EEG spectral topography in neurology: II. A new system and a theoretical comparison of interpolation techniques.

An EEG spectral topography system has been developed to produce maps of the time-averaged spectra of a 16-channel EEG. The design of this system is based upon the discussions of the contentious issues of topography presented in Part I and a theoretical study of two-dimensional interpolation techniques. The comparison of techniques is based upon simulated electric fields on a three-layer spherical head model. Four two-dimensional interpolation techniques are investigated: bilinear, nearest neighbour, bicubic splines, and thin-plate splines. The most accurate method, a 2nd degree thin-plate spline, is employed in the new topography system. The EEG spectral topography system adopts the following procedures: the EEG is recorded using the 16 channel ipsilateral-ears montage; spectral analysis is applied to several overlapping epochs, relatively free of artifacts, of length 5.12 s; the averaged frequency spectrum is divided into specified frequency bands; for each channel, the average spectral component of each band is mapped onto a plane representing the head; and the maps are completed using 2nd degree thin-plate spline interpolation. A particular aim of the development was to produce a topography system compatible with the present EEG recording system at Christchurch Hospital. This has been achieved and, consequently, the topography system is of immediate clinical use. The system is currently part of a major clinical study.