Figure 6. Simultaneous recordings of cortical and hippocampal EEG during burst suppression activity revealed a high degree of synchrony for burst discharges in both structures. (A) The amplitude and time course of the burst events were quite different and variable in each structure, indicating they were not driven by direct (monosynaptic) connections, nor simply a volume conducted reflection of activity. (B) The synchrony, seen as a Gaussian distribution in the upper event-time histogram (level 1), most likely arises from synaptically connected, but separate, inputs to cortex and hippocampus. This synchrony was lost as deeper levels of burst suppression were achieved (lower histogram; level 2) accompanied by further behavioral depression associated with anesthesia. Event-time histograms were constructed by detecting the time of occurrence of burst peak positivities using level discriminators for both cortical and hippocampal signals. Hippocampal event times were plotted relative to each cortical burst. For each histogram, 200 events were fitted using a Gaussian (normal) distribution with a Levenberg-Marquardt algorithm of nonlinear least-squares fitting.