Transcript Conscious freely-willed movements are predicted by changes

Conjunct COST B27 and SAN
Scientific Meeting,
Swansea, UK, 16-18 September 2006
www.swansea.ac.uk
Bispectral analysis of the
EEG: what does it add to the state
versus non-state debate in hypnosis?
Adrian Burgess,
University of Swansea
Helen Crawford,
Virginia Polytechnic Institute & State
University
www.swansea.ac.uk
Plan of Talk
• What is bispectral analysis?
• State –vs- Non-state theories of Hypnosis
• Why is bispectral analysis relevant to the State –vs- Nonstate debate?
• The EEG bispectrum in hypnosis and waking for high and
low susceptible participants
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What is bispectral analysis?
Bispectral analysis is a Fourier based method for
examining the coupling between frequencies in
different ranges
The bispectrum is defined as:


B ( f1 , f 2 )  E X ( f1 )  X ( f 2 )  X ( f1 , f 2 )

Where X(.)=Fourier Transform of the time series x(t) and * indicates the complex
conjugate
Bicoherence is the normalised bispectrum:
L ( f1 , f 2 ) 
B ( f1 , f 2 )
2
E X ( f1 )  X ( f 2 )  E X ( f1  f 2 )
2
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What is bispectral analysis?
Within Channels
Between Channels
1st order
Mean
2nd order
Fourier Spectrum
Coherence
3rd order
Bicoherence
Cross-bicoherence
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State -vs- Non-state theories of Hypnosis
• State theorists believe that hypnosis is an altered state of
consciousness,
• Non-state theorists believe that hypnotic effects are the
product of more-mundane psychological processes such as
expectancy & role-play
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Neurophysiological evidence in favour of the State-Theory
• More than 20 years of EEG/ERP research has shown that the hypnotic
state is associated neurophysioloigcal changes in
– Alpha
– Theta
– Gamma
– ERP (e.g. MMN, Somatosensory ERP) etc….
• However, the differences are
– quantitative not qualitative
•
cf other states of consciousness
– within the normal range
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Why is Bispectral Analysis relevant to the
State -vs- Non-state debate?
• Bispectral Analysis has been shown to be a useful measure
of level of consciousness
– ~1000 research papers on Bispectral Analysis and anaesthesia
• The Bispectral Index (BIS®) is a patented technology
produced by Aspect Medical Systems that uses
– the bicoherence in the EEG
– the ratio of EEG power in the delta (1–4 Hz) and beta (13–30 Hz)
frequency ranges
– the proportion of the EEG that is isoelectric (i.e. electrical silence)
to produce an index of depth of ‘hypnosis’
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Hypotheses
• Participants with high susceptibility to hypnosis will show a
significant change in the bispectrum of their EEG between
the waking and hypnotic states
• Participants with low susceptibility to hypnosis will NOT
show a significant change in the bispectrum of their EEG
between the waking and hypnotic states
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Method - Participants
• Healthy, young, right-handed volunteers
• Pre-selected using the Stanford Hypnotic Susceptibility
Scale (SHSSC)
–
12 high susceptible (SHSS-C ≥9)
• Age range 20-24
• 10 women, 2 men
–
12 low susceptible (SHSS-C ≤4)
• Age range 20-24
• 9 women, 3 men
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Method- EEG
Recorded EEG from young, healthy volunteers
– 32-channel Neuroscan Synamps
– 28 EEG Channels
– Sampling rate 500Hz
– Bandpass 0.1-150Hz
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Method- Procedure
Stanford Hypnotic
Susceptibility Scale
Pre-induction
Eyes Closed
Waking
Pre-induction
Memory test
Hypnotic
Induction
SHSS-C
Post-induction
Eyes Closed
Post-induction
Memory test
Hypnotised
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Calculation of the bispectrum
• Bispectrum was calculated on the Eyes Closed Condition in
– Waking (pre-induction)
– Hypnosis (post-induction)
• Calculated using the MATLAB toolbox ‘Higher Order
Spectral Analysis’
• Averaged Bispectrum from the mean of ~4 minutes of EEG
divided into epochs of 1.024s
• Range 0-100Hz with a resolution of ~1Hz.
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Example of an EEG bispectrum
Alpha Peak
(10Hz,10Hz)
Alpha-Delta
Coupling
(8Hz,2Hz)
Delta Peak
(2Hz,2Hz)
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Topography of the bispctrum
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Bispectrum by Group and Condition
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Partial Least Squares Regression
•
•
•
•
A combination & extension of:
• Multiple Regression
• PCA
Designed to identify simultaneously
i) Whether the experimental design has an
effect
ii) Where in the data the effect is seen
Used rotated PLS with
• Hypnosis -vs- Waking
• For High and Low susceptible groups
– 1000 randomizations
– 1000 bootstrap samples
Output
• Latent variables showing contrasts
i.e. is there an effect?
•
Saliences showing location of differences
i.e where is the effect
From Lobaugh et al., 2000
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1st Latent variable – PLS of Bispectrum
LV 1; 95.3% cross-block variance, p<0.01
HIGHS
LOWS
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Topography of reliable differences between
Waking & Hypnosis
Bispectrum higher in the
Waking condition
•Midline frontal
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•Temporo-occipital
Reliable differences between Waking &
Hypnosis – across all electrodes
Bispectrum higher in the Waking condition
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Summary
• PLS analysis showed significant differences in the
bispectrum between waking and hypnosis for the High
Susceptible group
• Bispectrum was higher in the waking condition esp at high
frequencies
– Midline frontal
– Temporo-occipital sites
• What about bicoherence?
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1st Latent variable – PLS of Bicoherence
LV 1; 64.6% cross-block variance, p<0.26
HIGHS
LOWS
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Why the discrepancy?
• The only difference between the bispectrum and
bicoherence is the normalisation
• Normalisation is by the power in the signal at the relevant
frequencies
• Therefore, the differences between Waking and Hypnosis
must be in the Fourier Spectrum
• However, with very low power levels, esp at high
frequencies, normalisation can give erroneous estimates of
bicoherence
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1st Latent variable – PLS of Fourier Spectrum
LV 1; 87.3% cross-block variance, p<0.045
HIGHS
LOWS
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Reliable differences in the Fourier Spectrum
by frequency Band
Left-right difference
Ant-Post difference
Global difference
Midline Parietal difference
Ant-Post difference
Ant-Post difference
RED: Waking>Hypnosis
BLUE: Hypnosis>Waking
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Reliable differences in the Fourier Spectrum
by frequency
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Summary
Frequency Band
Waking > Hypnosis
Hypnosis > Waking
Delta
Right side
Left Side
Theta
Frontal Midline
Occipital
Alpha
Global
-
Beta
Midline Parietal
-
Gamma
Midline Parietal
Frontal Midline
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Final Summary
• High susceptibles show significantly greater Bispectral
values in the waking condition than in hypnosis, esp
– High frequencies
– Midline frontal
– Temporo-occipital sites
• There are no differences in Bicoherence
• The differences in the Bispectrum are due to differences in
the power spectra of the EEG
• Calculation of the Bispectrum is problematic
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Conclusion
• Q. Does bispectral analysis add anything to the state versus
non-state debate in hypnosis apart from complexity?
– Probably not
– But, with improved estimation of bicoherence it might
– But, PLS analysis of the FFT was helpful in elucidating
the EEG power differences seen between the waking
and Hypnotic states seen in High Susceptibles
– Ho hum
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Thank you
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