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discussed in Section 2.3.1. Some of their corresponding BCI paradigms are discussed in Section
2.5. In the second approach users control their brain activity by concentrating on a specific mental
task, for example imagination of hand movement can be used to modify activity in the motor
cortex. In this approach feedback signals are often used to let users learn the production of easily
detectable patterns of neurophysiological signals. The types of signals resulting from the
concentration on mental tasks are described in Sections 2.3.2, 2.3.3, and 2.3.4.
2.3.1 Event-Related Potentials (ERPs)
ERPs are stereotyped, spatio-temporal patterns of brain activity, occurring time-locked to an
event, for example after presentation of a specific stimulus, before execution of a movement, or
after the detection of an unexpected stimulus [30]. Typically, ERPs are recorded with the EEG
and have been used in neuroscience for studying the different stages of apprehension, cognition,
and action.
P300 wave
The P300 is a positive deflection in the EEG signals that appears approximately 300 ms after
the presentation of rare or surprising, task-relevant stimuli [73]. To elicit the P300, subjects
are asked to monitor a random sequence of two types of stimuli. One stimulus type (the
oddball or target stimulus) appears only infrequently in the sequence, while the other
stimulus type (the normal or non-target stimulus) appears more frequent. Whenever the
target stimulus appears or presented, a P300 can be detected in the EEG signals. This
principle was exploited by Farwell and Donchin et al., [29] in a BCI system. They described
the P300 speller in which a matrix of symbols from the alphabet is displayed on a screen.
Rows and columns of this matrix are intensified in a random order, and intensifications of
the row or column containing the required symbol constitute the oddball stimulus, while all
other intensifications constitute non-target stimuli. Since Farwell and Donchin’s study,
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several studies about P300-based BCI systems have carried out. A general review of some of
these studies and a detailed description of the P300 wave are provided in Section 2.5.
Steady-State Visual Evoked Potentials (SSVEPs)
SSVEPs are oscillations noticed at occipital electrodes, generated by frequent visual
animation. Stimulation at a specific frequency leads to brain signals oscillations at the same
frequency and at harmonics and sub-harmonics of the stimulation frequency [22, 51]. In a
BCI, SSVEPs are used by simultaneously presenting several stimuli intensified at different
frequencies. Users can select one stimulus by focusing on it, which leads to an increment in
the amplitude of the signal in the frequency bands corresponding to the flashing frequency
of the stimulus. Some systems employing this principle are described in [22].
Motor-Related Potentials (MRPs)
Motor-related potentials (MRPs) are independent of the comprehension or processing of
stimuli. MRPs are related to the events of anticipation or imagination of movements or
actions. MRPs are slow negative potentials, noticeable over the sensorimotor cortex before
the beginning of movement or during movement imagination [51]. Because the sensorimotor
cortex has a somatotopic organization, the body part that will be moved, or for which a
movement is imagined, can be inferred from the location of the greatest amplitude of the
MRP. This principle has been employed and used in several BCI systems.
2.3.2 Oscillatory Brain Activity
It is a sinusoid-like oscillatory brain activity that occurs in many regions of the brain. It changes
according to the state of users, for example between wake and sleep or between aware and idling.
Oscillatory activity in the EEG signals is classified into different frequency bands. Typically, it is
observable in the delta, theta, alpha and mu, beta, and gamma rhythms (see table 2.1 for
frequency bands values) [51].
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Sensorimotor Rhythms
Mu-rhythm oscillations can be detected over the sensorimotor cortex when a user does not
movement. When movements of body parts are imagined or performed, the amplitude of
these oscillations is decreased. Additionally, imagined or performed movements of body
parts lead to changes in beta-rhythm amplitude. The changes in the mu- and beta-rhythm are
located over the part of the sensorimotor cortex corresponding to the moved body part, and
so the imagined movements of different body parts can be discriminated. For example
imagination of moving the right hand corresponds to a decrease in mu-rhythm amplitude
over the left sensorimotor cortex, while imagination of moving the left hand corresponds to
a decrease in the amplitude over the right sensorimotor cortex [51]. The changes in
sensorimotor rhythms occurring in untrained users are usually not high enough to be
detected by the classification algorithms. Consequently, feedback training has to be used to
let users acquire control over sensorimotor rhythms. Several BCI systems employing
imagined movements of hands, feet, or tongue have been introduced in the literature such as
by the BCI proposed in [12].
