Brain–Computer Interfaces - Index of

Neurofeedback Training for BCI Control 67
A complete neurofeedback training process usually comprises 25–50 sessions
that each lasts 45–60 min (cf. [22]). However, the number of sessions required
appears to vary from individual to individual. At the beginning, a blocked training
schedule (e.g., on a daily basis, or at least 3 times per week) may be preferable to
facilitate the neuromodulatory learning process. For improving attentional abilities
in healthy people, for instance, Gruzelier and coworkers established that the benefits
with neurofeedback training can be achieved within only 10 sessions [15, 23].
Further, the learning curve for EEG self-regulation is usually not linear, but often
shows a typical time course (see e.g. [24, 42]). That is, users tend to show the greatest
improvement early in training, just as 10 h of piano practice could make a big
difference to a novice, but probably not to an expert pianist.
2.1 Training of Sensorimotor Rhythms
Many BCI systems utilize sensorimotor EEG activity (i.e. mu and central beta
rhythms), since it can be modulated by voluntary imagination of limb movements
[25, 26] and is known to be susceptible to operant conditioning [5, 17, 27].
Experiments have confirmed that EEG frequency components recorded from central
areas (mu, central beta, SMR) can be enhanced during long-term feedback training
[5, 27, 28]. A further reason to use sensorimotor EEG in BCIs is that it is typically
modulated by both overt and covert motor activity (that is, both actual and imagined
movements), but unaffected by changes in visual stimulation [29]. Therefore, people
can use an SMR BCI while watching a movie, focusing on a friend, browsing
the web, or performing other visual tasks.
The operant conditioning of this type of EEG activity was discovered in the late
1960s in work with animals and replicated later in humans (for a review, see [17]).
Sterman and coworkers observed that, when cats learned to suppress a movement,
a particular brain rhythm in the range of 12–15 Hz emerged at the sensorimotor
cortex. They successfully trained the cats to produce this “sensory motor rhythm”
(SMR) through instrumental learning, by providing rewards only when the cats produced
SMR bursts. Since SMR bursts occurred when the cat did not move, the SMR
was considered the “idle rhythm” of the sensorimotor cortex. This is similar to the
alpha rhythm for the visual system, which is strongest when people are not using
their visual systems [30]. An unexpected observation was that cats that had been
trained to produce SMR more often also showed higher thresholds to the onset of
chemically induced seizures. A number of later studies in humans established that
SMR training resulted in decreased seizure activity in epileptic subjects [17].
Other studies suggested that the human mu rhythm is analogous to the SMR
found in cats, in terms of cortical topography, relationship to behavior, and reactivity
to sensory stimulation [27, 31]. However, it is not clear whether the neurophysiological
basis of the two phenomena is really identical (for a recent review of
oscillatory potentials in the motor cortex, see [32] as well as chapter “Dynamics
of Sensorimotor Oscillations in a Motor Task” in this book).