Page 136 - Handbook of Biomechatronics

Biomechatronic Applications of Brain-Computer Interfaces 133

The P300
The P300 is an electrical potential that appears about 300ms after the user
has observed a rare relevant stimulus (Nicolas-Alonso and Gomez-Gil,
2012). For example, if a person is told to listen for animal types and is then
read the words “house,” “apartment,” “shark,” and “building,” a P300
response can be expected about 300ms after the word “shark.” This method
of eliciting P300 responses by mixing a relevant stimulus with several other
irrelevant stimuli is known as the oddball paradigm.
Similarly to SSVEPs, the P300 is used to select among multiple possible
commands. Possible commands flash on the screen, and the command that
the user desires will evoke a P300 response since it is the relevant “oddball”
command. The timing of the P300 response can then be analyzed to deter-
mine what command likely triggered the response. When many possible
commands are available (e.g., the user is selecting the next possible letter
for an e-mail), the selection is generally done in a two-stage process. First,
all possible commands are displayed in a two-dimensional grid, and the col-
umns of the grid flash one after the other. The user’s brain generates a P300
in response to the column that contains the command of interest. Once the
correct column has been identified, the rows of the grid begin to flash one
after the other, and the user’s brain generates a P300 in response to the row
of interest, allowing the correct command to be identified as the intersection
of the correct row and column. This process is illustrated in Fig. 3. If the
system is unsure what command should be selected (e.g., two columns

Fig. 3 The principle of a P300-controlled spelling device. The user is thinking of the let-
ter “P.” The different columns of the grid flash one after the other, and the column con-
taining the relevant letter evokes a P300 response (A). The different rows then flash one
after the other, and the row containing the relevant letter evokes a P300 response (B).
The relevant letter can then be identified as the intersection of the column and row that
evoked the P300 (C).