6.2. Materials and Methods 6 95 Figure 6.2: The phosphene simulation that was used in Experiment 1. A) Simultaneous activation of all 700 possible phosphenes centered at the point of fixation. B) The virtual hallway environment that was used in the practice session of Experiment 1. The green rectangle indicates the region of interest that would be used for gaze-contingent image processing, if the participant looks at the fixation point indicated in red. C) The resulting phosphene simulation after processing the region of interest in panel B. Note that only the phosphenes are activated that are located on the surface boundaries. Table 6.1: Average age and height (± standard deviation) of the study participants. Exp. 1 Exp. 2 n 23 19 Age 24.3 (±1.9) 24.1 (± 2.9) Height 173.4 (± 8.0) 171.4 (± 14.3) 6.2.2. Materials For the VR simulation, we used the HTC VIVE Eye Pro (HTC Corporation, Taiwan) head mounted display (HMD), which has an inbuilt eye-tracker manufactured by Tobii eye tracking systems (Tobii AB, Sweden). Calibration procedures were performed using the software provided by the manufacturer. The same software was used for obtaining the gaze directions. The head position was tracked using the external cameras (base stations) provided with the VR system. As no body positions were measured except for the head position, we simulated the subject in the virtual environment as a vertical cylinder with a radius of 0.225 meters, centered around the head position. These virtual body dimensions were kept equal across all participants. The HMD was connected to a laptop (Dell Precision 7550 with NVIDIA Quadro RTX 4000 GPU) by wire. All wires were suspended from the ceiling to enable free movement. 6.2.3. Phosphene Simulation For the VR simulation of cortical prosthetic vision, we used the Unity game engine (Unity Technologies, CA, USA) with different virtual 3D environments. For the real time integration of sensor data (subject pose, and gaze direction), we used the SDK (SRanipal) provided by the manufacturer. The simulation of cortical prosthetic vision was based on(van der Grinten et al., 2024), adapted for Unity using the Cg shader programming language for graphics processing. To restrict the experimental complexity, we did not include temporal dynamics in the phosphene simulation, and we assumed full control over the phosphene brightness (with a high dynamic range). For the phosphene count and field of view we (arbitrarily) chose experimental parameters that reflect the quality of a hypothesized future cortical implant. In the first experiment, we used 700 possible phosphene locations (i.e., simulating 700 electrodes), covering a field of view of 35 degrees.
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