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

In conclusion, our results point towards a distributed alpha system with functional
relation to sensory processing and possibly to further processes.
4.6 Application to gamma responses of bisensory event-related
potentials
4.6.1 Introduction
During the last ten years gamma-band phenomena gained increasing popularity since
the reports of Gray and Singer (1989). Measuring the spike activity of cellular units
and multi-units of the cat's striate cortex they found burst activities to be in congruence
with oscillatory LFPs (local eld potentials) in orientation-specic columns. Later
ndings (Eckhorn et al., 1988, Gray etal. 1989) led to the proposal of a mechanism suf-
cient to explain the formation of distant neuronal units to assemblies. Such assembly
formation has been suggested as the brain's solution to the problem of encoding. The
so-called \binding-theorem" provided a solid explanatory base in congruence with the
experimental data.
Although being very elegant in solving the problem to encode a quasi-unlimited
number of real object features and combinations in a limited number of neural elements
interest has, nevertheless, not swayed from the experimental results, which were not
directly related to theoretical questions. As, for example, the studies of Tiitinen et al.
(1993) and Pulvermuller et al. (1995) could demonstrate, the 40Hz oscillatory activity
is not restricted to sensory processing, but can be rather modulated or triggered by
cognitive processes as well.
4.6.2 Material and Methods
Event-related potentials were measured in 15 healthy subjects who had neither any
known neurological decit nor reported intake ofdrugsknown to aect the EEG. Electrodes
were placed according to the 10 ; 20 system in F3, F4, Cz, C3, C4, T3, T4, P3,
P4, O1 and O2 locations, referenced to linked earlobes. Data were amplied with a time
constant of 1:5sec: andalow-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.
Subjects were instructed to view and listen passively to the stimuli. A recording
session consisted of3parts:
1. Registration of 120 sweeps covering 2s time-windows with application of unimodal
stimulation presenting a 2kHzsinusoidal tone-step binaurally.
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