5 60 5. Towards biologically plausible phosphene simulation artificial vision system that presents visual information from the surroundings to a blind subject using a penetrating multi-electrode array in the visual cortex (Fernández et al., 2021). Taken together, the previous literature provides strong evidence for the clinical potential of cortical prostheses for the blind. Perceptual reports on cortical prosthetic vision In our simulator, we integrate empirical findings and quantitative models from existing literature on electrical stimulation in the visual cortex. Stimulation in V1 with intracortical electrodes is estimated to activate tens to thousands of neurons (Histed et al., 2009), resulting in the perception of often ‘featureless’ white dots of light with a circular shape (Bak et al., 1990; Brindley & Lewin, 1968; Fernández et al., 2021; Niketeghad et al., 2020; Schmidt et al., 1996). Due to the cortical magnification (the foveal information is represented by a relatively large surface area in the visual cortex as a result of variation of retinal RF size) the size of the phosphene increases with its eccentricity (Bosking et al., 2017a; Winawer & Parvizi, 2016). Furthermore, phosphene size, stimulation threshold (defined as the minimum current to produce a visible phosphene 50% of the time) and brightness depend on stimulation parameters such as the pulse width, train length, amplitude and frequency of stimulation (Bak et al., 1990; Bosking et al., 2017a; Fernández et al., 2021; Niketeghad et al., 2020; Schmidt et al., 1996; Winawer & Parvizi, 2016). To account for these effects, we integrated and adapted previously proposed quantitative models that estimate the charge-dependent activation level of cortical tissue (Bosking et al., 2017a; Bruce et al., 1999; Geddes, 2004; Kim et al., 2017; Tehovnik& Slocum, 2007; Winawer & Parvizi, 2016). Furthermore, our simulator includes a model of the temporal dynamics, observed by (Schmidt et al., 1996), accounting for responseattenuation after prolonged or repeated stimulation, as well as the delayed ‘offset’ of phosphene perception. Simulated prosthetic vision A wide range of previous studies has employed SPV with sighted subjects to non-invasively investigate the usefulness of prosthetic vision in everyday tasks, such as mobility (Chaet al., 1992b; Dagnelie et al., 2007; de Ruyter van Steveninck et al., 2022b; Han et al., 2021; Vergnieux et al., 2017), hand-eye coordination (Srivastava et al., 2009), reading (Cha et al., 1992a; Sommerhalder et al., 2004) or face recognition (Bollen et al., 2019a; Thompson et al., 2003). Several studies have examined the effect of the number of phosphenes, spacing between phosphenes and the visual angle over which the phosphenes are spread (e.g., de Ruyter van Steveninck et al., 2022b; Parikh et al., 2013; Sanchez-Garcia et al., 2022; Srivastava et al., 2009; Thorn et al., 2020). The results of these studies vary widely, which could be explained by the difference in the implemented tasks, or, more importantly, by the differences in the simulation of phosphene vision. The aforementioned studies used varying degrees of simplification of phosphene vision in their simulations. For instance, many included equally-sized phosphenes that were uniformly distributed over the visual field (informally referred to as the ‘scoreboard model’). Furthermore, most studies assumed either full control over phosphene brightness or used binary levels of brightness (e.g. ‘on’ / ‘off’), but did not provide a description of the associated electrical stimulation parameters. Several studies have explicitly made steps towards more realistic phosphene simulations, by taking into account cortical magnification or
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