EEG Flashcards
EEG
Which neurons are the mains contributors to scalp-recorded EEG signals
Pyramidal cells
Arranged in the perpendicular orientation to the cortical surface
EEG
What is the phase convention of EEG waveforms?
Downward = +
Upward = -
EEG
What are the two possible EEG montages
Referential and bipolar
EEG
How is localisation of a discharge made using a :
a) bipolar montage
b) referential montage
a) analysing orientation and amplitude
b) analysing amplitude alone
EEG
What is a bipolar montage
Each electrode’s voltage is linked and compared to an adjacent one to form a chain of electrodes. In each chain, an electrode’s voltage is compared to that of the electrode behind it, so each tracing line is a pair of electrodes in which the voltage of the second electrode is subtracted from the voltage of the first. Because of this, in bipolar if the first electrode in the tracing line is more positive/higher than the second, you get a positive, downward deflection; if the second electrode is more positive/higher, you get a negative, upward deflection.
EEG
How to interprete phase reversal in a bipolar montage?
With phase reversals, the middle electrode of the pair that makes the reversal is the electrode of maximal voltage (ex. T3-T5 and T5-O1 phase reversal means T5 has the greatest voltage of them all).
Negative discharges cause the surrounding tracings to point toward the electrode of maximal voltage, while positive discharges cause surrounding tracings to point away from the electrode of max voltage (an easy way to remember this: positives can fit a plus sign, and negatives can only fit a negative sign)
EEG
In a bipolar montage, what is an end of chain phenomenon?
Occurs when potentials appear with greatest voltage in the last electrode in the chain of electrodes producing waveform deflections that are all in the same direction (no phase reversal). Solution: use another type of bipolar montage (e.g. circumferential montage).
EEG
What is the most common used bipolar montage:
Double banana, marked by the bilateral temporal chains over the bilateral parasagittal chains, and a central chain below that.
EEG
What is a referential montage
Compare all the electrodes to single reference point.
There are no phase reversals, and as such, the highest amplitude waveform is the one with the greatest voltage, be it downward or upward.
EEG
What is a major limitation to the reference montage?
When the active electrode is the reference electrode, all the traces will be deflected in the same direction (following the reference signal). Creates a reference electrode artifact.
EEG
Define low f (high pass) filter
filter out frequencies below a certain threshold
EEG
Define high frequency (low pass) filter
Filter out frequencies above a certain threshold
EEG
Define notch filter
selectively removes 60 Hz activity that arises from electrical interference such as wires and equipment
EEG
Define sensitivity
Affects the height of the waveforms.
Higher Se = Lower number = smaller waveforms
EEG
Define page speed or epoch
Determines how many seconds of the study are displayed across your computer monitor at one time. Higher speed makes tracing look more stretched, low speed makes tracing look more condensed
note: each division on the epoch represents 1 second
EEG
Name the 4 frequency bandwiths
from low frequency/high amplitude –> high frequency/low amplitude
- Delta (<4 Hz) - deep sleep
- Theta (4-8 Hz) - sleep
- Alpha (8-13 Hz) - Awake
- Beta (13-30 Hz) - Stimulated / mental activity
- Gamma, low (30-60 Hz)
- Gamma, high (>70 Hz)
- Ripples (> 100 Hz) - epileptiform discharges
EEG
Name the patient-origin, physiological artifacts
Scalp muscles (EMG)
Eye movements (EOG) - blink (up/down), REM (lateral)
Cardiac muscles (ECG)
Breathing
Blinking
Tongue and jaw mts (e.g. chewing)
https://www.learningeeg.com/artifacts
EEG
Name the external, non physiological artifacts
Intstrumental
Electrode
Environmental
Digital
EEG
Describe the Blink artifact. what is the underlying phenomenon?
Large positive (downward) deflection as the eye move upward in its orbit. Only seen in the frontal leads (F3 or F7).
Is a key component of normal, awake EEG.
It is caused by the Bell’s Phenomenon. The eyes’ cornea is positively charged and retina is negatively charged; when you blink, the eyes roll up slightly, and the cornea moves closer to the frontal electrodes Fp1 and Fp2, which thus see a positive signal that is reflected on EEG.
http://www.eegpedia.org/index.php?title=Eye_blink_artifact
EEG
Describe lateral eye movements (e.g. REM sleep)
Positive frontal deflection on the side to which you look, and a contralateral negative frontal deflection (opposite waveforms in bilateral frontal regions)
EEG
Describre muscle artifacts
High frequency (40-70Hz), low amplitude. Often dominant in the frontal and lateral tempora leads.
Describre chewing artifacts
Diffuse bursts of fast myogenic artifact; hypoglossal artifact is diffuse and slow, synchronized activity
EEG
Describe ECG artifacts
Look like negative spikes time-locked to the QRS complex
EEG
Describe an electrode pop
Single electrode showing a very sudden, steep upslope with a slower downslope and absolutely no field.
