## 2 Materials and Methods
### Experiments
#### 2.3.2 Experimental Setup  
A within-subjects experiment was developed to verify the null hypothesis by presenting novice participants with three HRI conditions (Baseline/2D SpaceMouse; MR direct control/MR SpaceMouse; and MR subspace) in a random order to guide the robotic arm-hand system and complete two spatial manipulation tasks: (1) pick-and-place and (2) assembly. A total of 24 participants took part in the user study, 13 males (54%) and 11 females (46%). Subjects were all from the University of Canterbury and ranged in age from 20 to 31. In general, participants were unfamiliar with robotic systems, HRI, or MR. In the pre-experiment questionnaire, subjects' experiences with VR and computer games were collected. Among the 24 participants, 13 had no previous experience with VR, 7 had used a VR headset once, and 4 were more frequent users. Most of them were had a lot of experience with computer games and they usually played once a week. None of the participants had experience with controlling a robot, and they were regarded as novice users of the teleoperated robotic system. The tasks for the user study were to guide the robotic arm-hand system to conduct pick-and-place and assembly telemanipulation processes through three different teleoperation intermediaries. Each participant completed the user study within 60 min, which includes pre-questionnaires, instructions, experiments, post-questionnaires and comments. The participants were informed of the right to stop and/or withdraw from the experiment at any time. The proposed MR-HRI mapping approach was compared to a SpaceMouse 6-DOF control input device sensor (3 Dconnexion, Munich, Germany) with 2D camera feeds, and the SpaceMouse with the 3D point cloud interface of telemanipulation. The 3D connexion SpaceMouse Compact is a 6-DOF control input component widely used for guiding robots, precise 3D navigation in Computer-aided design (CAD), or 3D analysis and review. The 6-DOF SpaceMouse was chosen as a baseline [49], because prior work indicated that, despite the complex operations, the SpaceMouse interface was highly effective for cartesian teleoperation of a robot end-effector [50; 51; 52]. The goal of the experiment is to test whether the isotonic-position control scheme with the MR subspace outperforms isometric-rate control using the SpaceMouse input with either 2D direct camera feeds or 3D visual feedback. The Baseline HRI is a typical teleoperation mode, providing participants with 2D visual feedback displaying the working view through the RGB camera in the Kinect V2 depth sensor. The user is presented with a 2D ego-centric view of the robot's working space on the monitor and unable to change perspective. The piloting metaphor was used for HRI with a stationary 6-DoF SpaceMouse. The user interaction included manipulating input from a SpaceMouse for TCP translation and rotation of the UR5 robot arm and the grasp and release of the robot hand. The MR direct control/MR SpaceMouse module can be regarded as an MR subspace without Velocity-Centric Motion Mapping (VCMM), and is a condensed version of the proposed MR-HRI system that enables the user to inspect the robotic arm-hand situation in the remote environment via a 3D point cloud in the MR subspace. The user can interact with the robotic system through an isometric-rate control scheme by using the SpaceMouse input with 3D visual feedback, but without perceived correspondence between the input device motion and the robot end-effector movement. The MR subspace experiment is a full MR-HRI system with VCMM using the tracked Vive motion controller in a grasping metaphor, and allows the user to use hand motion to directly guide the robotic arm-hand system through the isotonic-position control scheme,and view the robot's space through the point cloud generated within MR subspace, conducting a set of spatial manipulation tasks. The full MR-HRI system features the perceived movement correspondence between the input device (Vive controller) and the robot end-effector.