A new grid stimulus with subtle flicker perception for user-friendly SSVEP-based BCIs

Abstract

Objective. The traditional uniform flickering stimulation pattern shows strong steady-state visual evoked potential (SSVEP) responses and poor user experience with intense flicker perception. To achieve a balance between performance and comfort in SSVEP-based brain-computer interface (BCI) systems, this study proposed a new grid stimulation pattern with reduced stimulation area and low spatial contrast. Approach. A spatial contrast scanning experiment was conducted first to clarify the relationship between the SSVEP characteristics and the signs and values of spatial contrast. Four stimulation patterns were involved in the experiment: the ON and OFF grid stimulation patterns that separately activated the positive or negative contrast information processing pathways, the ON-OFF grid stimulation pattern that simultaneously activated both pathways, and the uniform flickering stimulation pattern that served as a control group. The contrast-intensity and contrast-user experience curves were obtained for each stimulation pattern. Accordingly, the optimized stimulation schemes with low spatial contrast (the ON-50% grid stimulus, the OFF-50% grid stimulus, and the Flicker-30% stimulus) were applied in a 12-target and a 40-target BCI speller and compared with the traditional uniform flickering stimulus (the Flicker-500% stimulus) in the evaluation of BCI performance and subjective experience. Main results. The OFF-50% grid stimulus showed comparable online performance (12-target, 2 s: 69.87±0.74 vs. 69.76±0.58 bits min-1, 40-target, 4 s: 57.02±2.53 vs. 60.79±1.08 bits min-1) and improved user experience (better comfortable level, weaker flicker perception and higher preference level) compared to the traditional Flicker-500% stimulus in both multi-targets BCI spellers. Significance. Selective activation of the negative contrast information processing pathway using the new OFF-50% grid stimulus evoked robust SSVEP responses. On this basis, high-performance and user-friendly SSVEP-based BCIs have been developed and implemented, which has important theoretical significance and application value in promoting the development of the visual BCI technology.

A large database towards user-friendly SSVEP-based BCI

“…This indicated that the participants felt comfortable about the stimuli despite the slight flicker and accepted it as a preferred BCI system. This finding aligns with the user experience reported by Ming et al [10]. Moreover, Ming et al [10] demonstrated that participants preferred grid stimuli over high- contrast flicker stimuli, and each contrast increment represented a step sacrifice in user experience.…”

“…This finding aligns with the user experience reported by Ming et al [10]. Moreover, Ming et al [10] demonstrated that participants preferred grid stimuli over high- contrast flicker stimuli, and each contrast increment represented a step sacrifice in user experience. The BCI system will be further evaluated systematically in the future (e.g., the tradeoff between the low-contrast stimuli and the effort required to focus on a target), thus establishing a standard evaluation of user experience.…”

“…Each grid pattern consists of flicking dark squares (10 × 10 pixels) arranged in an 8 × 8 matrix with a stationary background, contributing to a spatial frequency of 1.48 cycles/ degree. According to a previous study, the Weber contrast of the darkest squares and background is 0.5, whereas the gray scale of the background is 50% [10].…”

“…After the experiment, the participants were asked to complete a questionnaire regarding their feelings about the visual stimuli. The questionnaire included three aspects: comfort level, flicker perception, and preference level [10]. A lower score represented a better user experience.…”

“…Furthermore, reversal checkerboard stimuli showed a decreased flicker perception [9]. A recent study designed a grid stimulus with low and negative contrast, demonstrating comparable performance to the traditional flicker stimulus [10].…”