EEG Instrumentation and Calibration 4 — Questions and Answers
Question 1: What is the primary purpose of the 'biological calibration' signal used in EEG systems?
- To verify the amplifier gain using a known physiological signal (Correct answer)
- To eliminate 60 Hz interference from the recording
- To set the low-frequency filter cutoff automatically
- To calibrate the ink-jet speed of the chart recorder
Correct answer: To verify the amplifier gain using a known physiological signal
Biological calibration uses a known physiological signal (such as eye blink artifact) to verify that amplifier gain and channel balance are consistent across all channels.
Question 2: A technologist notices that channel 3 shows a 50% smaller deflection than all other channels during the calibration square wave. The most likely cause is:
- A broken or high-impedance electrode on channel 3
- Gain set too low on the channel 3 amplifier (Correct answer)
- The notch filter being activated on channel 3
- A 60 Hz artifact contaminating channel 3
Correct answer: Gain set too low on the channel 3 amplifier
When one channel shows a consistently smaller deflection than others during a known calibration signal, the amplifier gain for that channel is set too low.
Question 3: Which electrode placement system uses 10% and 20% distances from skull landmarks to determine electrode positions?
- The Queen Square system
- The International 10-20 system (Correct answer)
- The Modified Combinatorial Nomenclature
- The American Clinical Neurophysiology Society (ACNS) system
Correct answer: The International 10-20 system
The International 10-20 system divides skull circumference into intervals of 10% and 20% from fixed bony landmarks to standardize electrode placement.
Question 4: When performing a square wave calibration, the 'tilt' or slant on the flat top of the square wave most likely indicates a problem with:
- The high-frequency filter setting
- The low-frequency filter (time constant) setting (Correct answer)
- Electrode impedance being too high
- Amplifier gain being set too high
Correct answer: The low-frequency filter (time constant) setting
A tilted or slanted flat top on a square wave calibration signal indicates the low-frequency filter (time constant) is cutting off low-frequency components and distorting the sustained DC portion.
Question 5: The standard paper speed used in routine clinical EEG recording in the United States is:
- 10 mm/sec
- 15 mm/sec
- 30 mm/sec (Correct answer)
- 60 mm/sec
Correct answer: 30 mm/sec
30 mm/sec is the standard paper speed for routine clinical EEG in the US, providing optimal visualization of typical brain wave frequencies.
Question 6: Which component of the EEG amplifier circuit is responsible for rejecting signals common to both inputs (such as 60 Hz power line noise)?
- The low-frequency filter
- The differential amplifier (Correct answer)
- The analog-to-digital converter
- The electrode selector matrix
Correct answer: The differential amplifier
The differential amplifier amplifies the difference between two inputs while rejecting signals that appear equally on both (common mode), which is how 60 Hz interference is suppressed.
Question 7: An EEG amplifier has a CMRR of 100 dB. This means the amplifier rejects common-mode signals by a factor of approximately:
- 100 times
- 1,000 times
- 10,000 times
- 100,000 times (Correct answer)
Correct answer: 100,000 times
100 dB CMRR means common-mode signals are rejected by a factor of 10^(100/20) = 100,000 times relative to the differential signal.
EEG Instrumentation and Calibration 4
What is the primary purpose of the 'biological calibration' signal used in EEG systems?