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Conjunct COST B27 and SAN Scientific Meeting, Swansea, UK, 16-18 September 2006

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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

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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