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

7 General Discussion
In this chapter I will rst discuss how the results described in this thesis are related
with several questions of neurophysiology, so far still unresolved with the traditional
approaches. The joining of evidence obtained with the described methods sheds lighton
these topics and allows the conjecture of physiological mechanisms. In the second part of
this chapter I will compare these methods, stressing their advantages and disadvantages
when applied to the study of EEG signals.
7.1 Physiological considerations
7.1.1 Dynamics of Grand Mal seizures
Chaos analysis of epileptic seizures leads to the general result that during seizures a
transition from a complex system to a simpler one takes place.
On the other hand, by using the RIR dened from the Gabor Transform, I showed
and quantied a well dened frequency behavior during seizures. Grand Mal seizures
were dominated by alpha and theta frequencies. Delta oscillations decreased during them
and had an abrupt increase correlated with the clonic phase. With Wavelet Packets,
I showed with a better resolution the temporal evolution of these frequency patterns
and it was possible to establish that the low frequency activity (3 ; 4Hz) related with
the rhythmic contractions of the clonic phase, was in fact originated by the \slowing"
of higher frequencies (at least 8 ; 9Hz). Then, during Grand Mal seizures there is
a clear frequency dynamics: some seconds after the starting of the seizure alpha and
theta activity dominates, these oscillations later becoming slower and when they are
in the limit of the delta band (about 3 ; 4Hz) the clonic phase of the seizure starts
and delta activity has an abrupt increase dominating the EEG recording. Moreover,
it is reasonable to conjecture that the violent contractions of the clonic phase are the
response to brain oscillations that are generated in higher frequencies, but owing to
the fact that muscles cannot react so fast, muscle activity is then limited to a tonic
contraction (muscular tension) until brain oscillations become slower and muscles are
capable of contracting in resonance with them.
The frequency pattern described is in agreement with studies in animals and with
computer simulations. Furthermore, it would be very interesting to investigate possible
causes of this behavior. Neuronal fatigue is one of the most plausible explanations.
The ring of a neuron is produced as a response to excitatory inputs of neighboring
neurons. This connection is done mainly by means of synaptical processes generated
by neurotransmitters produced in the neurons. During an epileptic seizure there is an
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