 70 ms) and sharp waves (70-200 ms) that disrupt background and have an after-going slow wave |
| Sleep markers | Vertex waves and sleep spindles/K-complexes mark N2; slow-wave (delta) dominates N3 |
| Artifacts | Physiologic artifacts follow the patient; technical artifacts follow the equipment |
Instrumentation Limits in One Pass
The differential amplifier subtracts input 2 from input 1 and amplifies the difference. Signals common to both inputs (like 60 Hz line noise) are mostly canceled - this is common-mode rejection, quantified by the Common Mode Rejection Ratio (CMRR). Equal electrode impedances are what let the amplifier 'see' the noise as common; a single high-impedance electrode unbalances the inputs and lets 60 Hz through. The fix is re-prepping the electrode, not reaching for the notch filter.
Digital EEG must obey the Nyquist theorem: the sampling rate has to be at least twice the highest frequency you want to record, or fast activity aliases into false slow waves. A 256 Hz sample rate faithfully captures content up to 128 Hz.
Localization Logic
Phase reversal is the single most tested localization concept. In a bipolar (longitudinal 'banana') montage, electrodes are linked in chains. A negative cortical source produces an upward (by EEG convention, negative-up) deflection in the channel where it is input 1 and a downward deflection where it is input 2. The electrode shared by the two channels whose pens point toward each other (or away, for a positive field) is the site of maximal voltage. In a referential montage, the channel with the largest amplitude marks the maximum - no phase reversal is needed.
Normal Variants vs. Epileptiform
Benign variants mimic pathology but lack the disruptive, evolving character of true epileptiform discharges. Key distinguishing features:
- Wicket spikes: arciform temporal waves in drowsiness; no after-going slow wave, no background disruption.
- RMTD (rhythmic mid-temporal theta of drowsiness): 4-7 Hz notched theta runs, monomorphic, no evolution.
- 14-and-6 positive spikes: surface-positive arch bursts in sleep, often in adolescents.
- SREDA (subclinical rhythmic EEG discharge of adults): theta runs without clinical seizure or post-ictal slowing.
True epileptiform discharges disrupt the background, have a steep rising phase, and are typically followed by a slow wave.
Seizure and Clinical-Correlation Recap
Under the ILAE 2017 framework, seizures are classified by onset (focal, generalized, unknown). High-yield electroclinical pairings: 3-Hz spike-and-wave = childhood absence (HV-activated); 4-6 Hz polyspike-and-wave, photosensitive = juvenile myoclonic epilepsy; hypsarrhythmia = infantile spasms/West syndrome; slow (<2.5 Hz) spike-and-wave = Lennox-Gastaut; anterior temporal sharp waves (F7/F8, T1/T2) = mesial temporal lobe epilepsy.
For disorders: focal polymorphic delta = structural lesion; diffuse slowing = global/metabolic process; temporal LPDs/PLEDs = herpes encephalitis; triphasic GPDs (reactive) = hepatic/uremic encephalopathy; periodic sharp complexes ~1/s = CJD.
Neonatal/Pediatric Recap
Interpret neonates by conceptional (postmenstrual) age. Continuity matures: trace discontinu (very premature) -> delta brushes prominent (~28-34 wk) -> trace alternant in term quiet sleep -> continuous slow-wave sleep. Normal graphoelements (delta brushes, encoches frontales, occasional temporal sharps) are not epileptiform. Neonatal seizures are often subtle/electrographic-only and focal. The PDR matures ~4 Hz (4 mo) -> 6 Hz (12 mo) -> 8 Hz (3 yr) -> 9-10 Hz (~8 yr).
Post-Study and Ethics Recap
Post-Study (19%): clean before disinfect; single-use electrodes for non-intact skin/prion risk; write an accurate technical report of observations (not a diagnosis); save/back up/archive raw data (ACNS Guideline 4, HIPAA). Ethics (20%): HIPAA minimum-necessary disclosure to authorized care team only; one patient ground + isolated equipment + leakage-current limits; collodion/acetone are flammable - follow SDS/OSHA; the Code of Ethics forbids falsifying or omitting data.
Recap Self-Check
A technologist sees a single channel with a high, unbalanced electrode impedance and 60 Hz interference only on that channel. Which concept best explains why the noise is no longer rejected?
The Nyquist sampling rate was set too low
The unequal impedance unbalanced the differential amplifier inputs, reducing effective common-mode rejection
The low-frequency filter cutoff was set above 1 Hz
The sensitivity was set to a number that is too high
In a longitudinal bipolar montage, channels Fp1-F3 and F3-C3 show pen deflections pointing toward each other at F3. What does this indicate?
An artifact isolated to the C3 electrode
Maximal voltage (phase reversal) at the F3 electrode
A reference electrode contamination at Fp1
Aliasing from an inadequate sampling rate
A 16-year-old's drowsy recording shows brief surface-positive arciform bursts at about 14 Hz and 6 Hz over the posterior temporal regions. The most appropriate interpretation is:
Generalized epileptiform discharges requiring urgent notification
A 14-and-6 positive spike pattern, a benign normal variant
A photoparoxysmal response from photic stimulation
A 60 Hz technical artifact
Digital EEG is recorded at a 200 Hz sampling rate. According to the Nyquist theorem, what is the highest frequency that can be accurately represented, and what is the risk if higher-frequency activity is present?
400 Hz; the signal will be over-filtered
100 Hz; higher frequencies will alias into false slower waveforms
200 Hz; there is no risk because the rate equals the frequency
50 Hz; the data will saturate the amplifier
Sections you finish are checked off in the contents.