# Investigation of Visual Stimulus Signals Using Hue Change for SSVEP
## 4 Discussion  
The black-and-white flash stimuli of the conventional method achieved an algorithm accuracy rate of more than 75% for all frequencies in Figure 4. In the results of the experiment with the proposed color stimuli, the red stimulus signal achieved an accuracy rate of more than 75% for all hue differences in Figure 7. For the green stimulus signal, the results showed that the accuracy rate was approximately 40% when the hue difference was less than 15\({}^{\circ}\). When the hue difference was more than 20\({}^{\circ}\), the accuracy rate was approximately 70%. For the blue stimulus signal, the discrimination rate was 65% or higher for all hue differences. In the subjective questionnaire of the conventional method (black-and-white flash stimulation), shown in Figure 6, many subjects reported levels of blurred vision, eye strain, and irritated eyes to be slight (2) to moderate (3), with a mean of two or more in the questionnaire results. Some subjects experienced eye pain, slowed adjustment, or double vision, with a mean of 1.5 Subjects reporting that they felt certain about blurred  
Figure 11: Comparison of the average accuracy rate of the algorithm for black-and-white flash stimuli and blue signals, and the average flicker and eye strain in the questionnaire results. (Flicker rating: 1 = none, 2 = slight, 3 = clear. Measures of discomfort: 1 = none, 2 = slight, 3 = moderate, 4 = clear, 5 = severe.) Figure 12: Changes in CFF values with blue flash stimuli: (**a**) Right eye; (**b**) Left eye. Shaded bar graphs show the upper and lower limits of the score. Black bars indicate the mean score (0 = no change in fatigue, + = more fatigued, \(-\) = less fatigued). vision and irritated eyes. The subjective questionnaire results show that many participants experienced eye strain. In addition, from the result of the CFF value, the average value was approximately +2 for both eyes in the black-and-white flash stimulation of the conventional method (Figure 5). Therefore, it is clear that using the visual stimuli of the conventional method causes fatigue. For all the hues and hue differences of the proposed method, there was some variation in the CFF change, but the change in fatigue was less than that of the conventional method. The mean change in fatigue was almost always between \(-1\) and +1 (Figures 8, 10 and 12). The only visual stimuli that were above this range were R45 and G5. The results for these two signals were also closer to 1 than to 2. From these results, we can confirm that the proposed method reduces the accumulation of fatigue compared to the conventional method. From the results of the subjective questionnaire for each hue of the proposed method, it was confirmed that the eye strain of the proposed method was lower than that of the conventional method for red visual stimuli with a hue difference of 20\({}^{\circ}\) or less from R5 to R20. For blurred vision and irritated eyes, the eye strain was lower than that of the conventional method for all hue differences from R5 to R45. The average flicker rating for the lowest hue difference was approximately 2. However, we can confirm that the red visual stimuli were slightly more accurate than the conventional method. For green visual stimuli, eye strain, blurred vision and irritated eye were lower than the conventional method for all hue differences from G5 to G45. From G35 to G45, the flicker rating was the same as that of the conventional method, and from G5 to G15, the flicker rating was less than 2, and the discrimination rate was significantly reduced. For blue visual stimuli, eye strain was similar to the conventional method for B30 and B40, and any other hue difference was found to be reduced. In addition, blurred vision and irritated eyes were lower for all hue differences from B5 to B45 than with the conventional method. For flicker rating, it was the same as the conventional method for B15 to B45, and the discrimination rate was slightly lower than the conventional method for all hue differences.