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

ain activity during an epileptic seizure, we eliminated B 1 and B 2 bands, both containing
high frequency artifacts that obscure the EEG (see sec. x3.4). The relative band
intensity ratio (RIR) (dened as in sec. x3.2 but in this case from the wavelet scales)
had a similar behavior as the one showed with Gabor Transform in gure 12.
Frequency bands B 3 and B 4 were chosen for performing an analysis with Wavelets
Packets, these bands being important in the development of the tonic-clonic seizures as
showed in Chapter x3.4 (see also Quian Quiroga et al., 1997b).
B 3 band coecients were segmented with sliding windows of l = 32 samples corresponding
to time intervals of t =2:5 sec. Discrete sets of frequencies between 6:4 and
12:8 Hz with intervals of 0:4 Hz were obtained as showed in g. 4.4.2 (squared values
shown).
From second 50, we can see an increase of the activity in nearly all the packets. Due
to the good time-frequency resolution of the Wavelet Packets it is possible to follow the
evolution of the frequency peaks. For example, the peak marked with an arrow in the
wavelet packet corresponding to 8:4Hz at about second 50, is also visible in the packets
corresponding to 8:0, 7:6 and7:2Hz, appearing with higher amplitude and some delay.
Then, this peak originated in the 8:4Hz packet, or probably in higher frequencies but
with lower amplitude, is evolving with time to lower frequencies.
Figure 17 shows the Wavelet Packets corresponding to the B4 band (squared values
shown). They were generated by using l = 16 samples corresponding to time intervals
of t =2:5 sec and discrete sets of frequencies between 3:2 and 6:4 Hz with intervals
of 0:4 Hz. Note that the j = ;4 level has half dispersion in frequencies compared with
the j = ;3 level and for this reason we used a window of 16 samples in order to obtain
the same denition.
In the B4 band there is a very clear peak, marked with an arrow, after second 60
in the frequencies around 3 ; 4Hz, this increase being correlated with the starting of
the clonic phase of the seizure. This peak is also visible, but appearing earlier in time,
in the higher frequency packets (also marked with an arrow). Although this behavior is
predictable with a visual inspection of the EEG, it is very interesting to note that this
peak is in fact the same peak described in the gure 4.4.2 but appearing more delayed.
Analyzing both gures together, we can observe very clearly how this high amplitude,
low frequency peak (3 ; 4Hz) at about 65sec was in fact rst observed in the higher
frequencies (about 9Hz), then evolving with time to lower frequencies until reaching
a very high amplitude when the clonic phase starts, this evolution being very dicult
to identify from a visual inspection of the EEG or by using traditional methods as the
spectrograms (see discussion of sec. x3.5).
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