Other Oscillatory Activity
Cognitive tasks other than motor imagery can also be used to trigger changes in oscillatory
brain activity. Examples of such tasks are mental calculations, auditory imagery, spatial
navigation imagery, or imagination of rotating geometric objects [25]. The classification
accuracy for such cognitive tasks is comparable to that achieved with motor imagery.
Additionally, depending on the users preferences the alternative cognitive tasks may be
easier to perform than motorimagery [27]. Recently, Emotiv has developed a BCI that uses
cognitive signals to move and rotate a virtual cubic on the screen and this motivated many
researchers to use this BCI in several control applications such as in [7] where it was used to
control humanoid robot.
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2.3.3 Slow Cortical Potentials (SCPs)
SCPs are slow voltage changes in the EEG signals occurring in the frequency range of 1 to 2 Hz.
Positive SCPs correspond to a general increase in cortical excitability, while negative SCPs
correspond to a general decrease in cortical excitability. Subjects can learn to control their SCPs
using feedback training [51]. The voluntary generation of both negative and positive SCPs has
been used in one of the earliest BCI systems for disabled subjects [55]. They showed in their
work that patients suffering from amyotrophic lateral sclerosis (ALS) can use a BCI to control a
spelling device and communicate with their environment.
2.3.4 Neuronal Ensemble Activity
Generally, action potentials are the basic unit of information in the brain and they allow different
neurons in the brain to communicate. Number of action potentials per time (i.e. neuron firing rate)
can be used in a BCI to detect the behavior of a user [51]. For example, the firing rate of neurons
in the motor and premotor-cortices can be used to predict hand positions or hand movement
velocities. Usually the firing rates from ensemble of neurons are used to predict subject behavior
[51]. Additionally, by using feedback training, subjects can learn to modulate the firing rates of
neurons in the motor cortex. Consequently, neuronal ensemble activity can be used as
neurophysiological signal in BCIs.
2.4 P300-Based Brain Computer Interfaces
After the general discussion of BCI systems in the previous sections, now in this section we give
a more detailed review of the type of the BCI system that is used in this thesis, i.e. P300-based
BCIs. First, we give descriptions of the P300 from a neuroscientific point of view, then we list
some of the conditions under which the P300 wave can be elicited and some factors that influence
the characteristics of the P300 wave. Finally, the basic idea underlying P300-based BCIs is
introduced and some systems and applications implementing this idea are described.
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2.4.1 The P300 Event-Related Potential
Generally, ERPs can be divided into two classes; exogenous and endogenous ERPs. Exogenous
ERPs are the result of the initial and automatic processing of stimuli and have a latency,
amplitude, and topographic distribution that depend mainly on the physical characteristics of
stimulus. Endogenous ERPs are the result of later and more comprehensive processing of stimuli
and have characteristics that mainly depend on the stimulus context, i.e. on the task was given to
the subject and on the attention the subject pays to the stimuli [45 and 73]. An endogenous ERP
that has gained much attention in both, the neuroscientific and medical research communities is
the P300 (see figure 2.3). The P300 is an interesting research topic doe to the reliability in its
measurement and the characteristics of its waveform, such as amplitude and latency, where they
can be influenced by various factors, and its eliciting nature. Since the discovery of the P300 by
Sutton et al., [73] many studies have tried to discover the psychological and neurophysiological
meaning of the P300 wave by changing ways of stimuli presentations and consequently observing
the corresponding changes in the waveform of the P300. Other studies have linked the
characteristic of the P300 wave to some specific factors of subjects such as gender, age, or brain
diseases, for example Alzheimer or schizophrenia. Since it is difficult to review all of these
studies in details, the following discussion is restricted to the factors that are important for the
P300to be used in BCI systems.
