2 14 2. Real-world indoor mobility with simulated prosthetic vision Figure 2.3: Visualization of the image processing steps. A) Input image B) Blurred image, using Gaussian smooting C) Edge mask, produced using the Canny algorithm D) Simulated phosphene vision, based on Canny edge mask E) Surface normals prediction by the SharpNet deep learning model F) Surface boundary prediction by by the SharpNet deep learning model G) Surface boundary mask produced using the SharpNet predictions H) Simulated phosphene vision, based on surface boundary mask. arc (sigma: 2.0 pixels) on a rectangular grid in the center of the VR display (480 × 480 pixels; roughly 35 degrees of visual arc). The visual field of our phosphene simulation is kept constant throughout the experiment, as well as the phosphene sizes. The number of phosphenes, however, is varied across study conditions, which means, by consequence, that the phosphene density is also different across study conditions. Note that wherever we refer to the effects of the phosphene resolution, this should be interpreted as the combined effect of the number of phosphenes and the phosphene density. Phosphenes could only take binary values (‘on’ or ‘off’), as at this time, cortical visual prostheses do not allow for systematic control over phosphene brightness (Najarpour Foroushani et al., 2018; Troyk et al., 2003). To mimic biological irregularities in phosphene mapping, distortion was added to the grid locations and a minor (temporally constant) variation was applied to the brightness of individual phosphenes. 2.2.5. Experimental procedure The experiment was partitioned into three sessions, starting with a training session (ca. 20 minutes) containing practice trials with the full experimental setup, to allow the participants to get acquainted with the simulated phosphene vision and the experimental task. The remaining two sessions started with two control trials, in which normal vision was simulated by directly displaying the camera input on the VR-device, followed by 8 different phosphene conditions. The total duration of the experiment was 2.5 – 3 hours. The study conditions were designed to facilitate three types of comparisons, which correspond to our study aims: i) To obtain a general measure of restorability of mobility performance and an indication of the required number of implanted electrodes, we compared the mobility performance with SPV at six different phosphene resolutions to the performance in a control condition with normal camera vision (seeFigure2.4). ii) To examine the theoretically
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