Electroencephalogrphy (EEG) is a powerful method to observe brain activity. Due to its origin, EEG
is contrary to other imaging techniques like fMRI able to register brain activity directly. EEG is
registered noninvasively by electrodes placed on the scalp. The number of electrodes can vary from
a few up to more than 200. For research studies electrodes are placed in an international defined
way called 10-10 or 10-20 system. This standard systems ensures that the position and number of the
electrodes is equal across laboratories and that results are comparable all over the world.
Normal EEGs have a frequency range for 0.5Hz to 70Hz and typical Amplitudes of 10ÁV-100ÁV. Generally,
we can say that with increased attention the EEG is dominated by higher frequencies.
EEG can be used for several clinical applications such as to distinguish epileptic seizures from
other diseases. In research EEG is used to investigate neurophysiologic processes in a noninvasive way.
Compared to other techniques, e.g. fMRI, EEG investigations are cheap and EEG has a high temporal resolution.
On the other hand a disadvantage of EEG is its relatively poor spatial resolution. In order to be able
to investigate event related potentials (ERPs) it is necessary to average many of the potentials to improve
the SNR. The problem of averaging is that information of a single-trial gets lost by this technique.