Comparison of Visual Stimuli for Steady-State Visual Evoked Potential-Based Brain-Computer Interfaces in Virtual Reality Environment in terms of Classification Accuracy and Visual Comfort.

Kang-Min Choi,Seonghun Park,Chang-Hwan Im

doi:10.1155/2019/9680697

Abstract

Recent studies on brain-computer interfaces (BCIs) based on the steady-state visual evoked potential (SSVEP) have demonstrated their use to control objects or generate commands in virtual reality (VR) environments. However, most SSVEP-based BCI studies performed in VR environments have adopted visual stimuli that are typically used in conventional LCD environments without considering the differences in the rendering devices (head-mounted displays (HMDs) used in the VR environments). The proximity between the visual stimuli and the eyes in HMDs can readily cause eyestrain, degrading the overall performance of SSVEP-based BCIs. Therefore, in the present study, we have tested two different types of visual stimuli—pattern-reversal checkerboard stimulus (PRCS) and grow/shrink stimulus (GSS)—on young healthy participants wearing HMDs. Preliminary experiments were conducted to investigate the visual comfort of each participant during the presentation of the visual stimuli. In subsequent online avatar control experiments, we observed considerable differences in the classification accuracy of individual participants based on the type of visual stimuli used to elicit SSVEP. Interestingly, there was a close relationship between the subjective visual comfort score and the online performance of the SSVEP-based BCI: most participants showed better classification accuracy under visual stimulus they were more comfortable with. Our experimental results suggest the importance of an appropriate visual stimulus to enhance the overall performance of the SSVEP-based BCIs in VR environments. In addition, it is expected that the appropriate visual stimulus for a certain user might be readily selected by surveying the user's visual comfort for different visual stimuli, without the need for the actual BCI experiments.

Highlights

Electroencephalography (EEG) has been the most widely used neural signal for brain-computer interfaces (BCIs), whose main aim is to provide the paralyzed or disabled with new means of communication with the external environment [1]
All participants were informed of the details of the experiments and had given their written consent. e data of two participants were excluded in further analyses: the first was excluded owing to the frequent blinking of the eyes during the presentation of the visual stimuli and the second owing to the nonexistence of spectral peaks in the recorded EEG. e eye blinks were identified by visually inspecting vertical electrooculogram (EOG) recorded during the offline experiment. is so-called “BCI-illiteracy” is a well-known issue in EEG-based BCIs [21]. is experiment was approved by the institutional review board of Hanyang University, Republic of Korea (IRB HYI-14-167-11)
We hypothesized that the pattern-reversal checkerboard stimulus (PRCS), which are widely used in the state visual evoked potential (SSVEP)-based BCIs, might not be the optimal visual stimulus in a virtual reality (VR)-headmounted displays (HMDs) environment because the images displayed on the HMDs are closer to the eyes than those on the LCD monitors, and the PRCS might be too intense for the eyes. erefore, in this study, we tested another type of visual stimulus called the grow/shrink stimulus (GSS) that changes both size and luminance in VR environments and compared the BCI performances with the PRCS

Summary

Introduction

Electroencephalography (EEG) has been the most widely used neural signal for brain-computer interfaces (BCIs), whose main aim is to provide the paralyzed or disabled with new means of communication with the external environment [1]. Owing to its advantages over the other paradigms and recent development of advanced analysis methods [4, 5], the SSVEP-based BCIs have been implemented for a variety of applications including assistive and rehabilitation tools for the disabled [6] and practical applications for the healthy, such as car navigation [7] and entertainment [8]. The SSVEP-based BCIs implemented in VR environments have employed visual stimuli identical to those used in conventional LCD monitor environments, without any major modification. All SSVEP-based BCI studies performed in VR environments assumed that the presentation of visual stimuli on HMD is not significantly different from that on an LCD monitor. It is well known that an experiment in the VR environment is highly vulnerable to visual fatigue than that in the LCD environment; this is mainly due to the image distortion, or crosstalk, in the stereoscopic viewing [19] as well as the proximity between the source of illumination and the eyes [20]

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Discussion

Conclusion