# Investigation of Visual Stimulus Signals Using Hue Change for SSVEP
## 5 Conclusions  
In this paper, we highlighted the risk of visual discomfort or photosensitive epilepsy in response to intense light-induced flash stimuli and other visual stimuli used in noninvasive SSVEP-type BCI. The flash stimulus signal used as a visual stimulus was proposed to reduce visual discomfort by changing the color phase from a change in brightness to a change in hue. A measurement experiment of SSVEP with different hue changes and frequencies was performed to determine the flash stimulus signal with a similar degree of accuracy after reducing visual discomfort compared to conventional methods. The results are as follows.  
* For almost all hues and hue differences of the proposed visual stimuli, there was some variation in the CFF changes, but all fatigue changes were between \(-1\) and \(+1\). The only visual stimuli that were above this range were R45 and G5. For these two signals, the results were also closer to 1 than to 2. Therefore, the proposed method causes less fatigue than the conventional method, which has a CFF mean of \(+2\).
* As for the discrimination rate, we could confirm that the red visual stimuli were slightly more accurate than the conventional method. For green and blue visual stimuli, the accuracy depended on the hue difference, and the overall accuracy was slightly lower in the G15-G45 and B25-B45 ranges than the conventional method. However, for green and blue visual stimuli, the larger the hue difference, the much lower the eye strain was, compared with red.  
For the three items that were above 2 from the subjective questionnaire results of the conventional method, blurred vision and eye irritation were below 2 for all colors in the proposed method. Only eye strain varied in relation to the hue difference.
* From the results of the discrimination rates of the nine frequencies, for all hue differences in red visual stimuli, a discrimination rate of more than 60% was obtained for frequencies of 6 Hz and above, and a discrimination rate of more than 50% was obtained for frequencies of 5 Hz and below. For green visual stimuli, the discrimination rate was more than 50% for the frequencies from G20 to G45 that were of 6 Hz or higher. For blue visual stimuli, the discrimination rate of more than 50% was obtained when the frequencies from B10 to B45 were 5 Hz or higher.  
Conceptualization, Y.S., Y.K. and Y.B.; methodology, Y.S. and Y.K.; software, Y.S. and Y.K.; writing--original draft preparation, Y.S.; writing--review and editing, Y.S. and Y.B.; supervision, Y.B. and T.H.; project administration, Y.B. and T.H. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The study was conducted according to the guidelines of the Declaration of Helsinki. Ethical review and approval were waived for this study, due to the research use only anonymized processed information which the data published in the paper does not identify specific individuals and no correspondence table has been created.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
We would like to express our sincere gratitude to the participants who cooperated with the experiment in the midst of the epidemic of the new coronavirus infection (COVID-19).  
The authors declare no conflict of interest.