6 94 6. Gaze-contingent processing improves mobility performance screen with users of the Argus II retinal implant (Second Sight Medical Products, CA, USA) and found that gaze-contingent image processing resulted in higher precision and less head movements. Another study byParaskevoudi and Pezaris (2021) used a simulated prosthetic vision (SPV) paradigm with sighted subjects to non-invasively evaluate the functional quality of the artificial percept in a reading task. The authors found that gazecontingent processing drastically improved the reading speed and accuracy compared to the control condition with head-steered vision. These results are encouraging, but since these studies were performed in relatively controlled settings (for instance, participants sit behind a computer screen to identify target objects or letters), it remains unclear how these results translate to more natural settings and more complex activities. Given the opportunities for more immersive simulations to test more intricate tasks such as mobility and orientation (for example, seede Ruyter van Steveninck et al., 2022b; Rasla and Beyeler, 2022), it is relevant to explore the benefits of gaze-contingent processing for these tasks. The current study aims to investigate gaze-contingent image processing in complex mobility and orientation tasks, using a mobile eye tracker in a virtual reality (VR) simulation. Specifically, we compare gaze-contingent image processing with a gaze-locked simulation of prosthetic vision, and a second control condition with gaze-ignored headsteered vision. We hypothesize that, owing to the availability of gaze-steered visual scanning and reestablished spatial updating, gaze-contingent phosphene vision yields higher mobility and orientation performance compared to gaze-locked and gaze-ignored phosphene vision. Note that the current study focuses on cortical prosthetic vision in particular, which, like natural vision, is characterized by an inhomogeneous resolution of information across the visual field, due to cortical magnification. Ultimately the goal of the current work is to provide further insights in the functional improvements that can be gained with an eye tracking system for gaze-contingent processing in head-steered cortical visual prostheses. 6.2. Materials and Methods We performed two separate SPV experiments in indoor VR environments. A general description of the materials and methods used in our study and the specific details of Experiment 1 (obstacle avoidance) and Experiment 2 (scene recognition and visual search) are explained in the following sections. 6.2.1. Participants In total, we recruited 43 healthy adult participants (23 for Experiment 1, and 20 for Experiment 2), with normal or corrected to normal vision, no limiting mobility impairments and no relevant prior history of cybersickness / motion sickness. None of the participants were familiar with the experimental tasks. One of the participants in Experiment 2 experienced VR-induced nausea during the practice session, and had to be excluded from the experiment, leaving a total of 19 subjects that completed Experiment 2. A descriptive summary of the study populations in both experiments can be found inTable6.1. All participants provided written informed consent to participate in this study. This study was performed in accordance with the Declaration of Helsinki and the Nurenberg Code and was approved by the local ethical committee (ECSW, Radboud University).
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