**2. Theoretical Background**

## *2.1. BCI Types*

A BCI provides an interconnection platform that supports the full duplex communication between the brain and an external device. According to the way that BCIs use to set up the brain–device interconnection, they are classified as non-invasive or invasive. *Non-invasive* BCIs use electrodes placed on the scalp. They are easy and safe to use, low-cost, portable, and o ffer a relatively high temporal resolution. *Invasive* BCIs use electrodes implanted in the interior of the scalp. Comparatively to non-invasive BCIs o ffer higher values of amplitude, spatial resolution, and resistance to noise. However, they require neurosurgery operations and they are both unsafe and expensive. Furthermore, scar tissues decrease the quality of signals received. Practically, non-invasive BCIs are used more often.

There are various non-invasive methodologies used in BCI technology, such as *Positron Emission Tomography* (PET), *functional Magnetic Resonance Imaging* (fMRI), and *Near-Infrared Spectroscopy* (NIRS), which study changes made in the blood flow, *magnetoencephalography* (MEG), which monitors the magnetic action of the brain, and EEG, which records the electric activity of the brain. Both NIRS and fMRI BCIs o ffer high spatial resolution, but poor temporal resolution. Moreover, MEG and PET BCIs offer high spatial and temporal resolution. However, PET BCIs require the inoculation of a radioactive constituent into the bloodstream. Furthermore, both fMRI and MEG methods rely on the use of equipment which is not only costly, but also huge. EEG BCIs are by far the most popular type, because, despite their relatively poor spatial resolution, they have high temporal resolution, low-cost, and easy installation. [6].

Moreover, BCIs are classified as either exogenous or endogenous, according to the nature of the input signals. *Exogenous* BCIs analyze the brain activity created due to external stimuli. They are easy to set up and o ffer high bit rates, but they need the continuous response of the user to outward incitements which may be either tiring, or even unfeasible. *Endogenous* BCIs use self-regulation of brainwaves without external stimuli. They provide lower data transfer rates but they can be operated via free self-control even by users with sensory organs a ffected or su ffering from motor neuron diseases [10].

Similarly, BCI systems are classified, according to the method used for input data processing, as synchronous or asynchronous. *Synchronous* BCIs analyze the brain signals only after a specific prompt and during predefined time intervals. Thus, the overall process is better organized and the user is free to make any kind of movements, which would produce artifacts, when brain signals are not observed. They also require minimal training and have stable performance and high accuracy. *Asynchronous* BCIs inspect brain signals successively, thus letting the user act at free will. Therefore, they o ffer more natural human–machine interaction. However, they are more complex in design and

evaluation and require extensive training. Moreover, their performance may vary between users, and their accuracy is not very high [10].

#### *2.2. Brainwaves for EEG-BCIs*

The most commonly used types of brain waveforms to develop EEG-based BCIs are P300, SSVEP, ErrP, ERD/ERS, and alpha brainwaves [11].

*P300* is an event-related positive potential deflection which is caused by the reaction to a desired external stimulus of visual, auditory, or tactile modality. P300 waveforms are typically measured, with a latency of roughly 250 to 500 ms between stimulus and response, by using electrodes located over the parietal lobe of the scalp.

*Steady state visually evoked potentials* (SSVEP) are brain waveforms of exogenous type that are generated as responses to visual stimulation at specific frequencies ranging from 3.5 Hz to 75 Hz. Considering that SSVEP signals often have their highest values at medial occipital electrode sites, they are supposed to originate mostly from the primary visual cortex.

*Event-related desynchronization and event-related synchronization* (ERD/ERS) waves are endogenous brain signals, which are generated when performing mental tasks, such as motor imagery or mental arithmetic. They can be measured at di fferent cortical locations.

*Error-related potential* (ErrP) waveforms are brain signals which are activated every time that a subject identifies the commitment of an error which has been made either by himself/herself or by another individual during various choice tasks. Waves of this kind can be captured by applying electrodes on various brain regions including the anterior cingulate cortex, anterior insula, inferior parietal lobe, and intraparietal sulcus, as well as other regions of the cortex, subcortex, and cerebellum.

*Alpha brainwaves* are brain signals which have their amplitude increased whenever the eyes of an individual are closed during wakeful relaxation. In contrast, the amplitude of alpha waveforms is diminished for the duration of sleepiness and sleep and also when having eyes opened while mental e ffort is performed. This phenomenon is usually referred to as *alpha rhythm blocking*. Alpha brain waveforms can be monitored by applying a number of electrodes on both sides of the posterior segments of the scalp where the occipital lobe, which is the center of visual processing activities in the brain, is positioned.
