Brain–Computer Interfaces - Index of

Neurofeedback Training for BCI Control 71
}
ITR [bit/min]
20
15
10
5
0
5 4 3 2
single runs means for trial length
ITR=17 ITR = 17 bit bit/min
1 0
trial length [s]
( (falling time +1s)
Fig. 2 Left side: Graphical display of the “basket-paradigm”. The subject has to direct the ball
to the indicated goal (“basket”). The trial length varies across the different runs. Right side:
Information transfer rate (ITR) for one subject in relation to trial length. The black line represents
the maximum possible ITR for an error-free classification (modified from Krausz et al. [41])
output was adapted to each patient. The participant’s task was to hit the highlighted
basket (which changed side randomly from trial to trial) as often as possible. The
speed was increased run by run until the person considered it too fast. This way, we
attempted to find the trial length that maximized the information transfer rate. After
each run, users were asked to rate their performance and suggest whether the system
operated too slow or too fast. The highest information transfer rate of 17 bits/min
was reached with a trial length of 2.5 s ([41], see also Fig. 2 right side).
3.2 Impact of Feedback Stimuli
A well thought-out training protocol and helpful feedback signals are essential
to keep the training period as short as possible. The feedback provides the user
with information about the efficiency of his/her strategy and enables learning. Two
aspects of feedback are crucial. The first aspect is how the brain signal is translated
into the feedback signal (for advantages of continuous versus discrete feedback, see
[40]). The second aspect is how the feedback is presented. The influence of feedback
on the user’s attention, concentration and motivation are closely related to the
learning process, and should be considered (see also [42]).
As mentioned above, some BCI studies use different feedback modalities. In the
auditory modality, Hinterberger et al. [43] and Pham et al. [44] coded slow cortical
potential (SCP) amplitude shifts in the ascending and descending pitches on
a major tone scale. Rutkowski et al. [45] implemented an auditory representation
of people’s mental states. Kübler et al. [66] and Furdea et al. [67] showed that
P300 BCIs could also be implemented with auditory rather than visual feedback.
A BCI using only auditory (rather than visual) stimuli could help severely paralyzed
patients with visual impairment. Although the above studies showed that BCI
communication using only auditory stimuli is possible, visual feedback turned out to