Brain–Computer Interfaces - Index of

50 G. Pfurtscheller and C. Neuper
Fig. 2 Relationship between movement type band power changes in the 8–10 Hz (right side) and
the 10–12 Hz frequency bands (left side) over electrode locations C3, Cz and C4. A percentage
band power increase marks ERS, and a decrease reflects ERD. The band power was calculated in
a 500 ms window prior to movement-onset (modified from [26])
widespread over the entire sensorimotor cortex and probably reflects general motor
preparation and attention. Upper mu desynchronization (in the range of about
10–12 Hz), in contrast, is topographically restricted and is rather related to taskspecific
aspects. The lower mu frequency component displays a similar ERD with
hand and foot movement, while the higher components display a different pattern
with an ERD during hand and an ERS with foot movement (Fig. 2). This type of
reactivity of different mu components suggests a functional dissociation between
upper and lower mu components. The former displays a somatotopically specific
organization, while the latter is somatotopically unspecific.
Two patterns can develop during BCI training sessions with motor imagery: contralateral
desynchronization of upper mu components and a concomitant power
increase (ERS) over the ipsilateral side. In contrast to the bilaterally symmetrical
lower mu ERD, only the upper mu displays an ipsilateral ERS (Fig. 3).
These findings strongly indicate primary motor cortex activity during mental
simulation of movement. Hence, we can assume that the pre-movement ERD
and the ERD during motor imagery reflect a similar type of readiness or presetting
of neural networks in sensorimotor areas. Functional brain imaging studies
(e.g. [32–36]) and transcranial magnetic stimulation (TMS) show an increase
of motor responses during mental imagination of movements [37], which further
supports involvement of the primary sensorimotor cortex in motor imagery.
The observation that movement imagination triggers a significant ipsilateral ERS
along with the contralateral ERD supports the concept of antagonistic behavior
of neural networks (“focal ERD/surrounding ERS”, [38]) described in the next
section.
In a recent study, several motor imagery tasks were investigated, such as cuebased
left hand, right hand, foot, or tongue motor imagery [39]. Basically, hand
motor imagery activates neural networks in the cortical hand representation area,
which is manifested in the hand area mu ERD. Such a mu ERD was found in all
able-bodied subjects with a clear contralateral dominance. The midcentral ERD, in