## 3 Signal Processing
### Signal Acquisition  
The brain is composed of billions of neurons that communicate using electrical signals. These signals are produced at similar locations between individuals, yet it is not fully understood why they are emitted there and what their intentions are. However, it is still important to know when they are produced and their location, which reflects the normal or abnormal activity of the brain and user intentions.  
Many techniques have been developed to record brain activity (e.g., EEG, ECoG, single-neuron recording, PET, fMRI, MEG and FNIR) [14; 51]. Despite all of them being able to record brain activity, in this work, we will focus on EEG. The reason behind this is that the other enlisted techniques are either invasive, expensive or have high latency.  
EEG recordings are done using electrodes attached to the surface of the scalp, and each electrode measures the potential difference from a reference electrode and itself [51]. Correspondingly, these potentials reflect activity within the brain, and to avoid unwanted signal noise due to poor connection, the electrodes must have good contact with the area of interest. Furthermore, for understanding and repeatability, it is of great interest to know exactly where the electrodes are commonly located. For that reason, international 10-20 electrode systems are used, which consist of making an arc grid over the scalp that starts at specific locations, where the Nasion and Inion are the longitudinal references and the right and left preauricular are the lateral references (see Figure 3). The corresponding name of  
Figure 2: P300 wave and the classical P300 spelling paradigm described by [PERSON] [41].  
each arc crossing represents a location of the brain lobes, which is helpful for the spatial analysis of recorded signals.  
Furthermore, since EEG signals go through several layers of muscle, skin and bone, having a correct measure of the brain signal requires a process of amplification and filtering to improve the signal quality.  
First, the amplification helps increase the low-signal amplitude (\(\sim\)10-20 \(\upmu\)V), which is not easy to interpret using common displays, recorders or AC/DC converters. Notably, amplifiers must fulfill some requirements such as noise rejection and guarantee equipment and patient protection.  
Then, filtering is done to reduce either the environmental noise (e.g., power lines and electrical and/or surrounding medical equipment) or the physiological noise (e.g., muscle activation, eye movement, and/or blinking) [52]. Dealing with environmental noise is usually easier than dealing with physiological noise. Environmental noise can be avoided by removing most of the sources of electromagnetic signals from the recording room and its vicinity. Furthermore, one of the most common techniques is to use a notch filter at 50 or 60 Hz that helps by removing the noise of the electric power lines' artifacts. For physiological noise, one of the most common approaches is to incorporate physiological signals in the recordings and subtract them from the EEG. Other methods include telling the subject to remain still, not blink and hold the gaze during the study; however, this is usually difficult and can introduce even more noise because of the voluntary attention needed to control those body actions.