Breaking the performance barrier in deep learning-based SSVEP-BCIs: a joint frequency-phase training strategy

<i>Objective.</i>Deep learning (DL) exhibits considerable potential for steady-state visual evoked potential (SSVEP) classification in electroencephalography-based brain-computer interfaces (BCIs). SSVEP signals contain both frequency and phase characteristics that correspond to the visual stimuli. However, existing DL training strategies typically focus on either frequency or phase information alone, thus failing to fully exploit these dual inherent properties and substantially limiting classification accuracy.<i>Approach.</i>To tackle this limitation, this study proposes a joint frequency-phase training strategy (JFPTS), which comprises two complementary stages with distinct time-window sampling schemes. The first stage adopts a frequency prior-driven sampling scheme to improve frequency component utilization, whereas the second stage employs a phase-locked sampling scheme to enhance intra-category phase consistency. This design enables JFPTS to effectively leverage both frequency and phase properties of SSVEP signals.<i>Main results.</i>Comprehensive experiments on two well-established public datasets validate the effectiveness of JFPTS. The results demonstrate that the JFPTS-enhanced model achieves a marked superiority over the current state-of-the-art classification approaches, notably surpassing the long-standing performance benchmark set by task-discriminative component analysis (TDCA).<i>Significance.</i>Overall, JFPTS establishes a new training paradigm that advances DL approaches for SSVEP classification and promotes the broader adoption of SSVEP-BCIs.