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EEG Technical Fundamentals – Exam Prep
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EEG Technical Fundamentals – Exam Prep
EEG Technical Fundamentals – Exam Prep
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1
Question
What is analog-to-digital conversion (ADC) in EEG?
Answer
The process of converting a continuous analog signal (variable voltage and time) into a discrete digital signal. Each point on the waveform is sampled and assigned a binary value (0 or 1) based on its amplitude. The more samples/bins, the cleaner the digital reconstruction.
2
Question
State the Nyquist Criteria.
Answer
The sampling rate must be at least TWICE the highest frequency being recorded. Since EEG signals range from 1–100 Hz, the minimum sampling rate is 200 Hz (Canadian standard).
3
Question
What is aliasing and what causes it?
Answer
Aliasing is the distortion of a waveform's morphology caused by sampling at a rate below the Nyquist requirement (< 200 Hz). High-frequency signals falsely appear as lower-frequency signals — the waveform no longer reflects true brain activity.
4
Question
What is the minimum bit depth (vertical resolution) required for EEG, and why?
Answer
Minimum 12 bits = 4,096 numeric bins, with a voltage resolution of 0.5 µV. Too few bits blunts spike peaks (large changes go undetected) and overrepresents tiny noise fluctuations.
5
Question
What is the minimum screen resolution for digital EEG in Canada?
Answer
1024 × 768 pixels (some sources cite 1280 × 1024). Insufficient resolution mimics a reduced sampling rate and displays incomplete waveform data.
6
Question
What is sampling skew?
Answer
When data channels are not sampled simultaneously, causing a time-axis misalignment. Phase reversals may not appear perfect because waveforms are slightly offset in time.
7
Question
What is a High Frequency Filter (HFF) and what circuit produces it?
Answer
An HFF (low-pass filter) attenuates frequencies above the set cutoff. It is produced by an RC circuit. The cutoff point = 80% amplitude at the set frequency. Common settings: 15 Hz, 35 Hz, 70 Hz.
8
Question
What is a Low Frequency Filter (LFF) and what circuit produces it?
Answer
An LFF (high-pass filter) attenuates frequencies below the set cutoff. It is produced by a CR circuit. Common settings: 0.1, 0.3, 0.5, 1.0, 5 Hz.
9
Question
What is a time constant and how does it relate to the LFF?
Answer
Time constant = 1/f (where f = filter frequency). It is the measure of capacitor decay — the time for the signal to drop to 2/3 of its amplitude. A longer time constant allows more slow activity to pass through.
10
Question
What effect does setting a 15 Hz HFF have on a spike wave?
Answer
The spike wave decreases in amplitude and becomes rounded. High-frequency components of the spike are cut off, blunting its sharp morphology.
11
Question
What effect does changing the LFF from 1 Hz to 0.3 Hz have on a slow (delta) wave?
Answer
Opening the low-frequency band allows more slow activity to pass through, increasing the amplitude of slow waves — they appear taller.
12
Question
Is the use of a 60 Hz notch filter recommended? Why or why not?
Answer
No — it is only a last resort. It distorts waveform morphology, truncates spikes (makes them look rounded), and can make EMG (muscle) activity look like fake spikes, causing false interpretations.
13
Question
What is a FIR filter vs. an IIR filter?
Answer
FIR (Finite Impulse Response): digital, very precise, steeper slope, linear phase, always stable, no feedback in calculation. IIR (Infinite Impulse Response): more similar to analog filters, less steep slope, uses feedback, less stable but more efficient.
14
Question
What is a differential amplifier and how does it work?
Answer
Takes the difference between Input 1 and Input 2, amplifying only the signal that differs between them. Signals common to both inputs (noise/artifact) are rejected — this is called Common Mode Rejection.
15
Question
What is Common Mode Rejection Ratio (CMRR) and what is the acceptable value?
Answer
CMRR is a rating of amplifier quality — how well it rejects in-phase (common) signals and passes non-in-phase signals. The acceptable minimum is 10,000:1 (for every 10,000 units of shared signal, only 1 unit passes through).
16
Question
What is electrode impedance and what is the acceptable range?
Answer
Impedance = measure of how well the electrode is connected to the scalp. Normal range: 100–5,000 Ω. For brain death: 100–10,000 Ω. Balanced and equal impedance between electrodes is ideal.
17
Question
State the polarity rules for a differential amplifier.
Answer
• Input 1 more positive than Input 2 → downward deflection • Input 1 more negative than Input 2 → upward deflection • Equal inputs → flat line (common mode rejected)
18
Question
What are the advantages and disadvantages of a bipolar montage?
Answer
Advantages: localizes focal abnormalities via phase reversal; rejects noise common to both electrodes; allows hemisphere comparison. Disadvantages: cancellation effect (out-of-phase signals may cancel); end-of-chain phenomenon.
19
Question
What are the advantages and disadvantages of a referential montage?
Answer
Advantages: allows true amplitude comparison; highest amplitude recording. Disadvantages: no true reference electrode exists; voltage gradient issues; reference contamination (esp. Fp1, Fp2, O1, O2, midline electrodes).
20
Question
How do you create a common average reference?
Answer
Average all electrodes EXCEPT those prone to artifacts: exclude Fp1/Fp2 (eye movements), O1/O2 (alpha), and midline electrodes (sleep potentials). Most useful for enhancing focal abnormalities.
21
Question
What is the polarity of the cornea relative to the retina?
Answer
The cornea (front of the eye) is POSITIVE relative to the retina. This corneoretinal dipole is the basis of all ocular artifact production.
22
Question
How do you monitor VERTICAL eye movements?
Answer
Place Input 1 below the eye (or use Fp1/Fp2), with Input 2 as the reference. When the eye moves upward, the cornea (positive) approaches the electrode below, causing an upward deflection at that electrode.
23
Question
How do you monitor HORIZONTAL eye movements?
Answer
Place electrodes lateral to each eye (e.g., F7/F8 region). When eyes move left, F7 becomes more positive → F7-T3 shows a phase reversal. Opposite pattern for rightward gaze.
24
Question
Name 2 clinical situations where monitoring eye movements is useful.
Answer
1. Differentiating FIRDA (in-phase) from eye blinks (out-of-phase) 2. Patient with nystagmus 3. Identifying REM sleep
25
Question
What is a photoparoxysmal response (PPR)?
Answer
A generalized spike-and-wave discharge triggered by photic (flash) stimulation, typically elicited at flash frequencies between 14–18 Hz. The strobe lamp must be placed 30 cm from the nasion, symmetrically over both eyes.
26
Question
According to CSCN, why are 50 Hz and 60 Hz flash frequencies used in photic stimulation?
Answer
These frequencies correspond to the frequency of TV screens and common environmental stimuli — testing at these frequencies checks for photosensitivity to real-world conditions.
27
Question
What are the 4 landmarks for the 10-20 electrode placement system?
Answer
Nasion, Inion, Left pre-auricular point, Right pre-auricular point. All measurements are made as 10% and 20% intervals between these landmarks.
28
Question
Where is electrode F1 located?
Answer
Halfway between Fz and F3.
29
Question
Describe the anatomical locations of C3 and O1.
Answer
C3 = left parasagittal head region (central). O1 = left occipital region.
30
Question
What are the steps to troubleshoot an artifact in EEG?
Answer
1. Identify — determine which electrode/channel has the artifact 2. Eliminate — check impedance, electrode contact, turn off the source 3. Filter — use the appropriate filter for the artifact's frequency 4. Monitor — if physiologic (EMG, EKG, respiratory), you cannot eliminate it; add monitoring channels instead