Spectral analysis of stereo-electroencephalograms: preictal slowing in partial epilepsies

A substantial challenge in epileptology consists of reliable seizure anticipation ahead in time (minutes). We propose a procedure which derives from our demonstrating here that seizure outbreak shares the pretransitional behaviour characterizing nonequilibrium phase transitions with instabilities found in a number of domains. We investigated stereo-electroencephalograms, using chronically implanted cerebral electrodes, and found preictal (that is, preseizure) slowing. The method employed rests on reconstructing a high-dimensional phase orbit, projecting the orbit back to a scalar time series, namely Lerner's density function which is shown here to play the role of an order parameter, and Fourier-transforming the latter. This reduces the problem to comparing spectral characteristics of interictal and preictal states. The preictal states behaved alike for temporal-lobe and frontal-lobe partial seizures, as well as for the one subclinical temporal-lobe seizure studied. These signs of universality are in accordance with nonequilibrium phase-transitions being in essence the amplification up to a macroscopic size of fluctuations, which may be conjectured here to consist of synchronized neuronal assemblies. Received: 10 January 2000 / Accepted in revised form: 8 May 2000