Brain–Computer Interfaces - Index of

48 G. Pfurtscheller and C. Neuper
Further reports on movement-related gamma oscillations in man can be found in
Pfurtscheller et al. [12] and Salenius et al. [13].
Gamma oscillations in the frequency range from 60 to 90 Hz associated with
movement execution were observed in subdural recordings (ECoG) [14, 15]. Of
interest are the broad frequency band, short duration, and embedding of gamma
oscillations in desynchronized alpha band and beta rhythms. There is strong evidence
that the desynchronization of alpha band rhythms may be a prerequisite for
the development of gamma bursts.
2 Event–Related Potentials Versus ERD/ERS
There are two ways to analyze the changes in the electrical activity of the cortex
that accompany brain activities, such as sensory stimulation and motor behavior.
One change is time-locked and phase-locked (evoked) and can be extracted from
the ongoing activity by simple linear methods such as averaging. The other is
time-locked but not phase-locked (induced) and can only be extracted through
some non-linear methods such as envelope detection or power spectral analysis.
Which mechanisms underlie these types of responses? The time- and phase-locked
response can easily be understood in terms of the response of a stationary system
to the external stimulus, the result of the existing neuronal networks of the cortex.
The induced changes cannot be evaluated in such terms. The latter can be understood
as a change in the ongoing activity, resulting from changes in the functional
connectivity within the cortex.
A typical example of both phase-locked (evoked) and non-phase-locked
(induced) EEG activities is found during preparation for voluntary thumb movement.
Both, negative slow cortical potential shifts (known as Bereitschaftspotential)
and mu ERD start about 2 s prior to movement onset [4, 16]. Slow cortical potential
shifts at central electrode positions have also been reported after visually cued
imagined hand movements [17] in parallel to the desynchronization of central beta
and mu rhythms [18]. A BCI could even use both slow cortical potential shifts and
ERD/ERS for feature extraction and classification.
3 Mu and Beta ERD in a Motor Task
Voluntary movement results in a circumscribed desynchronization in the upper
alpha and lower beta band oscillations, localized over sensorimotor areas [4,
19–25]. The desynchronization is most pronounced over the contralateral central
region and becomes bilaterally symmetrical with execution of movement.
The time course of the contralateral mu desynchronization is almost identical
for brisk and slow finger movements, starting more than 2 s prior to movement
onset [22]. Finger movement of the dominant hand is accompanied by a pronounced
ERD in the contralateral hemisphere and by a very low ERD in the ipsilateral side,