## 5 Discussion
### fNIRS Results  
Previous research has shown that increases in \(\Delta\)HbO are often associated with decreases in \(\Delta\)HbR [16; 51]. This relationship is explained by the link between increased blood flow, higher \(\Delta\)HbO levels, and lower \(\Delta\)HbR levels [52]. Notably, \(\Delta\)HbO is typically more sensitive than \(\Delta\)HbR during neurovascular coupling [16; 51; 53]. These findings align with the results of this study, which found significant differences between conditions in the \(\Delta\)HbO metric but not in the \(\Delta\)HbR metric.  
[PERSON] et al. [54] explored the correlation between the intensity of acoustic stimuli and fNIRS responses, revealing that high-intensity stimulation elicits robust near-infrared spectral responses. However, they also suggested that brain activation in response to varying stimulus intensities is more influenced by individual perceptions of loudness rather than the physical properties of the stimulus. Our study yielded similar findings. Specifically, we observed a significant interaction effect between sound and brain area. High sound induced a notably higher \(\Delta\)HbO in both the left and right DLPFC compared to no sound.  
Figure 8: Cerebral activity of users in different experimental conditions: (**a**) \(\Delta\)HbO, (**b**) \(\Delta\)HbR, * depicts \(p\) \(<\) 0.05. HbO = oxygenated hemoglobin; HbR = deoxygenated hemoglobin.  
Previous studies have suggested that elevated cognitive workload can result in increased \(\Delta\)HbO in the DLPFC [55; 56]. One possible explanation is that users expend more cognitive resources when perceiving pictures accompanied by sound stimuli, particularly as the loudness of the sound increases. Additionally, variations in \(\Delta\)HbO could arise from different emotional responses elicited by the stimuli. Previous studies have shown that both positive and negative emotional states can trigger activation in the PFC [57; 58]. The introduction of appropriate incongruity in a product enhances enjoyment and favorability [50]. Hence, it is plausible that the high-sounding hair dryer, compared to the soundless one, evoked a positive emotional experience, leading to variations in \(\Delta\)HbO in the DLPFC, a hypothesis supported by the perceived quality score of the product.  
The study revealed that \(\Delta\)HbO in users' left DLPFC was higher than that in the right DLPFC when the product emitted low sound. However, no significant differences were observed between the left and right DLPFC when the product emitted high sound or no sound. Research on emotion processing suggests that activation in the left DLPFC is more pronounced during positive affective states, whereas activation in the right DLPFC is more prominent during negative affective states [59; 60; 61]. When users were exposed to soundless pictures or high sound stimuli, both hemispheres processed the information similarly. However, when confronted with low sound stimuli, certain positive feelings were elicited in users, resulting in increased activity in the left brain regions. While our findings reveal significant correlations between fNIRS signals and perceived product quality, these results do not imply causality. The observed neural activations may reflect processes associated with, but not necessarily causing, changes in perceived quality. Future studies employing causal inference methods or experimental manipulations would be needed to establish directional relationships.