Quantitative analysis of EEG signals: Time-frequency methods and ...

1.1.2 EEG in Epilepsy
One important application of the EEG is to the study of epilepsy. The appearance of
abrupt high amplitude peaks (spikes), abnormal rhythmicities, \slowing" of the recording
and several other paroxysms are a general landmark of it, helping to identify, classify
and localize the seizures.
Epilepsy is a disorder of the normal brain function that aects about 1% of the
population, characterized by an excessive and uncontrolled activity of either a part or
the whole central nervous system. Historically, the EEG has been the most useful tool for
its evaluation, now complemented with the advances in imaging techniques, especially
the ones achieved by the Magnetic Resonance Imaging (MRI).
Given that ictal recordings (i.e. recordings during a seizure) were rarely obtained,
EEG analysis of epileptic patients usually relied on interictal ndings. In those interictal
EEGs, seizures are usually activated with photostimulation, hyperventilation and other
methods. However, one disadvantage of these stimulation techniques is that provoked
seizures do not necessarily have the same behavior as the spontaneous ones. The introduction
of long term Video-EEG recordings has been an important milestone providing
not only the possibility to analyze ictal events, but also contributing with valuable information
in those candidates evaluated for epilepsy surgery (see Quian Quiroga et al.,
1997a Porter and Sato, 1993 Kaplan and Leser, 1990 Gotman et al., 1985 Meierkord et
al., 1991). In this setting and following strict protocols, seizures are elicited by gradually
reducing antiepileptic drugs.
Classication
The classication of epileptic seizures is a very dicult task and leaded to several
controversies. I will present a simplied classication (for further details see Niedermeyer,
1993c). Tonic-Clonic seizures will be described with more detail since these are
the ones to be studied in the following chapters. Seizures can be classied in:
1. Partial seizures: they have a focal origin they can also evolve to a generalized
seizure (secondarily generalized).
Simple partial seizures: consciousness is not impaired. Depending on the
zone of the brain involved, they are characterized by focal motor movements,
sensory symptoms (simple hallucinations), autonomic symptoms (epigastric
sensation, sweating, pupillary dilatation, etc.) or psychic symptoms.
Complex partial seizures: involve a loose of consciousness. They are characterized
by dierent psychical sensations and motor automatisms. Since there
are many types of these seizures, the EEG is very variable but it can be gen-
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