  The degree of synchronization and the amount of dynamical cluster formation
in electroencephalographic (EEG) signals are characterized by employing two
order parameters introduced in the context of coupled chaotic systems subject
to external noise. These parameters are calculated in EEG signals from a group
of healthy subjects and a group of epileptic patients, including a patient
experiencing an epileptic crisis. The evolution of these parameters shows the
occurrence of intermittent synchronization and clustering in the brain activity
during an epileptic crisis. Significantly, the existence of an instantaneous
maximum of synchronization previous to the onset of a crisis is revealed by
this procedure. The mean values of the order parameters and their standard
deviations are compared between both groups of individuals.
