Fisch EEG Primer Chapter 3 Flashcards
Digital and Analog EEG instruments: parts and functions
EEG frequency range of greatest clinical significance
0.5-30Hz
Aliasing results from..
sampling analog signals LESS than at least TWICE (Nyquist theorum and rate) as fast as the fasted freqency contained in the analog signal.
Difference between the electrode input board (aka jackbox, input box) of an analog vs. digital EEG instrument
analog: receives input and then transfers to amplifier
digital: receives AND amplifies signal before sending to computer monitor
Each channel of the EEG recording is producted by the output of one differential amplifier which measures…
difference in voltage between its two input terminals. Negative differences (greater negativity at input 1 compared to input 2)deflect UPWARDS and positive difference (greater negativey at input 2 compared to input 1) deflect DOWNWARDS.
Digital Filtering involves…
suppression of waveforms in selected frequency ranges
Montage reformatting involves..
retrospective changing of the displays montage
Purpose of EEG amplifiers (2)..
- differential discrimination of differences in electrical potnetial of input 1 and 2
- amplify size of potential differences
Differential discrimination/amplification (performed by amplifiers) involves
inputs 1 and 2 are essentially amplified in opposite directions from the amplifier ground, and effectively subtracted
In differntial amplification, a single waveform output can be the result of
an infinite number of different inputs
What is the difference between impedence and resistance
resistance involves direct (constant voltage) while impedence involves alternating currents.
60 Hz interference can be seen when…
unequal electrode impedence is present - most commonly results from loss of contact between electrode and scalp
Absent or ineffective ground connection results in…
lack of reference to polarize inputs 1 and 2 and results in amplication of artifacts that would otherwise be ignored
NOTE: does not protect patient from electrical shock
Amplification is characterized in terms of sensitivity which is measured by…
sensitivity: microVolts/mm (typical 7)
Low frequency filter (a.k.a. High pass filter)
reduces the amplitude of slow waves - decreases amplitude of selected frequency range 20-30% and then logarithmic filtering at even lower frequencies
High frequency filter (a.k.a. Low pass filter)
reduces the amplitude of fast waves - decreases amplitude of selected frequency range 20-30% and then logarithmic filtering at even higher frequencies