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EEG Voltage and Power Scales
Transcript of EEG Voltage and Power Scales
Definitions - Voltage... a Review...
for current to flow from one place to another (i.e. not the actual flow of ionized particles itself, but rather the "pressure" that pushes electricity through a conductor).
Voltage potentials measured on the scalp are thought to arise from postsynaptic activation of cortical pyramidal neurons.
EEG Voltage and Power Scales
Content and Prezi by Phil Vlisides
When these neurons are activated (as defined by postsynaptic potential detection) at a certain frequency (i.e. 8-12 Hz), a characteristic EEG waveform is rendered, with time on the x-axis and voltage on the y-axis:
unit of measurement for the above potential. EEG voltage signals are relatively small, measured in μV.
When large groups of cortical pyramidal neurons are activated simultaneously, discrete voltage signals can be detected...
Changing the voltage scale on EEG monitoring devices can then produce markedly different waveform appearances based on the chosen voltage scale (i.e. 1 μV, 10 μV, 100 μV, etc.).
Thus, it is important to choose an appropriate voltage scale for optimal wave resolution on the EEG device...
Too high of a scale can make low-amplitude slow wave anesthesia appear as isoelectricity...
On the other hand, too low of a scale can make the waveforms difficult to read...
Thus, it is important to choose an appropriate voltage scale for optimal wave resolution on the EEG device. This is often approximately 5-10 μV for most patients.
Spectrogram - a Review...
Different EEG frequency waveforms exist in the brain at any give time (see Module 2: Intro to Waveforms for more).
Depending on the state of arousal, different proportions of these frequencies exist at any given time. In general, there is a higher proportion of higher frequency waveforms during awake states and more slow-frequency waveforms with states of anesthesia.
The EEG spectrogram helps us to visualize this phenomenon. With a spectrogram, the visual spectrum of frequencies is visualized over time...
The density, or “amount” (represented by color intensity) of a particular frequency’s presence at any given time is quantified on the z-axis by power (dB)
Some EEG monitors with spectrogram capabilities allow modulation of the power scale color bar. In other words, color intensity can be re-assigned to different dB levels (i.e. red can be reassigned from 20 -> 5 dB or vice versa).
This scale modulation may either highlight or dull a particular spectrogram signature...
It is thus important to be aware of the calculated power of your signal and color spectrum represented when selecting your scale....
Note: scales in this presentation are for illustration purposes only and are not completely accurate.
Apical dendrite, pyramidal neuron
Presynaptic cortical (or
thalamic) axonal input
Cell body, pyramidal neuron
Basal dendrite, pyramidal neuron
Further, if these neurons are oriented along the same axis, and that axis is perpendicular to the scalp electrode, the voltage dipole will be recorded as a large, single spike...