“…These spatial filters or projection direction can be learned by exploiting individual training template (Bin et al, 2011), reference signal optimization (Zhang et al, 2013), interfrequency variation (Yin et al, 2015), and ensemble reference signals (Nakanishi et al, 2014;Chen et al, 2015a) in the framework of canonical correlation analysis (CCA). Recently, the task-related components (Nakanishi et al, 2018) and the multiple neighboring stimuli (Wong et al, 2020a) have been utilized to derive spatial filters in order to boost the discriminative power of the learned model further. On the other hand, the trainingfree methods perform feature extraction and classification in one step without the training session in the online BCI.…”