Different stimulus modalities and paradigms can be used to elicit the P300 wave. For the
stimulus modality, auditory, visual, tactile, gustatory, or olfactory stimuli can be used [73].
However, for some practical reasons, often visual, auditory, or both stimuli are used in most
studies in the literature. Basically, two paradigms have been employed in the most of the studies
in the literature; the oddball paradigm and the three-stimulus paradigm.
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Figure 2.3: Typical P300 wave. A series of negative and positive components (N1, P2 and N2) precedes
the P3. While the P3 reflects high-level processing of stimuli, the earlier components reflect low-level,
automatic processing of stimuli [45].
In the oddball paradigm, two different stimuli are used; a target (or oddball) stimulus and
a non-target stimulus. The two stimuli are presented in a random sequence. The target stimulus
appears only rarely, while the non-target appears more frequent. Subjects are directed to respond
only to the presentation or appearance of the target stimulus and to ignore the appearance of the
non-target stimuli.
The three-stimulus paradigm is a modified oddball paradigm in which a so-called
distracter stimulus appears infrequently in the sequence of target and non-target stimuli
presentations. Usually the subjects have no idea about the distracter stimulus when being given
the instructions. So, the subjects are surprised by the distracter when it first appears in a sequence.
Several unique distracter stimuli are used to increase the surprise and each distracter stimulus is
presented only once. The distracter stimuli should be perceptually different from both the target
and non-target stimuli. For example dog-barks or some other environmental sounds can be used
as distracters [30, 65 and 7].
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Different types of P300 can be observed in the two paradigms described above. In the
classical oddball paradigm, target stimuli elicit the so-called P3b. The P3b has a latency of
approximately 300-500 ms and can be detected mostly over centro-parietal regions of the brain.
The P3b appears only if users pay attention to stimuli and disappears if they do not pay attention
to stimuli. When users do not pay attention to stimuli, the target stimuli in the oddball paradigm
evoke a different type of P300 called P3a. The P3a has a latency of approximately 200-400 ms
and can be detected mostly over fronto-central regions of the brain. In the three-stimulus
paradigm, the target stimuli also elicit a P3b. However, the distracter stimuli evoke a P3a [29, 45,
56, 61 and 73]. The relation between the different paradigms and the P3a and P3b is summarized
in figure 2.4.
(a) (b)
Figure 2.4: Paradigms for evoking the P300: a) Oddball paradigm; b) Three-stimulus paradigm
2.4.2 Factors Influencing the P300
In addition to its dependence on different experimental paradigms as mentioned above, the P300
is also affected by many other factors. The reliance of the P300 wave on these factors shows that
the P300 is not a fixed or static phenomenon, but rather an actually variable response of the brain,
occurring in situations in which infrequent or rare and task-relevant stimuli have to be processed.
Below are some important factors influencing the P300 listed [9, 14, 30, 32, 47, 56 and 78]:
P3b
P3b
N
N
T
N
N
N
N
N
N
T
0 s 5 s 10 s
P3b
P3b
N
N
T
N
N
N
N
N
T
D
P3a
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Target Probability
The P3b peak amplitude is inversely proportional to the probability of the eliciting stimulus.
Low probability of the target stimulus leads to evoking high amplitude P3b waves, while
low amplitude P3b waves are evoked when the probability of the target stimulus is high.
Practically, the probability for target stimuli is usually set to values about 10% in order to
reliably evoke the P300 wave. The amplitude of the P3b is also influenced by local target
probability in addition to the effect of global target probability. This means that amplitude is
high when a target stimulus is preceded with many non-target stimuli and that amplitude is
low if a target stimulus is preceded with a small number of non-target stimuli.
