Source-free domain adaptation for SSVEP-based brain-computer interfaces

<i>Objective.</i>Steady-state visually evoked potential-based Brain-computer interface (BCI) spellers assist individuals experiencing speech difficulties by enabling them to communicate at a fast rate. However, achieving a high information transfer rate (ITR) in most prominent methods requires an extensive calibration period before using the system, leading to discomfort for new users. We address this issue by proposing a novel method that adapts a powerful deep neural network (DNN) pre-trained on data from source domains (data from former users or participants of previous experiments), to the new user (target domain) using only unlabeled target data.<i>Approach.</i>Our method adapts the pre-trained DNN to the new user by minimizing our proposed custom loss function composed of self-adaptation and local-regularity terms. The self-adaptation term uses the pseudo-label strategy, while the novel local-regularity term exploits the data structure and forces the DNN to assign similar labels to adjacent instances.<i>Main results.</i>Our method achieves excellent ITRs of 201.15 bits min^-1and 145.02 bits min^-1on the benchmark and BETA datasets, respectively, and outperforms the state-of-the-art alternatives. Our code is available athttps://github.com/osmanberke/SFDA-SSVEP-BCI.<i>Significance.</i>The proposed method prioritizes user comfort by removing the burden of calibration while maintaining an excellent character identification accuracy and ITR. Because of these attributes, our approach could significantly accelerate the adoption of BCI systems into everyday life.