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R. Wilke, U. Greppmaier, A. Harscher, H. Benav, E. Zrenner; Factors Affecting Perceptual Thersholds of Subretinal Electric Stimulation in Blind Volunteers. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2026.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate factors affecting voltage (VT) and charge (QT) thresholds for perception of as part of the Tübingen Subretinal Implant Project.
11 blind volunteers received a subretinal implant consisting of an active light sensitive chip comprising 1500 pixels and an additional, hard-wired 4x4 electrode array, for light-independent experiments. Using this array (TiN electrodes, (50-100µm)2) perceptual thresholds were determined using a staircase procedure. VT for single or multiple electrodes was recorded several days post-implantation while electrodes were driven with constant voltage pulses of variable duration, pulse forms, and repetition frequencies. Respective current was measured, and QT for the first pulse phase calculated.
Overall median VT was 1.50V (0%, 90% quantiles: 0.45V, 2.0V; N=1207), respective median QT per electrode 31.7nC (0%, 90% quantiles: 2.1nC, 91.8nC; N=514). Significant differences in mean VT and QT were found between implantees, and all further analyses were performed on a single person level. Factors significantly contributing to variance in intra-individual data were the individual electrode, number of simultaneously activated electrodes, number and frequency of repetitive pulses, duration of single pulses, time after implantation, and pulsform. A multivariate non-linear model was fit to describe these effects (Figure). Higher repetition frequencies lowered VT, but not QT, longer pulse duration decreased VT, but increased QT. The effect of pulse forms (cathodic or anodic first) was found to be a function of a given electrode rather than impacting thresholds in one particular way. Biphasic pulses tended to decrease VT (but not QT).
Various factors affect perceptual thresholds in an interdependent manner. In this study 9 factors were identified and described with this model. Others (ambient light variation, attention) affect performance as well. Including them in the model is still a challenging task in order to render precise pattern of percepts and their variation in blind subjects.
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