Brain–Computer Interfaces - Index of

58 G. Pfurtscheller and C. Neuper
Fig. 8 Mean ERD ± SD
over central regions during
viewing a static versus
moving hand or cube
(modified from [80])
neuron system. This system is an action observation/execution matching system
capable of performing an internal simulation of the observed action (for a review
see [81, 82]). This line of research started on the discovery of so-called mirror neurons
in cortical area F5 of macaque monkeys, which are active both in observing
and executing a movement [83–85]. Based on functional imaging studies, evidence
for a comparable mirror neuron system has been also demonstrated in humans
[86], showing a functional correspondence between action observation, internal
simulation or motor imagery, and execution of the motor action [87].
Specifically, it has been proposed that the mu rhythm may reflect the downstream
modulation of primary sensorimotor neurons by mirror neurons in the inferior
frontal gyrus [88, 89]. The underlying idea is that activation of mirror neurons by
either executed, imagined or observed motor actions produces asynchronous firing,
and is therefore associated with a concomitant suppression or desynchronization
of the mu rhythm [90]. This is supported by MEG findings about activation of the
viewer’s motor cortex [82]. Hari and coworker showed that the 20-Hz rebound after
median-nerve stimulation is not only suppressed when the subject moves the fingers
or manipulates a small object, but also – although significantly weaker – when the
subject just views another person’s manipulation movements [91]. Interestingly, the
suppression of the rebound is stronger for motor acts presented live than those seen
on a video [92].
The fact that similar brain signals, i.e. oscillations in the mu and beta frequency
bands, react to both motor imagery and observation of biological movement may