Fig. 2.

Shows seizure activity in the frontal cortex (top trace) as well as AN (bottom trace). It is very clear that both signals are tightly coupled to one another at the repetition frequency of 5 Hz. This would generate moderate coherence between time series. A casual glance at the signals reveals that although the coherence is close to one, the cortical signal has a wider bandwidth is there is plenty of spike-wave and higher frequency component presents. These are new frequency terms that do not have a subcortical origin, but are generated locally in the cortex. This harmonic generation causes loss of coherence. Newer procedures must be used to singularly isolate the nonlinearity.