# The Sense of Agency in Human-Machine Interaction Systems
## 6 The Proposed Route for the Future Development of HMI Systems  
Currently, most research on the operator's SoA in the HMI system focuses on shared control and transparency. To enhance the human SoA, machines are adjusted by humans. However, with the recent development of HMC technology, a key research question for the HMI system is, \"how can machines be enabled to accurately perceive human intentions and emotions?\" Machines achieve this by using sensors and data processing techniques to understand human intentions and emotions. Nevertheless, due to the machine's limited understanding capacity, operators can sense that they are communicating with a machine, which harms their SoA. Consequently, the key challenge in current HMC technology is achieving unconscious communication between operators and machines, minimizing negative impacts on the operator's SoA. Furthermore, if the SoA serves as a bond for HMC, it enables unconscious communication, allowing machines to adjust themselves according to the human SoA without causing any disruption to humans; therefore, a stronger SoA can be achieved. [PERSON] et al. [154] proposed a symbiotic human-machine system, and they collaborated on a multivariate perspective of symbiosis as the highest form of interaction in physically coupled HMSs, characterized by the oneness of the human and the machine. However, we believe that the ultimate goal of HMSs should be to achieve the spiritual coupling between the human and the machine, where the machine becomes the spiritual companion of the human. Summarizing the above ideas, we suggest a seven-level development route. In this development route, the maturity of the HMI system is divided into the following three stages, consisting of seven levels, as shown in Figure 6:  
* The first stage is machine-centric. This stage aims to increase the operator's SoA by adjusting the machine. To maintain the balance between the SoA and operation performance, the machine's weight in shared control is adaptively adjusted according to the difficulties of tasks (level 1). Furthermore, when the transparency of the HMI system is increased, the operator can understand the intention of the machine and, therefore, trust and accept the machine (level 2). Most of the existing HMI systems are in this stage. * The second stage is human-centric, which aims to enhance the SoA by HMC. In this stage, the machine begins to gradually act like a human. In the third level, \"Exist\" in Figure 6, the human operator gets used to and accepts the existence of the machine and, therefore, does not pay attention to the machine deliberately. Even if the human operator and machine communicate preliminarily through control instructions and feedback information in level 3, the existence of the machine does not affect the SoA. At the fourth level, \"Communicate\", the machine begins to have the ability to understand human intention preliminarily via motions, body language, linguistic information, and so on, and then gives feedback accordingly. At this level, the human operator also begins to slowly regard the machine as their collaborator. This level marks a change in the way the human and the machine interact. When it develops to the fifth level, \"Unconscious\", the machine fully understands human intention and emotions, with a certain logical judgment ability. So, it can communicate freely with the human operator like a person, and the human operator already regards the machine as their collaborator from the previous level. At level 5, when performing HMI tasks, the human operator is not aware that they are collaborating with a robot. * The third stage is to enhance the SoA by making machines \"think\". At the sixth level, \"Symbiosis\", human transparency begins to be considered. The machine begins to have \"thoughts\" and understand human emotions, intentions, and feelings, like a person. Then, the machine will adjust itself to adapt to humans based on the obtained human operator's emotions, intentions, and feelings. Therefore, mental human-machine integration is achieved.