## 2 Materials and Methods
### Experiments
#### 2.3.3 Experimental Procedure  
Each subject picked up two cubic and cylindrical items and placed them on target positions on the tabletop in the first pick-and-place manipulation task. By requiring both translation and rotation for placing the objects, the process assesses the translational and rotational functionality of the three HRI schemes. The second task was to grab one LEGO subassembly from a predefined spot and stack it onto a fixed LEGO base on the table. As an assembly task with increased complexity, this task required the subject to rearrange the in-hand object's orientation by rotating the red LEGO cuboid subassembly and aligning the two components before mating, which restricted the orientation of the end-effector in comparison to placing the cubical and cylindrical items. All participants were given a full information sheet and briefing and were informed of the right to stop and/or withdraw at any time, including withdrawing their data. All data and usability survey data were anonymized. Thus, the approach met all best-practice standards and covered all informed consent requirements. The trial began with each subject filling out a pre-experiment questionnaire to record age and gender, and assess prior knowledge and experience with MR, robotics, and HRI. Following the pre-experiment questionnaire, participants were given health and safety guidelines and provided with details on the experimental tasks and objectives. Prior to using each HRI method, each subject was presented with a tutorial video demonstrating the user interaction module. The video-based introduction was utilized to equalize the training that subjects were offered in each HRI module. The three HRI modules included the interaction scheme of the SpaceMouse input device with 2D camera feeds, the SpaceMouse with the 3D point cloud interface of telemanipulation, and the MR-subspace-integrated 3D mapping of motion and visualization scheme. After indicating preparedness, participants ran through a training task of grasping a cylindrical object with the robotic hand-arm to get used to each control method of the teleoperation system [53]. The object grasping task was selected due to its simplicity and the preparation offered for the experimental tasks, which allows the users to get familiarized with the HRI methods. Each participant completed a training activity once to standardize the training process, and they were still regarded as unskilled operators. Thus, this activity mainly serves as part of the instructions, and its influence on the bias of the results is minimized. When the user expressed their readiness to conduct the experimental tasks, the cumulative training time for each HRI module was recorded to assess the learnability and efficiency of each HRI module. The locations of target objects and the robotic pose changed, respectively. To reduce order effects during the execution of the experimental tasks, the sequence of the HRI modules and the target position of the objects were randomly allocated, and pick-and-place and assembly manipulation task orders were also randomized, which minimizes the learning effects between tasks. The teleoperation task was suspended when the subject placed the object in the goal position and mated the components together in an assembly. To standardize the initial position, the experimenter returned the robot to an identical home configuration after each trial. Participants completed a post-trial questionnaire on each HRI module after completing the training and experimental tasks. Participants remained outside the robotic platform operating zone at all times to guarantee physical safety, and one experimenter closely observed all operations with an emergency stop. If a collision occurred on the objects or tabletop, the experimenter immediately activatedthe emergency stop button to halt all robot motion. The robot was then sent back to its original pose, demanding the subjects restart the task. In the MR subspace, virtual safety grids were also established to surround and remind the user.