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M.J. McMahon, I. Fine, S.H. Greenwald, A. Horsager, G. Palmer, B.V. Mech, R.J. Greenberg, M.S. Humayun; Electrode Impedance as a Predictor of Electrode–Retina Proximity and Perceptual Threshold in a Retinal Prosthesis . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3184.
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Our goal was to determine if measurements of electrode impedance could be used to estimate both electrode–retina proximity and perceptual detection thresholds in humans implanted with an epiretinal prosthesis.
The FDA and USC–IRB approved the study protocol. Informed consent was obtained and 6 subjects were implanted with a Second Sight Medical Products16–electrode retinal prosthesis. In each session, electrode impedance was measured via back telemetry from the implant and the detection threshold was measured by varying stimulus current until the subject was able to see the visual percept on 50% trials, corrected for false alarms. In five subjects, cross sectional images through the array and underlying retina were obtained using optical coherence tomography. The scans were analyzed to determine the distance between the prosthesis electrodes and the epiretinal surface.
An inverse square relationship was found between impedance and electrode–retina distance, an inverse relationship was found between the impedance and threshold, and a linear relationship was observed between electrode–retina distance and stimulation threshold. We calculated the linear correlation of impedance vs. (1/(electrode proximity)²) (ρ=0.66, 148 measurements, 5 subjects), impedance vs. (1/threshold) (ρ=0.78, 493 measurements, 6 subjects) and threshold vs. (electrode proximity)² (ρ=0.56, 114 measurements, 5 subjects). All correlations were statistically significant (p<0.001).
The stimulus intensity required to elicit a percept is proportional to the square of the distance of the electrode from the retinal surface, consistent with the prediction that the electric field decreases with the square of the distance from the stimulating electrode. The correlations that we have observed suggest that impedance may be a useful tool to predict both the proximity of the prosthesis to the retina and stimulation thresholds in future subjects.
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