BR-SFDA: A Source-Target Bidirectional Refined SFDA for Privacy Preserving EEG-based BCIs

Due to significant inter-subject variability in feature distributions caused by the diversity of neural activity patterns, Electroencephalography (EEG)-based brain-computer interface (BCI) systems face considerable challenges in cross-subject EEG decoding. Though transfer learning has been widely introduced for knowledge transfer from source subject(s) to target subject and exhibited great success, a non-negligible issue is that source subjects' EEG data usually contains privacy information and should be protected. To address both issues, we propose a source-target bidirectional refined source-free domain adaptation (BR-SFDA) framework in this paper for privacy preserving cross-subject EEG classification. BR-SFDA makes improvements from two aspects under the popular 'pretraining and fine-tuning' paradigm. On one hand, it locally performs data augmentation and builds a multi-criteria fused metric to select representative EEG sample for model pre-training. On the other hand, a structured graph learning strategy is employed to better guide the model finetuning in a self-supervised manner. Both improvements collaborate respectively from the front-end and back-end, leading to a bidirectional refined SFDA framework. Extensive experiments are conducted on two tasks of cross-subject motor imagery decoding and emotion recognition, and the results on four datasets demonstrate that BR-SFDA achieves superior performance to some competitive models. Besides, the effectiveness of data augmentation and filtering, structured graph learning and domain adaptation is well evaluated.