Brain–Computer Interfaces - Index of

BCIs in the Laboratory and at Home: The Wadsworth Research Program 105
Fig. 8 Sample waveforms for attended (black) and unattended (gray) stimuli for electrodes PO7,
Pz, and PO8. Row 1: Data were collected using a 6 × 6 P300 matrix paradigm with stimulation
every 175 ms. Rows 2 and 3: Data were collected using a two-target oddball paradigm with stimulation
every 200 ms or every 1.5 s, respectively. The P300 response is evident at Pz (at about
400 ms), and a negative deflection at approximately 200 ms is evident at locations PO7 and PO8
response (i.e., Fz, Cz, and Pz) provide valuable information for classifying matrix
data. This is consistent with previous data showing that occipital electrodes improve
classification [20, 35]. These electrode locations discriminate attended from nonattended
stimuli, as measured by r 2 , the squared value of Pearson’s r that measures
the relationship between two variables [52]. Examination of the waveforms suggests
that a negative deflection preceding the actual P300 response provides this
additional unique information (see Fig. 8, row1).
The unique classification information provided by the occipital electrodes is
probably not due simply to the user fixating the target symbol. Several previous
studies have established that attentional factors increase the amplitude of the occipital
P1, N1, and N2 components of the waveform [15, 18, 19, 26]. For example,
Mangun et al. [26] had participants maintain fixation on a cross while stimuli were
flashed in a random order to the four visual quadrants. The participants’ task was
to attend to the stimuli in one quadrant and ignore the other three quadrants. The
amplitudes of the P1, N1, and N2 responses to stimuli in a given location were significantly
larger when the location was attended than when the location was ignored,
even though fixation remained on a central cross throughout. In addition, eye movements
and blinks were monitored by recording vertical and horizontal EOG, and