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

6.3 Application to visual event-related potentials
6.3.1 Methods and Materials
In 9 voluntary healthy subjects (no neurological decits, no medication known to aect
the EEG) two types of experiments were performed:
1. No-task visual evoked potential (VEP): subjects were watching a checkerboard
pattern (sidelength of the checks: 50'), the stimulus being achecker reversal.
2. Oddball paradigm (NON-TARGET/TARGET stimuli): subjects were watching
the same pattern as above. Two dierent stimuli were presented in a pseudorandom
order. NON-TARGET stimuli (75%) were pattern reversal, and TARGET
stimuli (25%) consisted in a pattern reversal with horizontal and vertical displacement
of one-half of the square side length. Subjects were instructed to pay
attention to the appearance of the target stimuli.
In both cases, 200 stimuli were presented and the duration of each stimulus was
1 second. Recordings were made following the international 10 ; 20 system in seven
dierent electrodes (F3, F4, Cz, P3, P4, O1, O2) referenced to linked earlobes. Data
were amplied with a time constant of 1:5sec: and a low-pass lter at 70Hz. For each
single sweep, 1sec: pre- and post-stimulus EEG were digitized with a sampling rate of
250Hz and stored in a hard disk.
After visual inspection of the data, 30 sweeps free of artifacts were randomly selected
for each type of stimuli (VEP, NON-TARGET and TARGET) for future analysis. A
Wavelet Transform was applied to each single sweep using a quadratic B-Spline function
as mother wavelet. The multiresolution decomposition method (Mallat, 1989) was used
for separating the signal in frequency bands, dened in agreement with the traditional
frequency bands used in physiological EEG analysis. After a ve octave wavelet decomposition,
the components of the following bands were obtained: 62 ; 125Hz,31; 62Hz
(gamma), 16 ; 31Hz (beta), 8 ; 16Hz (alpha), 4 ; 8Hz (theta) and the residues in the
0:5 ; 4Hz band (delta).
For each subject the results of the wavelet decomposition of the 30 single sweeps
were averaged, obtaining the mean components (C ij ). Finally, from this coecients
the Wavelet-entropy was calculated as described in the previous section. In this case,
since the WS is calculated from the averaged wavelet coecients, only phase-locked
oscillations will contribute to it, the others being cancelled (for the same data, an analysis
of phase-locking in the alpha band was done in Quian Quiroga et.al., 1999d).
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