**1.1. P300 event-related potential**

This event-related potential is a function of uncertainty of the external stimuli, and major changes in the positive amplitude of the EEG waveform appears at about 300 ms after the stimulus which is called P300 component of ERP, first used by Sutton et al. [3]. The P300 component of ERP was tested in human by Farwell and Donchin, and their experiment revealed that the rare event elicits P300 which can be used to develop mental prosthesis [4]. Farwell and Donchin proposed alphanumeric BCI speller consisting of 26 alphabets and 10 numbers (0–9) arranged in 6×6 matrix of rows and columns as shown in **Figure 1a** [4]. In this row-column (RC) paradigm, rows and columns are flashed randomly and the subject is asked to count the number of flashings of rows and columns corresponding to the target character. Flashed row/ column containing target stimulus elicits P300 from parietal, occipital, and temporal regions (majorly in parietal) of the brain based on Oddball Paradigm, i.e., occurrence of rare (odd ball) event. The higher amplitude P300 is evoked from stimulus with higher strength and low probability (rare event). However, this paradigm suffers from low information transfer rate (ITR) due to multiple trials.

Various changes in visual aspects of RC paradigm in terms of background color, character distance, and character size is done [5] to test the system performance. In this experiment, various visual protocols such as black background, white background, large symbol size, small symbol size, larger inter-symbol distance, and smaller inter-symbol distance are tested to observe the performance in RC BCI speller. Visual protocol with white background outperformed all the other protocols, while small symbol size resulted in poor performance.

A region-based (RB) BCI paradigm was designed by [6] to reduce human perpetual errors such as attentional blink, repetition blindness, habituation, and other spatial errors such as crowding effect and adjacency problems. Human perpetual errors in P300 speller was demonstrated by [7] to show the effect of adjacency problems. RB paradigm shown in **Figure 1b** and **c** uses seven regions flash at two levels instead of rows and columns. The region containing

**Figure 1.** (a) RC paradigm with second row flashing [4]. (b) and (c) RB paradigm where seven sets of characters in level 1 (b) is expanded in level 2 (c) to spell a character "B" [6].

target character is selected at first level and the target is selected at second level which elicits P300. The number of characters in the RB speller is 49 and the probability of hitting a target is 1/7 which evokes higher amplitude of P300. Thus, accuracy, user acceptability, and ITR are enhanced in RB paradigm than traditional RC paradigm [6].

Only one character flashes in single character (SC) paradigm rather than all the six characters in row or column in RC paradigm [8]. In [9], SC paradigm was compared with RC paradigm for 19 subjects and observed that the classification accuracy was better for RC (85.3%) than SC (77.90%). Further, in [10], four P300 BCI spellers: RC, SC and two RB paradigms were compared, in which characters were based on alphabetical order in one and frequency of use in another. It was observed that accuracy of RB with characters in alphabetical order was highest and SC, the least for six subjects to spell two words WATER and LUCAS in three trials. In addition, whereas, user acceptability was highest for both RB paradigms than RC and SC, and region accuracy was least for central character on seven regions [6].

A checker board (CB) paradigm was proposed in [11], having 8×9 matrix of alphanumeric characters and keyboard commands, and compared the performance with traditional RC paradigm. Eighteen healthy subjects were used for the experiment and it was found that mean online accuracy, mean bit rate, and user acceptability were significantly higher for CB than RC but it suffers from adjacency errors. Other various modifications on standard RC paradigm have been done like a constant character flashing and shape changing which enhances the performances of P300 to some extent [12].
