Interelectrode coherences from nearest-neighbor and spherical harmonic expansion computation of laplacian of scalp potential

Section of Electroencephalography, Mayo Clinic, Rochester, MN 55905, USA.
Electroencephalography and Clinical Neurophysiology 10/1995; 95(3):178-88. DOI: 10.1016/0013-4694(95)00025-T
Source: PubMed

ABSTRACT Interchannel coherence is a measure of spatial extent of and timing relationships among cerebral electroencephalogram (EEG) generators. Interchannel coherence of referentially recorded potentials includes components due to volume conduction and reference site activity. The laplacian of the potential is reference independent and decreases the contribution of volume conduction. Interchannel coherences of the laplacian should, therefore, be less than those of referentially recorded potentials. However, methods used to compute the laplacian involve forming linear combinations of multiple recorded potentials, which may inflate interchannel coherences. WE compared 3 methods of computing the laplacian: (1) modified Hjorth (4 equidistant neighbors to each electrode), (2) Taylor's series (4 nonequidistant neighbors), and (3) spherical harmonic expansion (SHE). Average interchannel coherence introduced by computing the laplacian was less for nearest-neighbor methods (0.0207 +/- 0.0766) but still acceptable for the SHE method (0.0337 +/- 0.0865). Average interchannel coherence for simulated EEG (random data plus a common 10 Hz signal) was less for laplacian than for referential data because of removal of the common referential signal. Interchannel coherences of background EEG and partial seizure activity were less with the laplacian (any method) than with referential recordings. Laplacians calculated from the SHE do not demonstrate excessively large interchannel coherences, as have been reported for laplacians from spherical splines.

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    • "When averaged across pairs of electrodes, the index R ij represents a measure of global synchronization strength R À Á . For this analysis, before computing the wavelet-coefficients, the raw EEG trials were first transformed with a modified version of the nearest-neighbor Hjorth Laplacian algorithm computed by Taylor's series expansion (Lagerlund et al., 1995 "
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    • "The average of this index across pairs of electrodes represents a measure of global synchronization strength (R). For the bivariate synchronization analysis, a modified version of the nearestneighbour Hjorth Laplacian algorithm computed by Taylor's series expansion (Lagerlund et al., 1995 "
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    • "For each single subject the averaged MMN waveform was cross-correlated to the MMN grand-averaged envelope — defined as the minimum value among all electrodes for each time frame, of the control group, and the maximum delay value resulting from the cross-correlation was then used to align the individual MMN waveform. Prior to SCD estimation by mean of a Laplacian operator (Perrin et al., 1989), individual data were interpolated in a spherical scalp model using 1000 pixels by means of a Spherical Harmonics Expansion (Lagerlund et al., 1995), according to the method described by Ruffini et al. (2002). A scalp area was considered activated on the grand-average SCD map if 4 adjacent pixels showed p b 0.05 relative to prestimulus interval during 6 consecutive time frames using the Holmes non-parametric method, that is corrected from multiple comparisons (Holmes et al., 1996). "
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