Montage Types
What is a bipolar montage and how does it help localize brain activity?
It compares two electrodes to detect voltage differences, helping localize focal abnormalities using phase reversal.
How many channels are recommended for EEG recordings according to guidelines?
At least 16 channels; more are encouraged.
What does “reformatting data” allow EEG reviewers to do?
View the same data in different montages, filters, and sensitivities—even in real time.
What is aliasing in EEG recordings?
Distortion caused by low sampling rates, making fast waves appear slower.
Which montage is best for awake recordings?
Longitudinal A/P bipolar (Double Banana).
What is the difference between longitudinal and transverse bipolar montages?
Longitudinal runs front-to-back (Double Banana), transverse runs side-to-side across the head.
What are the three classes of montages that should be represented in each record?
Longitudinal bipolar, transverse bipolar, and referential.
Why is time-linked video important in LTM recordings?
in LTM recordings?It allows correlation of clinical behavior with EEG discharges.
What is the Nyquist Theorem and how does it prevent aliasing?
Sampling rate must be at least twice the signal frequency to avoid aliasing
Which montage is preferred for sleep recordings and why?
Transverse bipolar; captures midline vertex activity.
What is a circumferential montage and when is it most useful?
Electrodes are connected around the head’s perimeter; useful for detecting lateralized abnormalities and temporal lobe activity.
Why should anterior electrodes be placed above posterior ones?
To maintain consistent orientation and improve localization accuracy.
What is the purpose of averaging in EP recordings?
To extract time-locked responses and reduce background noise.
What is an anti-aliasing filter and when should it be applied?
A filter that removes high-frequency components before digitizing to prevent aliasing.
What montage would you use to localize temporal lobe seizures?
Transverse or circumferential bipolar montage.
What is a referential montage and how does it differ from bipolar?
Each electrode is compared to a common reference (e.g., ear or Cz); bipolar compares two active electrodes.
What does “left above right” mean in montage derivations?
Electrode pairings should be ordered with left hemisphere electrodes above right hemisphere ones.
What is the signal-to-noise ratio and how is it improved?
It’s the clarity of the signal over noise; improved by averaging, which increases S/N by √number of sweeps.
What happens if the sampling rate is too low in EEG?
Fast brain waves are misrepresented as slower waves, causing distortion.
Why might Cz reference be avoided during sleep studies?
Cz picks up sleep activity, contaminating the reference.
What is a common average montage and what are its advantages and limitations?
Each electrode is compared to the average of all electrodes; reduces noise but can be affected by noisy electrodes.
Why is it important to use both bipolar and referential montages in a study?
Each provides different localization methods—bipolar uses phase reversal, referential uses amplitude.
What are hostile electrical environments and how can they affect EEG recordings?
ICUs/ORs with electrical leakage can cause 60Hz interference; unplugging equipment and proper grounding helps.
How does digital filtering in EEG help reduce noise without affecting the signal, and what is the risk of improper filter settings?
Digital filtering smooths out high-frequency noise while preserving the shape of the EEG waveform. If filters are set improperly (e.g., too narrow or too aggressive), they can distort the signal, mask important clinical features, or introduce phase shifts that misrepresent timing and location of brain activity
How do you choose a montage based on expected symptoms or events?
Use montages that cover the area of interest and avoid references near expected activity; bipolar for localization, referential for amplitude.