## 2 Related Studies  
### Natural User Interfaces (NUIs)  
Natural User Interfaces (NUIs) have been designed to enhance user interactions by utilizing instinctive human actions like hand gestures, eye gaze, and voice commands, aiming to make the perception of the user experience smoother and more intuitive [13]. A NUI focuses on reducing the learning curve by aligning interactive behaviors with the user's physical and cognitive habits--for example, employing mid-air gestures to browse or manipulate digital contents [14]. Research on tactile and gesture-based systems has shown that physically interacting with digital elements improves users' spatial recall and lessens their cognitive strain, as users feel that the interaction is like a natural extension  
Figure 1: Images of participants (wearing a smart ring mockup) interacting with the UI components of a landing page interface by performing predefined gestures under two experimental conditions: a TV-screen-based interface (TV-type projector: (**A-1,A-2**)) and an XR-based spatial interface (**B-1,B-2**).  
of their body movements [15; 16]. Similarly, a NUI promotes interacting with digital contents through intuitive actions--for instance, swiping or waving a hand to dismiss contents rather than clicking a delete button [17]. This method reflects the concept of hands-on interaction, enabling users to feel as if they are physically engaging with and handling digital components [18]. Additionally, NUI-based systems typically respond to environmental cues, offering context-aware suggestions and adjusting in real time to users' interaction patterns [19].  
In terms of their real-world application, NUI's have been adopted across numerous technological sectors. Since the early 2010s, Microsoft has played a pioneering role in NUI development, especially through its Kinect series and accompanying SDKs for Xbox, which enabled users to interact with games using full-body gestures [20; 21]. Likewise, Apple's touchscreen interfaces on iPhones and iPads have set the industry standards for intuitive gesture interactions like finger-based pinching or dragging gestures [22]. Especially since 2020, major big IT companies have actively been advancing NUI technologies and integrating intuitive multimodal interaction methods like gesture recognition, eye tracking, and voice commands into their products and platforms. As previously mentioned, the Apple Vision Pro and the Meta Quest series have pioneered seamless interactions in mixed and virtual reality environments, harnessing sophisticated hand tracking, eye tracking, and voice control to enhance user immersion [3; 23]. In consumer smart home devices, Google Nest Hub utilized Soli radar technology to enable touch-free gesture interactions, demonstrating the integration of NUI's into daily life [24; 25]. Meanwhile, Amazon Alexa's voice-interaction-driven NUI has shown new usability in that the AI assistant applies contextual awareness and responds to user conversations with human-like responsiveness [26]. Considering these technological and UX trends, these examples from major tech companies illustrate how NUI-driven technologies and principles can be embedded into products to transform how we engage with digital interfaces.