Before revision:  Nonetheless, due to high intra- and inter-subject variabilities, many studies on decoding EEG were designed in a subject-specific manner by using calibration samples, with no much concern on its less practical, time-consuming, and data-hungry process.
After revision:  Nonetheless, there still exist BCI-illiterate subjects who struggle to use BCI systems, showing high intra-subject variabilities. Several methods have been proposed to enhance their performance via transfer learning. In such a case, high intra- and inter-subject variabilities, many studies on decoding EEG were designed in a subject-specific manner by using calibration samples, with no much concern on its less practical, time-consuming, and data-hungry process.