Brain–Computer Interfaces - Index of

Brain Signals for Brain–Computer Interfaces 33
cortex (area V1). It is followed by a positive component at ~100 ms (P1 or P100)
and a negative complex at ~145 msec (N1 or N145). While the neural generators for
P1 and N1 are still debated, they are probably striate and extrastriate visual areas.
The steady-state visual evoked potential (SSVEP) is elicited by repetitive patternreversal
stimulation [12]. It is thought to arise from the same areas that produce the
VEP, plus the motion sensitive area MT/V5.
The VEP and SSVEP depend mainly on the properties of the visual stimulus.
They have not been shown to vary with intent on a trial-by-trial basis. Nevertheless,
the first BCI systems [13–15] and some modern systems [16, 17] are VEP- or
SSVEP-based. For example, the BCI user can modulate the SSVEP by looking at
one of several visual stimuli, each with different stimulus properties (i.e., different
flash rates). SSVEP features vary according to the stimulus properties and are
measured to determine which target is being looked at. The computer then executes
the command associated with the target that the user is looking at. This communication
system is equivalent to systems that determine gaze direction from the eyes
themselves. Since it depends on the user’s ability to control gaze direction, it is
categorized as a dependent BCI system, that is, it requires muscle control. Recent
evidence suggests that it is possible to modulate SSVEP features by shifting attention
(as opposed to gaze direction), and thus that the SSVEP might support operation
of an independent BCI (i.e., a BCI that does not depend on neuromuscular function)
[16, 18, 19].
P300
When an auditory, visual, or somatosensory (touch) stimulus that is infrequent,
desirable, or in other ways significant is interspersed with frequent or routine stimuli,
it typically evokes a positive peak at about 300 ms after stimulus onset (i.e., a
P300 ERP) in the EEG over centroparietal cortex [20–22]. A stimulation protocol of
this kind is known as an ‘oddball’ paradigm [23, 24]. Evocation of the P300 in the
oddball paradigm requires that the subject attend to the target stimulus. While the
underlying neural generators of the P300 are debated, it is thought that the signal
reflects rapid neural inhibition of ongoing activity and that this inhibition facilitates
transmission of stimulus/task information from frontal to temporal-parietal cortical
areas [25, 26].
In a P300-based BCI system, the user is presented with an array of auditory,
visual, or somatosensory stimuli, each of which represents a particular output (e.g.,
spelling a particular letter), and pays attention to the stimulus that represents the
action s/he desires. That attended stimulus elicits a P300 and the other stimuli do
not (Fig. 2a). The BCI recognizes the P300 and then executes the output specified
by the eliciting stimulus. Since it requires only that a user modulate attention, rather
than any muscular output, a P300-based BCI is an independent BCI.
The first P300-based BCI system was developed by Donchin and his colleagues
[27, 28]. It presents the user with a 6 × 6 matrix of letters, numbers, and/or
other symbols. The individual rows and columns of the array flash in succession
as the user attends to the desired item and counts how many times it flashes. The