Radial coherence, wave velocity and damping of electrocortical waves

Mean squared coherence was calculated as a function of frequency (1-32 Hz) and electrode separation (1-8 mm) from 64-channel extradural arrays on occipito-parietal association cortex of cats. A 2-parameter theoretical function was then fitted to sets of pooled estimates. The theoretical function described coherences between recording sites of small separation for linear, non-dispersive, dissipative waves moving on an infinite homogeneous plane medium, and driven by spatio-temporally noisy inputs. Residuals of fit were then plotted as a function of frequency and distance, and were found to show no systematic trends with frequency, but an irregular and generally increasing relation to distance. This was the result predicted for linear non-dispersive waves on a surface actually folded, and with significant additional wave action generated between electrodes. Further recordings of coherence from more widely separated electrodes indicated that boundary conditions were absorbing or remote, rather than closed or reentrant. The phase velocity for electrocortical waves obtained from autoregression estimates of temporal damping and parameters of fit to coherence, was found in the range 0.1-0.29 m/sec, and appeared independent of the direction of electrode alignment. This compares with the velocity of 0.33 m/sec for alpha waves earlier found by Lopes da Silva and Storm van Leeuwen.