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Copy of Log Squared

Neurophysiology of EEG
by

Scheib Christopher

on 18 October 2015

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Transcript of Copy of Log Squared

Thalamocortical Circuit
Large amplitude, low frequency activity in EEG during anesthesia is due to slow oscillations in the activity of cortical neurons.
Cortical Slow Waves
The same EEG filtered to show thalamic spindle waves
The same EEG filtered to show cortical slow waves
Raw EEG during propofol anesthesia
Thalamic Spindle Waves and
Cortical Slow Waves
1) Post synaptic potentials on cortical neurons create the EEG.
2) The action potentials that create the PSP’s come from thalamic or other cortical neurons.
Pyramidal dendrite PSP’s and the EEG
During anesthesia spindle waves are paced by the reticular thalamic nucleus.
EEG Spindle Waves
EEG and the
Neurophysiology
of Anesthesia

Neurophysiology
EEG Signal Analysis Methods
This is a spectrogram of an EEG.

The x-axis is frequency in Hertz.

The arrows point to peaks at 1 and 10 hertz.
Frequency Domain Methods
A set of filters can be used to separate the raw EEG signal into frequency bands.

This is a “time domain” method because the x-axis is time.

The amplitude scale changes as the frequency changes because the higher frequencies have a much lower amplitude.
Filtering the EEG signal
The raw EEG is a complex signal that can be analyzed to reveal multiple signals that are superimposed.

There are several methods that can be used to separate the component signals.
EEG Signal Analysis Methodology
Bicoherence shows the synchronization between frequencies.

This graph shows highly synchronized activity at 10 Hertz.

The horizontal and vertical lines imply that the 10 Hertz activity synchronizes with higher frequencies.
Bicoherence
Using a log frequency scale has advantages over a linear scale.
The long curved line from 10 – 40 Hz becomes a short straight line.
The details of the lower frequencies become more apparent.
The traditional presentation of the EEG spectra is log-linear (log amplitude & linear frequency scales).
Spectrogram on a Log-Log Scale
The lower spectra y-axis is a log scale.
The amplitude of low frequency components is so much greater than the high frequency components that a log scale is required to show both.
Both spectra are the same data.
The upper spectra y-axis is a linear scale.
Spectrogram with Log Scale
Gamma Band
25-45 Hz
Beta Band
14-25 Hz
Alpha Band
7-14 Hz
Theta Band
3.5-7 Hz
Delta Band
0.5-3.5 Hz
Raw EEG
Filtering the EEG Signal
EEG Spindles and Propofol Level
This is a Log-Log Spectra showing a peak at 10 Hertz. The frequency scale goes from 1 Hertz to 79 Hertz.
Spindles in the Frequency Domain
Bicoherence at the Maximum Spindle level shows 10 hertz spindle activity effecting other frequencies up to 40 Hertz.



At a lower propofol level the spindle is mostly effecting activity at 5 and 20 Hertz.
Bicoherence and Propofol Levels
Maximum spindle spectra (yellow upper line) is compared to the spectra at a lower propofol level. There is a loss of amplitude particularly at frequencies below 10 Hertz and the spindle peak is shorter.


At a still lower level of propofol there is a further loss of amplitude.
EEG Spectra and Propofol Levels
(30 seconds) (4 seconds)
Decreasing Propofol and Smaller Spindles
Maximum spindle size


Decreasing propofol level, decreasing spindle cycle length, increasing inter-spindle time
This presentation shows the number of cycles in a spindle in the 4 second window and the number of spindles in the 30 second window.
(30 seconds) (4 seconds)
Raw EEG
7-14 Hertz
Spindles in the Time Domain
Spindles and Propofol Level
The red line is the Bispectral Index, the green line is the spectral edge, and the yellow line is EMG. None of these parameters changed as cortical slow wave activity decreased and spindle activity increased with decreased propofol infusion rate.
BIS did not change with csw and spindles
The yellow line spectra was recorded at the higher propofol infusion rate. The white line spectra shows less amplitude in the cortical slow wave frequency range, a larger spindle peak, and no change in the frequency range between them.
Changing spectra track csw and spindles
EEG filtered to pass only 7 to 14 Hertz for 30 second periods. The spindle bursts become longer and more frequent as the propofol level decreases.
Spindles at three propofol levels
EEG filtered to pass only 0.5 to 6 Hertz for 30 second periods. The oscillations have lower amplitude and higher frequency as the propofol level decreases.
CSW at three propofol levels
After the propofol infusion rate is decreased, csw decreases, spindles increase, and theta is unchanged.
Blue is activity between 7 and 14 Hertz (spindles). Green is between 3.5 and 7.5 Hertz (theta).
Red is activity between 0.5 and 6 Hertz which is the frequency range of cortical slow waves (csw).
Filtered bands can be used to track cortical slow waves and spindles
Tracking Cortical Slow Waves and Spindles
The blue line tracks the spindle peak height above baseline and the green is frequency.
DSA plot shows decreased activity at 1Hz and increased activity at the spindle frequency later in the tracing.
DSA plot and peak parameters
Tracking Cortical Slow Waves
and Spindles

The amplitude of the EEG spectra decreases with age. The pattern of the maximum spindle pattern changes little as increasing age leads to decreasing amplitude of all frequencies in the EEG spectra.
EEG spectra amplitude and age
The consistency of the shape of the maximum spindle spectra is illustrated by comparing the highest amplitude, the lowest amplitude and the average spectra from 13 TIVA cases.
Maximum Spindle Spectra
For this TIVA case the red line is the maximum spindle spectra.

Increasing (blue line) or decreasing (green line) the propofol reduces the spindle peak and changes the spectra in predictable ways.
Maximum Spindle Spectra pattern
For any given patient there is an anesthetic level which produces the most EEG spindle activity.
Maximum Spindle Anesthetic Level
An initial EEG spectra from a patient may look like the maximum spindle pattern. To be certain one must raise and lower the anesthetic and compare the spectra.
Finding MACeeg
The pattern of the maximum spindle point is similar for TIVA, inhaled agents, and combination propofol and inhaled agent. This pattern could be a reference point for the level of anesthesia that is based on the EEG.
MACeeg
The Maximum spindle spectra of 13 TIVA Diagnostic Laparoscopy cases have the same shape but different amplitudes.
Maximum Spindle Spectra
Color can be used to show changes between the new spectra and the reference spectra. Green shows an increase and red a decrease in amplitude.
Comparing EEG Spectra
Maximum Spindle Level
Full transcript