EEG Instrumentation and Calibration 5 — Questions and Answers
Question 1: During EEG setup, electrode impedance is measured and found to be 28 kΩ on electrode F3. The appropriate action is to:
- Proceed with recording as this is within acceptable range
- Re-prep the skin and reapply the electrode to reduce impedance (Correct answer)
- Switch to a higher-sensitivity amplifier setting for that channel
- Apply a notch filter to channel F3 only
Correct answer: Re-prep the skin and reapply the electrode to reduce impedance
Standard acceptable electrode impedance is below 5 kΩ (some guidelines allow up to 10 kΩ); 28 kΩ is far too high and requires skin re-preparation and electrode reapplication.
Question 2: The 'sensitivity' setting on an EEG machine refers to:
- The frequency range captured by the amplifier
- The voltage per unit of pen or display deflection (e.g., µV/mm) (Correct answer)
- The number of channels recorded simultaneously
- The speed at which the paper or display advances
Correct answer: The voltage per unit of pen or display deflection (e.g., µV/mm)
Sensitivity in EEG describes how many microvolts of signal produce a given deflection (e.g., 7 µV/mm), controlling the amplitude scale of the recording.
Question 3: Which filter setting should be used to reduce sweat artifact (very slow baseline drift) without significantly affecting EEG frequencies of interest?
- Increase the high-frequency filter cutoff to 70 Hz
- Decrease the time constant (increase the low-frequency filter cutoff) (Correct answer)
- Enable the 60 Hz notch filter
- Decrease the sensitivity to 10 µV/mm
Correct answer: Decrease the time constant (increase the low-frequency filter cutoff)
Sweat artifact is a very low-frequency (sub-1 Hz) drift; decreasing the time constant (raising the low-frequency filter) attenuates these slow potentials while preserving standard EEG frequencies.
Question 4: In digital EEG systems, the Nyquist theorem states that to accurately digitize a signal, the sampling rate must be at least:
- Equal to the highest frequency of interest
- 1.5 times the highest frequency of interest
- Twice the highest frequency of interest (Correct answer)
- Ten times the highest frequency of interest
Correct answer: Twice the highest frequency of interest
The Nyquist theorem requires a sampling rate of at least twice the highest frequency component to avoid aliasing artifacts in the digitized signal.
Question 5: A technologist is setting up a 32-channel digital EEG system. If the high-frequency filter is set to 70 Hz, what is the minimum adequate sampling rate to avoid aliasing?
- 70 samples/sec per channel
- 100 samples/sec per channel
- 140 samples/sec per channel (Correct answer)
- 256 samples/sec per channel
Correct answer: 140 samples/sec per channel
Per the Nyquist theorem, sampling at least twice the highest frequency (2 × 70 Hz = 140 samples/sec) is the minimum to prevent aliasing.
Question 6: Which of the following best describes the function of a referential (monopolar) montage in EEG recording?
- Each channel records the difference between two adjacent active electrodes
- Each channel records the difference between one active electrode and a common reference (Correct answer)
- All electrodes are linked together to form a single average
- Channels are arranged to show voltage gradients across the scalp
Correct answer: Each channel records the difference between one active electrode and a common reference
In a referential montage, each channel compares one active scalp electrode to a single common reference electrode (such as Cz, ipsilateral ear, or average reference).
Question 7: When a square wave calibration signal shows 'ringing' or oscillations after the rising edge, this most likely indicates:
- The low-frequency filter cutoff is set too high
- The high-frequency filter cutoff is set too high, allowing high-frequency resonance (Correct answer)
- The electrode impedance is too low
- The amplifier gain is set too high
Correct answer: The high-frequency filter cutoff is set too high, allowing high-frequency resonance
Ringing after the rising edge of a square wave indicates the high-frequency filter cutoff is set too high, allowing the amplifier to pass and amplify resonant high-frequency components at the step transition.
EEG Instrumentation and Calibration 5
During EEG setup, electrode impedance is measured and found to be 28 kΩ on electrode F3.
The appropriate action is to: