Brain–Computer Interfaces - Index of

54 G. Pfurtscheller and C. Neuper
6 Induced Beta Oscillations after Termination of a Motor Task
The induced beta oscillation after somatosensory stimulation and motor behavior,
called the beta rebound, is also of interest. MEG [57] and EEG recordings [58]have
shown a characteristic feature of the beta rebound, which is its strict somatotopical
organization. Another feature is its frequency specificity, with a slightly lower
frequency over the lateralized sensorimotor areas as compared to the midcentral
area [59]. Frequency components in the range of 16–20 Hz were reported for the
hand representation area and of 20–24 Hz for the midcentral area close to the vertex
(Fig. 5). The observation that a self-paced finger movement can activate neuronal
networks in hand and foot representation areas with different frequency in both areas
[26] further shows that the frequency of these oscillations may be characteristic for
the underlying neural circuitry.
The beta rebound is found after both active and passive movements [23, 25,
60]. This indicates that proprioceptive afferents play important roles for the desynchronization
of the central beta rhythm and the subsequent beta rebound. However,
electrical nerve stimulation [59] and mechanical finger stimulation [61] can also
induce a beta rebound. Even motor imagery can induce a short-lasting beta ERS or
beta rebound [27, 62, 63, 10], which is of special interest for BCI research. Figure 6
shows examples of a midcentral induced beta rebound after foot motor imagery.
The occurrence of a beta rebound related to mental motor imagery implies that this
activity does not necessarily depend on motor cortex output and muscle activation.
A general explanation for the induced beta bursts in the motor cortex after movement,
somatosensory stimulation, and motor imagery could be the transition of the
beta generating network from a highly activated to an inhibited state. In the deactivated
state, sub-networks in the motor area may reset, motor programs may be
cancelled and/or updated by a somatosensory feedback. The function of the beta
rebound in the sensorimotor area could be therefore understood as a “resetting
function”, in contrast to the “binding function” of gamma oscillations [64].
Fig. 5 Frequency of the
post-movement beta rebound
measured over the foot
area/SMA and the hand area
in response to movement and
(electrical) stimulation of the
respective limb. The boxplots
represent the distribution of
the peak frequencies in the
beta band (14–32 Hz) at
electrode positions Cz and C3
(modified from [59])