A new spatial contrast coding approach for SSVEP-based BCIs

Background

Steady-state visual evoked potential (SSVEP)-based brain-computer interface (BCI) systems mainly adopt the frequency, phase, and hybrid coding approaches in previous studies. This study proposes a new encoding approach based on spatial contrast, which is one of the spatial properties of visual stimuli.

New method

First, this study designed checkerboard-like stimuli with 11 kinds of background contrast to explore the effect of background contrast on stimulus-response characteristics of SSVEPs. Based on the spatial contrast related modulation of responses, this study conducted offline simulations to evaluate the feasibility of a multi-target contrast coding approach. Finally, this study designed a four-target SSVEP-BCI system to demonstrate the contrast coding approach.

Results

Checkerboard-like stimuli with the same frequency and initial phase but different background contrasts have different SSVEP responses in terms of amplitude, topography, and phase. Taking advantage of the characteristics, both offline simulations and online verifications indicated that the proposed BCI system achieved good classification performance. Online BCI experiments found that the four-target SSVEP-BCI system achieved averaged information transfer rates of 59.58 ± 0.42 bits/min at the 15 Hz condition and 52.54 ± 2.32 bits/min at the 30 Hz condition, respectively.

Comparison with existing method

Different from previous frequency, phase, and spatial coding approaches, this study adopts a background contrast-based coding approach to achieve a four-target BCI system.

Conclusion

This study proposes a new spatial contrast coding approach, which will enrich the encoding approach of the SSVEP-BCI systems and promote the applications of the SSVEP-BCI systems in more scenarios.