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www.swansea.ac.uk
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- Non-
state debate?
• The EEG bispectrum in hypnosis and waking for high and
low susceptible participants
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What is bispectral analysis?
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Bispectral analysis is a Fourier based method for examining the coupling between frequencies in different ranges
The bispectrum is defined as:
Where X(.)=Fourier Transform of the time series x(t) and * indicates the complex conjugate
Bicoherence is the normalised bispectrum:
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Within Channels Between Channels
1st order Mean
2nd order Fourier Spectrum Coherence
3rd order Bicoherence Cross-bicoherence
What is bispectral analysis?
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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
Pre-inductionMemory test
Waking
Hypnotised
Hypnotic Induction
Post-induction Eyes Closed
Post-inductionMemory test
SHSS-C
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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 simultaneouslyi) Whether the experimental design has an
effectii) 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
HIGHS LOWS
LV 1; 95.3% cross-block variance, p<0.01
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Topography of reliable differences between Waking & Hypnosis
Bispectrum higher in the Waking condition
•Midline frontal•Temporo-occipital
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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
HIGHS LOWS
LV 1; 64.6% cross-block variance, p<0.26
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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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HIGHS LOWS
1st Latent variable – PLS of Fourier Spectrum
LV 1; 87.3% cross-block variance, p<0.045
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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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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
Summary
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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