Before revision:  In such a case, high inter- and intrasubject variabilities are the points to be considered. Transfer learning , especially as a domain adaptation technique, has drawn increasing attention in various fields. However, the adaptation of approaches into BCI faces two challenging limitations. (i) Most domain adaptation  methods are designed for labeled source and unlabeled target domain whereas BCI tasks generally have multiple annotated domains.
After revision:  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 concern of its practical use, hampered by time-consuming steps and a large data requirement. To this end, recent studies adopted a transfer learning strategy, especially domain adaptation methods are designed for labeled source and unlabeled target domain whereas BCI tasks generally have multiple annotated domains.