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

Electrode F3 F4 Cz P3 P4 O1 O2
Mean 6.27 6.72 9.80 8.96 8.85 12.65 11.63
SEM 0.56 0.64 0.99 0.85 0.85 1.36 1.22
F3 XXX - < 0.05 - - < 0.01 < 0.01
F4 XXX < 0.05 - - < 0.01 < 0.01
Cz XXX - - < 0.05 -
P3 XXX - < 0.01 -
P4 XXX < 0.01 < 0.05
O1 XXX -
O2
XXX
Table 2: Multiple factor ANOVA comparison of the maximum alpha band wavelet
coecients for the factor electrode. Note: SEM means standard error of the mean, -
means no signicance
for the three stimulation types, best dened in the occipital electrodes. The multiple
factor ANOVA test showed no signicant dierences between stimuli type. Electrode
dierences, instead, were signicant, conrming the predominant localization of the
enhancements in the occipital locations, with a lower response in the anterior electrodes
(see table 2).
It is also interesting to note that in some of the subjects, responses in posterior electrodes
upon TARGET stimulation are prolonged in comparison with the NON-TARGET
and VEP ones. This coherent alpha activity extended up to a second post-stimulation.
With the other stimulus types, enhancements have an abrupt decay after 200 ; 300ms
post-stimulus. However, we should remark that this result was not consistent for the
whole group.
The delay of the maximum response in occipital electrodes was about 180ms after
stimulation (see table 3). In parietal electrodes the maximum appeared about 30ms
later, and in central and frontal electrodes between 50 ; 100ms after the occipital one.
After applying a multiple factor ANOVA test, we veried statistically that frontal and
central responses were signicantly delayed in comparison to the occipital ones (p <
0:05).
56