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consuming for the user. It does this by building up a history of the last n responses
for each available choice during interaction.
Importantly, in our user test, each choice was
flashed on the screen one at a time,
instead of as a grid. Currently winning choices would
flash red, providing feedback
to the user. Stimuli are
flashed randomly on the screen at regular intervals. For this
test, we used the P300 scale player interface method detailed above to navigate a
3D world. Three arrows indicate left, right and straight ahead. These are arranged
parallel to the
flash is
triggered, a 400 ms chunk of EEG data is stored and tagged to the stimulus. At the
end of any given number of windows (
n
windows), results for each stimulus are
averaged together. Following this, each time a
floor of the 3D virtual environment (see Fig.
3.4
). Each time a
flash is triggered, the previous n
results are averaged for each stimulus. Each stimulus therefore always has an
averaged EEG signal associated with it representing the EEG response to the
previous n windows. If at any time the averaged signal contains an amplitude peak
between 200 and 600 ms after the onset of the stimulus, and the average area is
greater than that of every other averaged peak area, it is judged to be a possible
P300 target signal, as the target would be the peak with the highest average area
under the peak. This target signal is judged to be the winner, and the stimulus
changes from white to red to re
ect the change in direction.
The movement system functions by increasing the amount of force in the
direction of the current winner. In cases where subjects wish to move in a new
direction, the system automatically cancels all force in the current direction in
favour of generating force in the direction of the new winner.
Fig. 3.4
The visual stimulus interface for the scale player (
above
). This interface has the
advantage of being re-usable for P300 controlled game interactions (
below
). In the above example,
to mitigate against distractions, each
arrow
is presented one at a time. The currently selected
direction appears a different colour to provide visual feedback to the user (see 3.8)
