Information Technology Reference
In-Depth Information
against one of the central design problems of the P300 speller paradigm
that at
some point, detection must momentarily stop and a decision must be reached, before
beginning again. We discuss a separate solution to this problem later in this chapter.
The P300 composer is a good
—
first step in the creation of P300 BCMIs. However,
there are still a range of problems with the approach that users need to be made
aware of. Signi
cantly, if the user moves, blinks, loses attention or otherwise
disturbs the signal, this will lead to inaccurate results. However, it is very chal-
lenging for users to avoid this without practice, and can occur even with experi-
enced users. Various methods can be deployed to prevent this. For example, if the
signal peak is very high, this may indicate facial or bodily movement. These trials
can and should be discarded as and when they occur, as they will otherwise bias the
signal average, creating inaccurate results.
3.5
P300 Scale Player
The P300 Scale Player is a simpli
cation of the P300 Composer intended to be used
as a close-to-real-time improvisation and performance system. I
rst used the
system in 2008 as part of a demonstration to international media, and have used it
on stage on a few occasions. The basic premise is that the system has only three
visual targets
arrows. The left arrow indicates a drop in note pitch, the right arrow
indicates an increase in note pitch, and the middle arrow indicates that the note
pitch should remain the same. The system decides which note to play based on the
outcome of a given number of trials, and testing is more or less continuous, with a
new test beginning as soon as the previous test has completed.
The advantage of this approach is that with an experienced user, the time it takes
for the system to make a decision is greatly reduced. For example, with a total inter-
stimulus interval of 70 ms, and a total number of trials being no more than 5,
decisions on note direction are reached in just over a second. It
—
s of course accepted
that this is slow when compared to playing an instrument, but it does allow for the
control of slow moving melody lines with some reliability.
The primary disadvantages of this approach are that it only allows for adjacent
notes to be played, and only within a particular prede
'
ned scale. So, for example, if
the user is on D3, they can only choose to move to E3 or C3, or to stay on D3.
However, it is precisely these restrictions that give the system its speed.
Although the scale player is certainly not an instrument with the capacity for
varied melodic output, it is at least a BCMI that can be used in real-world scenarios,
played as an instrument with an ensemble, and even to play the occasional well-
known tune (for example, the main theme from Beethoven
'
s Ode to Joy).
3.5.1 Using the Scale Player as a Game Controller
This approach can be modi
ed to allow its use as a game controller to play simple 3D
games for example. In this context, with a reaction time of around a second, it is
