A Sequential Multilevel Frequency Stimulation Paradigm for SSVEP-Based Brain–Computer Interfaces

This study evaluated a discrete staircase implementation of sequential frequency stimulation for steady-state visual evoked potential (SSVEP)-based brain–computer interfaces (BCIs). Five adjacent frequencies separated by 0.2 Hz were presented in consecutive 2 s epochs as ascending-frequency flicker patterns (AFFPs) or descending-frequency flicker patterns (DFFPs) within low-frequency (6–10 Hz) and high-frequency (41–45 Hz) ranges. Offline EEG responses from 15 participants were compared with conventional fixed-frequency stimulation using eight established signal-processing methods: fast Fourier transform (FFT), power spectral density (PSD), canonical correlation analysis (CCA), filter bank CCA (FBCCA), dynamic PSD, short-time Fourier transform (STFT), continuous wavelet transform (CWT), and discrete wavelet transform (DWT). Low-frequency stimulation achieved a higher average normalized classification accuracy than high-frequency stimulation (90.8% vs. 80.6%), while CWT yielded the highest descriptive average accuracy among the evaluated methods; however, post hoc comparisons did not establish CWT as significantly superior to the other evaluated methods. The fixed-frequency, AFFP, and DFFP conditions achieved average normalized accuracies of 89.7%, 85.6%, and 89.5%, respectively. Statistical analysis showed a significant main effect of the signal-processing method but no significant main effect of the stimulation pattern. Three consecutive commands achieved an average accuracy of 91.5 ± 9.6%, whereas the accuracy decreased as the command sequence became longer. The results show that AFFP and DFFP staircase sequences can be detected offline, but they do not prove better classification than fixed-frequency stimulation. The small, uniform sample limits how widely these findings can be applied. Their main value may lie in assisting flexible sequential command design, which needs further testing with larger, more diverse groups and within an online closed-loop BCI system.