These methods include the motion-onset VEP (m-VEP), code VEP (c-VEP), and motor imagery (MI). To improve the performance of BCI spellers, a number of spellers based on other triggering methods other than that mentioned above have been proposed. In addition, the amplitude of the potential of the P300 decreases with increasing use time and reduces its accuracy of classification. As every output needs to traverse all stimuli of a holonomic trial, users need to wait for a long time. A problem with the P300 speller is its slow speed. In addition to the visual P300, the auditory P300 and tactile P300 were added to the design of the BCI. For a few years after its development, most BCI spellers that were proposed were based on the P300. This potential is prominent in the central partial region of the brain. The potential of the ERP P300 in EEG signals is positive, and occurs at about 300 ms, with differences in the specific time of its occurrence, after stimulus caused by an event with a small probability. It was based on event-related potential (ERP) P300, and featured a row and column (RC) paradigm. The first BCI speller was proposed by Farewell and Donchin. The BCI speller is a typical visual application of the BCI, and was among the earliest implementations of the concept. Finally, prevailing challenges and prospective research directions are discussed to promote the development of BCI spellers. Factors influencing the performance and visual fatigue of BCI spellers are provided. Furthermore, hybrid spellers that involve the use of the SSVEP are presented in parallel. SSVEP-based BCI spellers, where only the SSVEP is used, are classified by stimulus paradigms and described in chronological order. The fundamentals of BCI spellers are first briefly described. This paper reviews these stimulus paradigms and analyzes factors influencing their performance. Various paradigms of visual stimuli, including the procedure of target selection, layout of targets, stimulus encoding, and the combination with other triggering methods are used and considered to influence on the BCI speller performance significantly. Many BCI spellers that harness the SSVEP have been proposed over the past two decades. Matching the SSVEP to the stimulus allows users to select target letters and numbers. In BCI spellers, the targets of alphanumeric characters are assigned different visual stimuli and the fixation of each target generates a unique SSVEP. The steady-state visual evoked potential (SSVEP), measured by the electroencephalograph (EEG), has high rates of information transfer and signal-to-noise ratio, and has been used to construct brain–computer interface (BCI) spellers.
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