Brain–Computer Interfaces - Index of

52 G. Pfurtscheller and C. Neuper
regions. Once the movement sequence has been learned and the movement is performed
more “automatically,” the ERD is reduced. These ERD findings strongly
suggest that activity in primary sensorimotor areas increases in association with
learning a new motor task and decreases after the task has been learned [41].
The involvement of the primary motor area in learning motor sequences was also
suggested by Pascual-Leone [42], who studied motor output maps using TMS.
The opposite of desynchronization is synchronization, when the amplitude
enhancement is based on the cooperative or synchronized behavior of a large number
of neurons. When the summed synaptic events become sufficiently large, the
field potentials can be recorded not only with macro electrodes within the cortex,
but also over the scalp. Large mu waves on the scalp need coherent activity
of cell assemblies over at least several square centimeters [43, 44]. When patches of
neurons display coherent activity in the alpha frequency band, active processing of
information is very unlikely and probably reflects an inhibitory effect.
However, inhibition in neural networks is very important, not only to optimize
energy demands but also to limit and control excitatory processes. Klimesch [45]
suggested that synchronized alpha band rhythms during mental inactivity (idling)
are important to introduce powerful inhibitory effects, which could block a memory
search from entering irrelevant parts of neural networks. Combined EEG and TMS
studies demonstrated that the common pathway from motor cortex to the target hand
muscle was significantly inhibited during the movement inhibition conditions, and
the upper mu components simultaneously displayed synchronization in the hand
area. In contrast, the TMS response was increased during execution of the movement
sequence, and a mu ERD was observed in the hand representation area [46].
Adrian and Matthews [47] described a system that is neither receiving nor processing
sensory information as an idling system. Localized synchronization of
12–14 Hz components in awake cats was interpreted by Case and Harper [48] as
a result of idling cortical areas. Cortical idling can thus denote a cortical area of at
least some cm 2 that is not involved in processing sensory input or preparing motor
output. In this sense, occipital alpha rhythms can be considered idling rhythms of
the visual areas, and mu rhythms as idling rhythms of sensorimotor areas [49]. Also,
sleep spindles during early sleep stages can occur when signals through the thalamus
are blocked [40].
5 “Focal ERD/Surround ERS”
Localized desynchronization of the mu rhythms related to a specific sensorimotor
event does not occur in isolation, but can be accompanied by increased synchronization
in neighboring cortical areas that correspond to the same or to another
information processing modality. Lopes da Silva [38] introduced the term “focal
ERD/surround ERS” to describe this phenomenon. Gerloff et al. [50] reported an
antagonistic behavior with desynchronization of central mu rhythm and synchronization
of parieto-occipital alpha rhythms during repetitive brief finger movement.