Bispectral analysis gives us more information than power spectral-based analysis

S. Hagihira1, M. Takashina1, T. Mori1, T. Mashimo1, J. W. Sleigh2, J. Barnard2, A. Miller2 and D. A. Steyn-Ross2

1 Osaka, Japan 2 Hamilton, New Zealand

Editor–We read the article by Miller and colleagues1 with great interest. They examined the changes of the SynchFastSlow, bispectrum derived component in the BIS calculation, and concluded that bispectral analysis did not give any more information than power spectral-based analysis. They could not find any changes in EEG bicoherence, which directly indicates the degree of phase coupling.

In contrast, we found significant changes in EEG bicoherence during isoflurane anaesthesia combined with epidural anaesthesia.2 What was the difference between the study by Miller and colleagues and our own? We used the EEG recorded from a unilateral lead (Fp1-A1), while they used the EEG recorded from a bilateral lead (F7–F8). This is an important point in bispectral analysis of the EEG. We demonstrated the EEG bicoherence patterns obtained from a unilateral lead (Fig. 1A, Fp1-A1) and from a bilateral lead (Fig. 1B, Fp1-Fp2). Both EEG signals were recorded simultaneously during isoflurane 0.9% anaesthesia. EEG bicoherence obtained from a unilateral lead showed two significant peaks in the low frequency region, but EEG bicoherence obtained from the bilateral lead was quite low in all bifrequency planes. When we used the bilateral lead, we could not find any increase in EEG bicoherence during either isoflurane, sevoflurane or propofol anaesthesia. Thus, if we want to obtain more information than that provided by power spectral-based analysis from bispectral analysis, we must use the unilateral lead. As our previous report3 suggested, the increase in EEG bicoherence during anaesthesia seems to be related to the spindle wave and delta wave, whose rhythms are generated in the thalamic nuclei. During anaesthesia, those waves become predominant and EEG waveforms in both hemispherical leads, for example Fp1-A1 and Fp2-A2, show a similar pattern. However, those waves were cancelled out and disappeared when a bilateral lead was used. The BIS monitor uses a unilateral lead (Fpz-Ta1), so the conclusions of Miller and colleagues1 would not be applicable for the BIS calculation.



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Fig 1 EEG bicoherence patters. (A) Unilateral lead. (B) Bilateral lead.

 
These important issues in applying bispectral analysis are relevant in other situations. Even when using a unilateral lead, the major changes in the bispectrum are caused by changes in the power spectrum. As we pointed out, as far as using bispectral values, there is little advantage in bispectral analysis. We would use bicoherence values. We have also shown that we can obtain other facts from phase information.4 Bispectral analysis would have given more information than power spectral-based analysis, if the authors had used the EEG recorded from unilateral leads.

S. Hagihira

M. Takashina

T. Mori

T. Mashimo

Osaka, Japan

Editor—We thank Professor Hagihira and colleagues for their interest in our work, and their elegant demonstration of how the use of higher order EEG analysis can be extremely sensitive to the subtleties of electrode montage. We used a frontal bipolar electrode placement because of its convenience and its relative resistance to electromyographic interference. An EEG measure that is critically dependent on accurate lead placement is not likely to be of much use in the hustle and bustle of daily clinical practice. The peaks in the bicoherence detected by Professor Hagihara during sleep probably reflect the well-described coupling between slow waves (~4 Hz) and spindles (~10 Hz).5 EEG spindles are largely driven by the thalamus and are distributed synchronously and bilaterally to the cortex. Typically, spindles occur after a slow wave of cortical depolarization, thus phase-coupling will be seen between these two frequencies of cortical activity. It would seem that the bicoherence is a complicated way to detect sleep spindles and that a unipolar frontal electrode system would be superior to a bipolar frontal electrode system when collecting EEG data for this style of analysis. We agree that spindle detection and phase information analysis in general are potentially very important,6 and at present are missing from most EEG analysis of depth of anaesthesia. More original work is needed in this area.

In contrast to the bicoherence, the bispectrum is sensitive to amplitude changes of the components of the standard EEG frequency spectrum. Clearly, there are gross changes in amplitude of the various frequency components of the EEG in the awake vs the asleep or the anaesthetized state. Potentially, these changes of amplitude in the standard spectrum may mask any real changes in the amplitude of the true bispectral components of the bispectrum. The true bicoherence contribution to the bispectral power is minimal. The observations from Professor Hagihira do not change the other conclusions from our paper:

(i) The bispectrum is not significantly better than normal frequency spectrum in quantifying state of consciousness.

(ii) The calculation of higher order statistics from non-stationary short time series may be misleading because long EEG data segments of at least 180–360 s are necessary for accurate statistical averaging.

J. W. Sleigh

J. Barnard

A. Miller

D. A. Steyn-Ross

Hamilton, New Zealand

References

1 Miller A, Sleigh JW, Barnard J, Steyn-Ross DA. Does bispectral analysis of the electroencephalogram add anything but complexity? Br J Anaesth 2004; 92: 8–13[Abstract/Free Full Text]

2 Hagihira S, Takashina M, Mori T, Mashimo T, Yoshiya I. Practical issues in bispectral analysis of electroencephalographic signals. Anesth Analg 2001; 93: 966–70[Abstract/Free Full Text]

3 Hagihira S, Takashina M, Mori T, Mashimo T, Yoshiya I. Changes of electroencephalographic bicoherence during isoflurane anesthesia combined with epidural anesthesia. Anesthesiology 2002; 97: 1409–15[ISI][Medline]

4 Hagihira S, Takashina M, Ueyama H, Mori T, Mashimo T. Electroencephalographic bicoherence is sensitive to noxious stimuli during isoflurane or sevoflurane anesthesia. Anesthesiology, in press.

5 Timofeev I, Contreras D, Steriade M. Synaptic responsiveness of cortical and thalamic neurones during various phases of slow sleep oscillation in cat. J Physiol 1996; 494: 265–78[Abstract]

6 Freeman WJ, Burke BC, Holmes MD. Aperiodic phase re-setting in scalp EEG of beta-gamma oscillations by state transitions at alpha-theta rates. Human Brain Mapping 2003; 19: 248–72[CrossRef][ISI][Medline]