Interstimulus Interval
The amplitude of the P3b wave is directly proportional to the interstimulus interval (the
amount of time between two consecutive stimuli). The longer are ISIs the higher are
amplitudes and the shorter are ISIs the smaller are amplitudes.
Habituation
The amplitude of the P3a habituates. After the presentation of many distracter stimuli, these
stimuli become familiar to the subjects and P3a amplitude decreases. For the P3b, its
amplitude is mostly unaffected by long-term repetition of stimuli.
Attention
The amplitude of the P3b wave mainly depends on the attention that users pay to stimuli
during the presentation process and on the concentration of the users. Actually, the P3b
wave entirely disappears if users are not actively engaged in an oddball task. On the contrast,
the P3a wave remains mainly unaffected by changes in users attention and can be observed
even if subjects entirely ignore the stimuli.
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Task Difficulty
The latency of the P3b wave is directly proportional to the difficulty of the task being
processed while its amplitude is inversely proportional. For example, target stimuli that are
very different from non-target stimuli lead to higher P3b amplitudes than target stimuli that
are a little different from non-target stimuli [32]. For the P3a, the effect of task difficulty is
different. Increasing the difficulty of discrimination between target and non-target stimuli in
a three-stimulus paradigm will lead to high P3a amplitudes. Consequently, in addition to
stimulus surprise, the P3a also seems to be related to perceptual discrimination difficulty
between target and non-target stimuli. In such a setup, the P3b amplitude decreases as
expected.
After the review of the paradigms used for evoking both the P3a and P3b waves and the factors
influencing the shape of these waves, conclusions can be drawn about the psychological and
physiological meanings of these ERPs. Generally, the P3a is mainly related to frontal lobe
function and elicited by stimuli that require attention and subsequent processing. Specifically, it
has been stated that the P3a wave is a part of the so-called orienting response, i.e. the response of
the human body to rare, surprising or potentially threatening situations, consisting of rapid
changes in heart rate, skin conductance, and other physiological parameters [23]. The P3b is
proposed to be related to processes for context updating, processes for updating “models of the
environment”, and to the assessment of the stimulus. Contrary, immediate responses to a stimulus
and the P3b are thought to be part of high-level, metacontrol processing [29]. In summary, more
investigations and studies about the role of P3a and P3b in human information processing still
need to be established.
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2.5 P300-Based BCI Systems and Applications
The basic idea underlying P300-based BCI systems is to use an oddball or oddball-like paradigm
and let the user decide which stimulus plays the role of the target stimulus to be related to
performing some action. As the P300 (P3b) occurs only if a user voluntarily responses to a target
stimulus, the target chosen by the user can be automatically inferred from the EEG signals
recorded during stimulus presentation. More specifically, the sequence of events in a P300-based
BCI is usually as follows [29]. First, the user decides on a command he wants to execute or
perform with the help of the BCI then stimuli are randomly presented. The user concentrates on
the stimulus associated to the desired command. After stimulus presentation the recorded EEG is
analyzed and classified through specific steps of an algorithm (see Chapter 3).The main goal of
this analysis is to find which stimulus was chosen as target by the user if there was any. If the
analysis is successful the command corresponds to the chosen stimulus is executed by the BCI
system. In the next sections we present some widely used P300-based BCI systems that
implement this idea for different control applications.
2.5.1 P300 Speller
As mentioned before, the first P300-based BCI has been presented by Farwell and Donchin et al.,
[29]. In their work a 6×6 matrix containing the letters of the alphabet and some other characters
or symbols was displayed on a computer screen. Rows and columns of the matrix were flashed in
a random order or sequence, and subjects could choose a symbol from the matrix by counting
how often it was intensified. Flashes of the row or column containing the desired symbol
constituted target stimuli and elicited a P300 wave while all other flashes of rows and columns
constituted non-target stimuli did not elicit a P300 wave. Finding out which flashes evoked a
P300 wave is thus sufficient to infer which symbol the user wanted to select. Figure 2.5 indicates
the principle underlying the P300 speller.
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