  Feedback has been shown to affect performance when using a Brain-Computer
Interface (BCI) based on sensorimotor rhythms. In contrast, little is known
about the influence of feedback on P300-based BCIs. There is still an open
question whether feedback affects the regulation of P300 and consequently the
operation of P300-based BCIs. In this paper, for the first time, the influence
of feedback on the P300-based BCI speller task is systematically assessed. For
this purpose, 24 healthy participants performed the classic P300-based BCI
speller task, while only half of them received feedback. Importantly, the
number of flashes per letter was reduced on a regular basis in order to
increase the frequency of providing feedback. Experimental results showed that
feedback could significantly improve the P300-based BCI speller performance, if
it was provided in short time intervals (e.g. in sequences as short as 4 to 6
flashes per row/column). Moreover, our offline analysis showed that providing
feedback remarkably enhanced the relevant ERP patterns and attenuated the
irrelevant ERP patterns, such that the discrimination between target and
nontarget EEG trials increased.
