Brain–Computer Interfaces - Index of

BCIs in the Laboratory and at Home: The Wadsworth Research Program 103
3 P300-Based Item Selection
We have also been developing a BCI system based on the P300 event-related potential.
Farwell and Donchin [17] first introduced the BCI paradigm in which the user
is presented with a 6 × 6 matrix containing 36 symbols. The user focuses attention
on a symbol he/she wishes to select while the rows and columns of the matrix are
highlighted in a random sequence of flashes. Each time the desired symbol flashes,
a P300 response occurs. To identify the desired symbol, the classifier (typically
based on a stepwise linear discriminant analysis (SWLDA)) determines the row and
the column to which the user is attending (i.e., the symbol that elicits a P300) by
weighting and combining specific spatiotemporal features that are time-locked to
the stimulus. The classifier determines the row and the column that produced the
largest discriminant values and the intersection of this row and column defines the
selected symbol. Figure 6 shows a 6 × 6 P300 matrix display and the average eventrelated
potential responses to the flashing of each symbol. The letter O was the
target symbol, and it elicited the largest P300 response. The other characters in the
row and the column containing the O elicited a smaller P300 because these symbols
flashed each time the O flashed.
The focus of our P300 laboratory studies has been on improving classification
accuracy. We have examined variables related to stimulus properties and presentation
rate [47], classification methods [21], and classification parameters [22]. Sellers
et al. [47] examined the effects of inter-stimulus interval (ISI) and matrix size on
classification accuracy using two ISIs (175- and 350-ms) , and two matrices (3 × 3
Fig. 6 (A)A6× 6 P300 matrix display. The rows and columns are randomly highlighted as shown
for column 3. (B) Average waveforms at electrode Pz for each of the 36 symbols in the matrix. The
target letter “O” (thick waveform) elicited the largest P300 response, and a smaller P300 response
is evident for the other symbols in column 3 and row 3 (medium waveforms) because these stimuli
are highlighted simultaneously with the target. All other responses are to non-target symbols (thin
waveforms). Each response is the average of 30 stimulus presentations