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

4.4.3 Discussion
The high time-frequency resolution achieved with Wavelet Packets allowed a very detailed
study of the time evolution of the frequency peaks during the seizure. In fact, it
was possible to establish that the high amplitude peaks of about 3 ; 4Hz,characteristic
of the clonic activity, were generated in higher frequencies. This result is in agreement
with the dynamic described with the Gabor Transform. In this case, with the RIR (see
sec. x3.4) I showed how during the seizure the relevant brain activity was dominated by
alpha and theta bands, until the ending of the seizure, when delta activity dominated
again.
Then, it is reasonable to conjecture that the violent clonic contractions characteristic
of the clonic phase of the Grand Mal seizures are the response to brain oscillations
generated in higher frequencies, but owing to the fact that the muscles can not contract
so fast, the muscle activity is limited to a tonic contraction until the oscillations become
slower and the muscles are capable to oscillate in resonance with them.
It is interesting to note that the frequency behavior described is in agreement with
simulations of thalamic slices performed by Wang and coworkers (Wang et al., 1995
Golomb et al., 1996), who observed a slowing from 10Hz to 4Hz in their simulations
after the suppression of GABA A inhibitors. This result is also in agreement with in vitro
experimental results of Bal et al. (1995a,b). These experiments suggest that it would
be very interesting to investigate if the \slowing" of the frequencies observed during
the seizures can be related with a variation in the concentration of neurotransmitters,
especially of the GABA inhibitors, possible due to a neuronal fatigue provoked by the
abnormal ring rate of the neurons during the seizure.
4.5 Application to alpha responses of visual event-related potentials
4.5.1 Introduction
EEG alpha rhythms can be dened as oscillations between 8 and 13 Hz, with an amplitude
usually below 50V and localized over posterior regions of the head. Alpha
rhythms appear spontaneously during wakefulness, best seen with eyes closed, under
relaxation and mental inactivity conditions (Niedermeyer, 1993a).
Although alpha oscillations have been widely studied, their sources and functional
correlates are still under discussion. Sources of alpha rhythms have been investigated
leading to controversies whether they are thalamic, cortical or whether other structures
are involved in their generation (Adrian, 1941 Andersen and Andersson, 1968 Lopes da
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