Abstract:The proximity of stimulus electrodes to neural tissue in fluid-filled spaces can be estimated from access resistance changes in the stimulus pulse waveform. Because many prosthetic devices allow back telemetry communication of the stimulus electrode waveform, it is possible these series resistance increases observed with retinal proximity could be used as a metric of stimulus electrode placement.
“…It is possible that these electrodes were the ones that were closest to the retinal surface, since the surgical tack used to attach the implant to the retina was located next to the first row of electrodes. However, we did not have access to optical coherence tomography (OCT) or impedance data, which would have allowed us to estimate electrode-retina distance 35,36 (see Discussion).…”
Section: Resultsmentioning
confidence: 99%
“…Furthermore, low electrode-retina distances could explain why Subject 3 reported predominantly thin and elongated phosphenes (small errors in the tactile target control task argue against drawing bias; see Figures S2–S3). Unfortunately, we did not have access to OCT images or impedance measurements for our subjects, which would have allowed us to infer electrode-retina distances for each electrode 35,36 . Future studies could use such data to constrain the values of ρ and λ .Figure 8Simulated phosphenes as a function of electrode-retina distance, z .…”
Degenerative retinal diseases such as retinitis pigmentosa and macular degeneration cause irreversible vision loss in more than 10 million people worldwide. Retinal prostheses, now implanted in over 250 patients worldwide, electrically stimulate surviving cells in order to evoke neuronal responses that are interpreted by the brain as visual percepts (‘phosphenes’). However, instead of seeing focal spots of light, current implant users perceive highly distorted phosphenes that vary in shape both across subjects and electrodes. We characterized these distortions by asking users of the Argus retinal prosthesis system (Second Sight Medical Products Inc.) to draw electrically elicited percepts on a touchscreen. Using ophthalmic fundus imaging and computational modeling, we show that elicited percepts can be accurately predicted by the topographic organization of optic nerve fiber bundles in each subject’s retina, successfully replicating visual percepts ranging from ‘blobs’ to oriented ‘streaks’ and ‘wedges’ depending on the retinal location of the stimulating electrode. This provides the first evidence that activation of passing axon fibers accounts for the rich repertoire of phosphene shape commonly reported in psychophysical experiments, which can severely distort the quality of the generated visual experience. Overall our findings argue for more detailed modeling of biological detail across neural engineering applications.
“…It is possible that these electrodes were the ones that were closest to the retinal surface, since the surgical tack used to attach the implant to the retina was located next to the first row of electrodes. However, we did not have access to optical coherence tomography (OCT) or impedance data, which would have allowed us to estimate electrode-retina distance 35,36 (see Discussion).…”
Section: Resultsmentioning
confidence: 99%
“…Furthermore, low electrode-retina distances could explain why Subject 3 reported predominantly thin and elongated phosphenes (small errors in the tactile target control task argue against drawing bias; see Figures S2–S3). Unfortunately, we did not have access to OCT images or impedance measurements for our subjects, which would have allowed us to infer electrode-retina distances for each electrode 35,36 . Future studies could use such data to constrain the values of ρ and λ .Figure 8Simulated phosphenes as a function of electrode-retina distance, z .…”
Degenerative retinal diseases such as retinitis pigmentosa and macular degeneration cause irreversible vision loss in more than 10 million people worldwide. Retinal prostheses, now implanted in over 250 patients worldwide, electrically stimulate surviving cells in order to evoke neuronal responses that are interpreted by the brain as visual percepts (‘phosphenes’). However, instead of seeing focal spots of light, current implant users perceive highly distorted phosphenes that vary in shape both across subjects and electrodes. We characterized these distortions by asking users of the Argus retinal prosthesis system (Second Sight Medical Products Inc.) to draw electrically elicited percepts on a touchscreen. Using ophthalmic fundus imaging and computational modeling, we show that elicited percepts can be accurately predicted by the topographic organization of optic nerve fiber bundles in each subject’s retina, successfully replicating visual percepts ranging from ‘blobs’ to oriented ‘streaks’ and ‘wedges’ depending on the retinal location of the stimulating electrode. This provides the first evidence that activation of passing axon fibers accounts for the rich repertoire of phosphene shape commonly reported in psychophysical experiments, which can severely distort the quality of the generated visual experience. Overall our findings argue for more detailed modeling of biological detail across neural engineering applications.
“…However no information is available beyond 500 µm. Majdi reports a similar rise in impedance when approaching the retina with a 1.6mm outer diameter (OD) insulated probe over a range of 1mm [13]. Majdi explains this impedance rise by a decreasing electrically excited neuronal surface, hence causing an increased access resistance of the retina.…”
Section: B Proximity Sensing For Eye Surgerymentioning
Retinal Vein Occlusion (RVO) is a widespread eye vascular disease leading to vision loss because of clots obstructing a retinal vessel. Recent research investigated retinal vein cannulation, a promising treatment in which a microneedle injects a thrombolytic agent inside the clotted vessel. During this surgical procedure, the surgeon only relies on a microscope, looking through the patient eye's lens. Such visual feedback gives poor depth perception, with a risk to inject the agent underneath the retina. Such situation endangers the patient's eyesight and is to be avoided. This paper explores bioimpedance as a means to estimate the proximity between an insulated electrode and a retinal vessel. Experiments on 5 exvivo pig eyes showed that the impedance phase decreases as the electrode tip approaches the retinal vessel. Specific patterns in the impedance magnitude were detected. A dedicated algorithm showed 98 % sensitivity and 100 % specificity at a distance of 775µm±275µm away from the retinal vessels. Knowledge of the proximity can help the surgeon prevent damaging fragile retinal structures during retinal surgeries such as cannulation and scar tissue peeling. A minimally-invasive measurement principle of this technique is demonstrated on an ex-vivo pig head to show the feasibility of such technique in more realistic conditions.
“…Electrode-retina distance is correlated with perceptual thresholds and thus visual perception [16][17][18][19]. Though the correlation between electrode-retina distance and electrical thresholds of stimulation has been shown across numerous animal models including mammals [20][21][22] and rodents [23][24][25], detailed studies of a range of electrode-retina proximity and its impact on cortical activation are limited.…”
Section: Introductionmentioning
confidence: 99%
“…Optical coherence tomography (OCT) is a non-invasive imaging technique that has been widely used to estimate the proximity between the retina and epiretinal implants [26][27][28][29][30] in clinical settings. However, it is not easy to monitor insertion depth with OCT during stimulation [19,21,31]. In vivo impedance spectroscopy has been shown to be a robust monitoring tool for characterizing retinal tissue [32,33] and assessing performance of electrode array [34][35][36].…”
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