EEG Instrumentation and Calibration 2 — Questions and Answers
Question 1: What is the standard calibration signal used to verify EEG amplifier function?
- A 10 µV sine wave at 10 Hz
- A 50 µV square wave at 1 Hz (Correct answer)
- A 100 µV triangle wave at 5 Hz
- A 200 µV sine wave at 60 Hz
Correct answer: A 50 µV square wave at 1 Hz
The standard calibration signal is a 50 µV square wave at 1 Hz. This verifies amplifier gain accuracy, frequency response, and channel matching across the system.
EEG calibration uses a 50 µV, 1 Hz square wave signal applied simultaneously to all channels. The square wave is ideal because its sharp transitions test the entire frequency response of the amplifier — both the low-frequency components (flat portions) and high-frequency components (sharp edges). During calibration, all channels should show identical waveforms with the correct amplitude (matching the gain/sensitivity setting). Any channel showing different amplitude, distorted shape, or timing differences indicates equipment malfunction. Biological calibration (having the patient open and close eyes to produce alpha blocking) supplements the electrical calibration to verify electrode function.
Question 2: The common mode rejection ratio (CMRR) of an EEG differential amplifier measures its ability to:
- Amplify all signals equally across the frequency spectrum
- Reject signals that are identical at both inputs while amplifying their difference (Correct answer)
- Filter out frequencies above 70 Hz
- Maintain constant impedance across all electrodes
Correct answer: Reject signals that are identical at both inputs while amplifying their difference
CMRR quantifies how effectively the differential amplifier rejects common-mode signals (those appearing equally at both inputs, such as 60 Hz noise) while preserving the differential signal (brain activity).
The common-mode rejection ratio (CMRR) is expressed in decibels and represents the ratio of differential gain to common-mode gain. A CMRR of 100 dB means common-mode signals are attenuated by 100,000 times relative to differential signals. EEG amplifiers typically achieve CMRR of 80-120 dB. Environmental noise (60 Hz power line, fluorescent lights) appears as common-mode signal at both inputs and is rejected. The brain signal of interest is differential — slightly different at each electrode — and is amplified. Balanced electrode impedances are critical because impedance mismatch converts common-mode noise into differential signal, degrading effective CMRR.
Question 3: The high-frequency filter (low-pass filter) in EEG recording at a typical setting of 70 Hz will:
- Block all frequencies below 70 Hz
- Attenuate frequencies above 70 Hz while passing lower frequencies (Correct answer)
- Only display the 70 Hz component
- Amplify frequencies above 70 Hz
Correct answer: Attenuate frequencies above 70 Hz while passing lower frequencies
The high-frequency (low-pass) filter attenuates signals above its cutoff frequency. At 70 Hz, it progressively reduces muscle artifact and electrical noise above 70 Hz while preserving brain activity which is predominantly below 70 Hz.
The high-frequency filter (HFF), also called a low-pass filter, attenuates frequencies above the set cutoff point. The standard clinical EEG setting is 70 Hz (sometimes 35 Hz for reducing muscle artifact). At the cutoff frequency, the signal is reduced by 3 dB (approximately 30%). Attenuation increases at higher frequencies at a rate determined by the filter's roll-off (dB/octave). Setting HFF too low (e.g., 15 Hz) distorts spike morphology by removing the sharp components. The 70 Hz setting provides a good balance between reducing high-frequency noise (EMG, electrical interference) and preserving clinically relevant waveform features.
Question 4: What is the minimum recommended sampling rate for routine digital EEG recording?
- 64 Hz
- 128 Hz
- 256 Hz (Correct answer)
- 1024 Hz
Correct answer: 256 Hz
A sampling rate of at least 256 Hz is recommended for routine EEG to adequately capture frequencies up to 70-100 Hz according to the Nyquist theorem (sampling rate must be at least twice the highest frequency of interest).
The Nyquist-Shannon sampling theorem states that the sampling rate must be at least twice the highest frequency component of interest. For routine EEG with a bandwidth of 0.5-70 Hz, the minimum sampling rate should be at least 140 Hz, but 256 Hz is recommended to provide adequate oversampling for waveform fidelity. Many modern systems use 256, 512, or 1024 Hz. Higher sampling rates provide better temporal resolution and waveform reproduction but require more storage. The ACNS recommends a minimum of 256 Hz. For high-frequency oscillation research, sampling rates of 2000+ Hz are needed. Anti-aliasing filters must be applied before digitization.
Question 5: The time constant of an EEG amplifier determines:
- How quickly the amplifier reaches maximum gain
- The rate at which a DC-shifted signal decays back to baseline through the low-frequency filter (Correct answer)
- The maximum amplitude the amplifier can display
- The number of channels that can record simultaneously
Correct answer: The rate at which a DC-shifted signal decays back to baseline through the low-frequency filter
The time constant represents the time it takes for a DC-shifted square wave calibration signal to decay to 37% of its initial value. It is directly related to the low-frequency filter setting.
The time constant (TC) is the time required for a square wave signal to decay to 37% (1/e) of its initial amplitude. It is inversely related to the low-frequency filter (LFF): TC = 1/(2π × LFF). For a standard LFF of 1 Hz, TC = 0.16 seconds. A longer time constant (lower LFF) allows more slow activity to pass but also admits more slow artifacts (sweat, movement). A shorter time constant (higher LFF) reduces slow artifacts but may filter out slow pathological activity like delta waves. For routine EEG, a TC of 0.3 seconds (LFF = 0.53 Hz) or LFF of 1 Hz is standard. Understanding this relationship helps technologists choose appropriate filter settings.
Question 6: During a routine EEG, the standard display sensitivity is set to:
- 3 µV/mm
- 7 µV/mm (Correct answer)
- 15 µV/mm
- 50 µV/mm
Correct answer: 7 µV/mm
The standard display sensitivity for routine EEG is 7 µV/mm, which provides adequate visualization of normal brain rhythms. Sensitivity may be adjusted for high-amplitude (increase µV/mm) or low-amplitude (decrease µV/mm) activity.
The standard EEG display sensitivity of 7 µV/mm (sometimes expressed as the reciprocal: approximately 140 mm/50µV) provides optimal visualization of normal EEG amplitudes. Normal adult posterior dominant rhythm (alpha) ranges from 20-60 µV, producing deflections of 3-9 mm at this sensitivity — visible but not overlapping adjacent channels. The technologist adjusts sensitivity as needed: lower µV/mm (e.g., 3-5) for low-amplitude recordings or electrocerebral inactivity studies, higher µV/mm (e.g., 10-15) for high-amplitude activity like seizures or pediatric recordings. Digital systems allow post-hoc sensitivity adjustment, but the recording should be acquired with optimal dynamic range.
EEG Instrumentation and Calibration 2
What is the standard calibration signal used to verify EEG amplifier function?