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of all channels from each individual channel.

1.5.3

Frequency bands

The EEG signal is processed in specific frequency bands. Different specific EEG frequency bands are associated with various physiological processes. The EEG frequency band used in this thesis ranges from 0.5 to about 30 Hz. This range includes the δ, θ, α, µ and the β frequency bands. The δ-band has frequencies less than 4 Hz. It is found in the frontal areas in adult during sleep and during some continuous attention tasks [24].

The θ-band has a frequency range of 4-8 Hz. The θ-band can occur during action inhibition, drowsiness and it is found in locations that are not associated to a task at hand [24].

The α-band has a frequency range of 8-13 Hz [24]. It is found around the occipital and frontal areas of the head and is associated with process inhibition. The α-band amplitude is enhanced when the eyes are closed or during relaxation [24]. The µ-band often referred as the Rolandic µ rhythm has frequency range of 8-12 Hz which overlaps with that of the α-band [24]. For this reason the two may be confused and the α-band which has a larger am- plitude and spatial distribution may overshadow the µ-band. Electrode derivation methods that enhances local activities can be exploited to differentiate between the two. The µ-band is found in the central cortical representation areas that are concerned with sensory input and movement control. Consequently the µ-band is associated with movement related corti- cal activities. Its amplitude in the contralateral hemisphere is suppressed during movement related cortical processes like movement imagination, execution and observation of a body part [28]. The µ-band has several applications in this thesis.

The β-band has a frequency range of about 12-30 Hz [24]. It can be found in the frontal and the central areas. It is associated with active mental state, focus and alertness. Its activities during motor processes varies. Often the lower frequency band (12-16 and 16-24 Hz) de- creases in amplitude during motor processes (in a similar manner as the µ-band) while the the upper band may show increased amplitude in the brain hemisphere contralateral to the one performing the motor processes [28]. The β-band has several application in this thesis.

1.6

Brain computer interface

A brain computer interface (BCI) is a system that allows direct communication between the brain and a computer. In a typical BCI system, signal is acquired from the brain and used by the computer to determine the state of the brain. After determining the state of the brain by processing the recorded signal, the computer can translate the state into an action command.

1.6. Brain computer interface 14 The action command can include moving a computer cursor, control of a computer game or control of external devices like a wheelchair, control of a robotic hand and so on. A diagram showing the basis of a BCI is shown in Fig 1.8.

Applications Brain

activity Signal

acquisition _extractionFeature _{classification}Feature

Signal processing

Commands Digitisation

Figure 1.8: The schematic diagram of a BCI system.

Brain computer interface was originally developed for communication purposes. It has only recently been applied to rehabilitation. There are various signals modalities that can be used to build a BCI system used for the purpose of communication. The signal modalities are described in Table 1.2. This thesis focuses on BCIs that can be used for movement rehabili- tation following neurological injury. In movement rehabilitation, only EEG from Table 1.2 is widely used. The EEG can be easily recorded and it is portable compared with fMRI for ex- ample. It provides a measure of neuronal electrical activity and not hemodynamic response in the case of fMRI, the later having latency between a task and the related brain activity. Also in BCI for communication, different EEG control signals are used. These are described in Table 1.3. The Event related desynchronisation (ERD) and event related synchronisa- tion (ERS), ERD/ERS which can be effectively modulated by users even following many neurological injuries is widely used in movement rehabilitation. These features are easy to compute and require minimum number of EEG electrodes minimizing setup time. This ad- vantage makes the ERD/ERS method suitable especially for therapeutic BCI where setup time must be minimized given that patients are not normally available for experiments for a long period of time. The features are well established and researchers can target physiologi- cally relevant frequency bands when using it as features for therapeutic BCI.

1.6. Brain computer interface 15 Table 1.2: Signal modalities used in brain computer interface.

Modality Description Non-

invasive

EEG EEG (described in Section 1.5) is the extracellular electri- cal field potential recorded from the scalp with milliseconds temporal but poor spatial resolution [24]. It is a direct mea- sure of neuronal electrical activity recorded by placing elec- trodes on the scalp [29].

MEG Magnetoencephalography (MEG) is the recording of mag- netic field arising from neuronal electrical activities. It has similar temporal but higher spatial resolution than EEG [29].

fMRI Functional magnetic resonance imaging (fMRI) is a method of measuring brain activity by measuring hemodynamic re- sponse during mental activity. It has a lower temporal (≈1 s) but a higher spatial (≈1 mm) resolution compared with EEG. Since it determines brain activity by measuring the associated hemodynamic response, it is an indirect measure [29].

NIRS Near infrared spectroscopy (NIRS) can be used to indirectly measure brain activity exploiting hemodynamic response to brain activity in a similar manner as fMRI [29]. Hemody- namic changes are determined from changes in attenuation of light of near infrared wavelength. It has similar temporal but about five times lower spatial resolution than fMR [29]. Semi in-

vasive

ECoG Electrocorticography (ECoG) is a technique that measures direct brain electrical activity through semi invasive elec- trodes placed on the surface of the cortex. It provides signal of higher amplitude and is not artefact prone compared to EEG [29]. It also has higher spatial resolution than EEG [29].

Invasive

IcNR Intracortical neuron recording (IcNR) is an invasive tech- nique for measuring direct electrical brain activity by plac- ing electrodes inside the gray matter of the brain [29]. It has similar temporal resolution as ECoG but because it can capture spike signal from single or a small pool of neurons it allows for the highest spatial resolution.

1.6. Brain computer interface 16

Table 1.3: EEG-based control signal used in brain computer interfaces Feature Description

P300 A P300 potential occurs at about 300 ms following a visual, auditory and other stimuli. The P300 signal is used in BCI by exploiting the oddball paradigm. In this paradigm, when two stimuli are presented interchangeably at different rate, the infrequent stimulus is associated with a larger potential [29].

VEP Visual evoked potentials (VEP) are EEG potentials occur- ring as a result of processes in the visual cortex in response to visual stimuli [30]. It includes steady state visual evoked potential (SSVEP). Following visual stimulation at a spe- cific frequency, SSVEP with fundamental frequency equal to that of the stimuli is generated in EEG [31].

ERD/ERS Event related desynchronisation (ERD) and event related synchronisation (ERS) [28, 32] refer to decrease and in- crease respectively of EEG power relative to a baseline pe- riod within a narrow frequency band. Movement related cortical processes like those during MI and physical exe- cution can be quantified with ERD across the sensorimotor cortex. ERD/ERS, sometimes referred to as event related spectral perturbation [33], is used as a general term to refer to both ERD and ERS.

MRCP Movement related cortical potential is a negativity in EEG signal that occurs as a consequence of movement execution or imagination [34].

SCP Slow cortical potential (SCP) is a slow change in EEG be- low 1 Hz lasting from one to several seconds [35]. An in- crease in brain activity is marked by negative SCP while a decrease in brain activity is marked my positive SCP.