Interelectrode coherences from nearest-neighbor and spherical harmonic expansion computation of laplacian of scalp potential.
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.
- SourceAvailable from: Olivier Bertrand[show abstract] [hide abstract]
ABSTRACT: Description of mapping methods using spherical splines, both to interpolate scalp potentials (SPs), and to approximate scalp current densities (SCDs). Compared to a previously published method using thin plate splines, the advantages are a very simple derivation of the SCD approximation, faster computing times, and greater accuracy in areas with few electrodes.Electroencephalography and Clinical Neurophysiology 03/1989; 72(2):184-7.
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ABSTRACT: Small time differences between the EEG activities of two channels were measured by a method based on the use of coherence and phase spectra over a certain frequency range. In many cases this method allowed to establish that time differences of the order of 5-50 msec were actually present between two channels which appeared synchronous on visual inspection. The method was applied to seizure activity from the penicillin and kindling models in the cat and to seizures recorded from scalp and intracerebral electrodes in epileptic patients. When the seizure activity was widespread but was known to be related to an epileptic focus, it was found that the area of the focus had a consistent time lead over the other recording sites. It was concluded that the method could frequently allow to: (1) assess the presence of an epileptic focus even when only widespread seizure activity could be recorded; (2) make inferences about the possible routes of propagation of seizure activity.Electroencephalography and Clinical Neurophysiology 12/1983; 56(5):501-14.
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ABSTRACT: We determined locations of 33 scalp electrodes used for electroencephalographic (EEG) recording by placing markers in the positions determined by the 10-20 system and performing magnetic resonance image (MRI) scanning on volunteer subjects. Small Vaseline-filled capsules glued on the scalp with collodion produced easily delineated regions of increased signal on standard MRI head images. Measurements of each capsule's coordinates in 3 dimensions were made from MRI scans. A spherical surface was fitted through the marker positions, giving an average radius and an origin (center of sphere). The coordinate axes were rotated to ensure that electrode Cz was on the z-axis and that the y-axis was oriented in the posterior-anterior direction. Two spherical (angular) coordinates were determined for each electrode. Spherical electrode coordinates for different subjects differed by less than 20 degrees in all cases. An average and standard deviation of the spherical coordinates were calculated for each electrode. Standard deviations of several degrees were obtained. The average spherical coordinates obtained were close to those expected on the basis of applying the 10-20 system of placement to an ideal sphere. These measurements provide data necessary for various analyses of EEG performed to help localize epileptic foci.Electroencephalography and Clinical Neurophysiology 02/1993; 86(1):7-14.