Periodic EEG features in light sleep and propofol sedation
Frederic von Wegner,
Milena Wiemers, Gesine Hermann, Inken Tödt, Enzo Tagliazucchi,
Helmut Laufs
DFG project: Objective EEG bed side
assessment of impaired consciousness in epilepsy
DGKN 2023
EEG background
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EEG records electrical activity from several
cm2 of cortex
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each electrode records PSPs of (tens of) millions of neurons
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volume conduction averages (`blurs`) source activity
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EEG reflects excitability, neuronal synchrony, and connectivity
Preliminary observations
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EEG appears 'disorganzed' in both drowsy conditions
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Topographical organization seems to be lost
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Spectral organization seems to be lost
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Aim:
quantify these observations
Data sets
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Wake/sleep data set
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n=32 healthy subjects
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Simultaneous EEG-fMRI recordings (fMRI not used here)
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30 channel EEG (subset of 10-10 system)
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Sleep stages manually scored (AASM rules)
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Conditions:
wake (W), light sleep (N1)
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Example publications:
Brodbeck et al., 2012,
Kuhn et al., 2014
Tagliazucchi et al., 2012,
von Wegner et al., 2017
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Propofol data set
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n=20 neurologically healthy subjects
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128-channel EEG ('clean' subset of 91 channels used)
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Sedation levels monitored by propofol plasma concentration
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Conditions:
baseline, mild and moderate sedation, recovery.
Note:
Moderate sedation: still responsive, reaction time delays
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Published by:
Chennu et al., PLoS Comp Biol 2016, data
repository:
https://www.repository.cam.ac.uk/handle/1810/252736
Main questions
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Is EEG activity periodic at the global (scalp-wide) level?
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Is periodicity a marker of reduced vigilance (consciousness) ?
EEG microstates
(Lehmann, 1972)
How to summarize spatio-temporal information?
Step 2: Microstate sequence
EEG microstates and brain networks
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Resting-state microstate maps can be mapped to their
likely (brain-wide) current sources
(Custo et al., Brain Connectivity, 2017)
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Map C seems to correspond to the default mode network (DMN),
others are less clear
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Take-home:
when you see a microstate label (A..D), think "brain network"
Microstates and data compression
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Initial data set: 30 channels x 16 bit (for instance)
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Microstate sequence: single letter (A...D) = 2 bit
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Compression factor: 240
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Question: can this (0.4%) encode enough information to capture
subtle differences in alertness?
Analysis-1
(von Wegner, 2017)
Analysis-2
(von Wegner, in prep.)
Comment
Wiener-Khintchine theorem
Previous results
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EEG topographies at rest are periodic
(von Wegner, NeuroImage 2017, 2022)
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Their frequency is approx. twice the alpha frequency (20 Hz)
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Periodicity is quantified by information-theoretical measures
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Remaining question: frequency doubling?
(10 Hz EEG → 20 Hz microstates)
Frequency doubling
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Explained by the microstate algorithm - ignores polarity
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EEG polarity inverts every 50 ms (half an alpha phase),
both are mapped to microstate C (e.g. 48, 96 ms)
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Remember:
the microstate algorithm doubles the dominant EEG frequency
Results
Question:
Loss of consciousness - loss of topography?
Results
Microstate maps in reduced consciousness
Surprise:
Microstate maps don't change much in sleep and propofol sedation.
Results
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Autoinformation analysis - propofol
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A, baseline (no propofol): microstates occur
periodically (50 ms, 100 ms, ...) ~ 20 Hz
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B, mild sedation (approx. 0.6 µg/ml):
periodicity not affected
(subjects behaviourally relatively normal, see Chennu et al.)
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C, moderate sedation (approx. 1.2 µg/ml): periodicity attenuated
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D, recovery: effects are reversible
Results
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Spectral analysis - propofol
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A, baseline (no propofol): EEG main frequency (blue) 10 Hz,
microstate sequences (black) 20 Hz
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B, mild sedation (approx. 0.6 µg/ml):
periodicity not affected
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C, moderate sedation (approx. 1.2 µg/ml): EEG channels
have additional beta and delta frequencies, alpha attenuated,
microstate frequency peak at 20 Hz disappears
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D, recovery: effects are reversible
Results
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Autoinformation analysis - sleep
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W, wakeful: EEG main frequency (left) 10 Hz,
microstate sequences (black) occur periodically
(50 ms, 100 ms, ...) ~ 20 Hz
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N1, light sleep: EEG channels (left) lose their periodicity,
microstate sequences (right) too
Results
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Spectral analysis - sleep
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W, wakeful: EEG main frequency (left) 10 Hz,
microstate sequences ~ 20 Hz
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N1, light sleep: EEG channels (left) lose 10 Hz alpha
periodicity, microstate sequences (right) too
Conclusions
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EEG microstate sequences capture spatial (topographic) and temporal
characteristics simultaneously
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EEG patterns occur periodically, closely linked to the
individual's alpha frequency during wakefulness (explain
frequency doubling?)
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Loss of periodicity as a common feature of reduced consciousness
in light sleep and propofol sedation
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Single electrode level is not a good predictor of global
synchrony (propofol, moderate sedation)
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In the analyzed conditions, periodic, coherent scalp-wide
EEG patterns are a signature of intact vigilance (awareness,
consciousness)
Explanations & Outlook
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EEG - biophysical models - coupled oscillators
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Coupled oscillators close to criticality produce coherent but
changing patterns, leading to 'global patterns'
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Microstates represent momentarily phase-coupled oscillator
populations
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Loss of consciousness - loss of critical coupling
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Role of information processing and sharing across brain
regions
Thank you!
This project is a collaborative effort funded by the DFG
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Helmut Laufs
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Gesine Hermann
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Inken Toedt
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Enzo Tagliazucchi
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Frederic von Wegner (f.vonwegner@unsw.edu.au)