INTERELECTRODE COHERENCES FROM NEAREST-NEIGHBOR AND SPHERICAL HARMONIC EXPANSION COMPUTATION OF LAPLACIAN OF SCALP POTENTIAL

Interchannel coherence is a measure of spatial extent of and timing re lationships among cerebral eleclroencephalogram (EEG) generators. Inte rchannel coherence of referentially recorded potentials includes compo nents due to volume conduction and reference site activity. The laplac ian of the potential is reference independent and decreases the contri bution of volume conduction. Interchannel coherences of the laplacian should, therefore, be less than those of referentially recorded potent ials. However, methods used to compute the laplacian involve forming l inear combinations of multiple recorded potentials, which may inflate interchannel coherences. We compared 3 methods of computing the laplac ian: (1) modified Hjorth (4 equidistant neighbors to each electrode), (2) Taylor's series (4 nonequidistant neighbors), and (3) spherical ha rmonic expansion (SHE). Average interchannel coherence introduced by c omputing 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 coh erences of background EEG and partial seizure activity were less with the laplacian (any method) than with referential recordings. Laplacian s calculated from the SHE do not demonstrate excessively large interch annel coherences, as have been reported for laplacians from spherical splines.