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Factors affecting two-point discrimination in Argus II patients.

ABSTRACT: Two of the main obstacles to the development of epiretinal prosthesis technology are electrodes that require current amplitudes above safety limits to reliably elicit percepts, and a failure to consistently elicit pattern vision. Here, we explored the causes of high current amplitude thresholds and poor spatial resolution within the Argus II epiretinal implant. We measured current amplitude thresholds and two-point discrimination (the ability to determine whether one or two electrodes had been stimulated) in 3 blind participants implanted with Argus II devices. Our data and simulations show that axonal stimulation, lift and retinal damage all play a role in reducing performance in the Argus 2, by either limiting sensitivity and/or reducing spatial resolution. Understanding the relative role of these various factors will be critical for developing and surgically implanting devices that can successfully subserve pattern vision.

SUBMITTER: Yucel EI

PROVIDER: S-EPMC9448992 | biostudies-literature | 2022

REPOSITORIES: biostudies-literature

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Factors affecting two-point discrimination in Argus II patients.

Yücel Ezgi I EI Sadeghi Roksana R Kartha Arathy A Montezuma Sandra Rocio SR Dagnelie Gislin G Rokem Ariel A Boynton Geoffrey M GM Fine Ione I Beyeler Michael M

Frontiers in neuroscience 20220824

Two of the main obstacles to the development of epiretinal prosthesis technology are electrodes that require current amplitudes above safety limits to reliably elicit percepts, and a failure to consistently elicit pattern vision. Here, we explored the causes of high current amplitude thresholds and poor spatial resolution within the Argus II epiretinal implant. We measured current amplitude thresholds and two-point discrimination (the ability to determine whether one or two electrodes had been s ...[more]

PMID: 36090266

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Project description:PurposeThe Argus II epiretinal prosthesis has been developed to provide partial restoration of vision to subjects blinded from outer retinal degenerative disease. Participants were surgically implanted with the system in the United States and Europe in a single arm, prospective, multicenter clinical trial. The purpose of this investigation was to determine which factors affect electrical thresholds in order to inform surgical placement of the device.MethodsElectrode-retina and electrode-fovea distances were determined using SD-OCT and fundus photography, respectively. Perceptual threshold to electrical stimulation of electrodes was measured using custom developed software, in which current amplitude was varied until the threshold was found. Full field stimulus light threshold was measured using the Espion D-FST test. Relationships between electrical threshold and these three explanatory variables (electrode-retina distance, electrode-fovea distance, and monocular light threshold) were quantified using regression.ResultsRegression analysis showed a significant correlation between electrical threshold and electrode-retina distance (R2 = 0.50, P = 0.0002; n = 703 electrodes). 90.3% of electrodes in contact with the macula (n = 207) elicited percepts at charge densities less than 1 mC/cm2/phase. These threshold data also correlated well with ganglion cell density profile (P = 0.03). A weaker, but still significant, inverse correlation was found between light threshold and electrical threshold (R2 < 0.52, P = 0.01). Multivariate modeling indicated that electrode-retina distance and light threshold are highly predictive of electrode threshold (R2 = 0.87; P < 0.0005).ConclusionsTaken together, these results suggest that while light threshold should be used to inform patient selection, macular contact of the array is paramount.Translational relevanceReported Argus II clinical study results are in good agreement with prior in vitro and in vivo studies, and support the development of higher-density systems that employ smaller diameter electrodes. (clinicaltrials.gov identifier: NCT00407602).

Project description:PurposeRetinal implants use electrical stimulation to elicit perceived flashes of light ("phosphenes"). Single-electrode phosphene shape has been shown to vary systematically with stimulus parameters and the retinal location of the stimulating electrode, due to incidental activation of passing nerve fiber bundles. However, this knowledge has yet to be extended to paired-electrode stimulation.MethodsWe retrospectively analyzed 3548 phosphene drawings made by three blind participants implanted with an Argus II Retinal Prosthesis. Phosphene shape (characterized by area, perimeter, major and minor axis length) and number of perceived phosphenes were averaged across trials and correlated with the corresponding single-electrode parameters. In addition, the number of phosphenes was correlated with stimulus amplitude and neuroanatomical parameters: electrode-retina and electrode-fovea distance as well as the electrode-electrode distance to ("between-axon") and along axon bundles ("along-axon"). Statistical analyses were conducted using linear regression and partial correlation analysis.ResultsSimple regression revealed that each paired-electrode shape descriptor could be predicted by the sum of the two corresponding single-electrode shape descriptors (p < .001). Multiple regression revealed that paired-electrode phosphene shape was primarily predicted by stimulus amplitude and electrode-fovea distance (p < .05). Interestingly, the number of elicited phosphenes tended to increase with between-axon distance (p < .05), but not with along-axon distance, in two out of three participants.ConclusionsThe shape of phosphenes elicited by paired-electrode stimulation was well predicted by the shape of their corresponding single-electrode phosphenes, suggesting that two-point perception can be expressed as the linear summation of single-point perception. The notable impact of the between-axon distance on the perceived number of phosphenes provides further evidence in support of the axon map model for epiretinal stimulation. These findings contribute to the growing literature on phosphene perception and have important implications for the design of future retinal prostheses.

Project description:BackgroundLimited evidence informs whether pre-operative values of two-point discrimination (2PD) in patients with carpal tunnel syndrome predict response to surgery.Questions/purposesThe primary aim of this study was to determine the predictive value of pre-operative 2PD scores on outcomes following carpal tunnel release (CTR). In particular, we sought to evaluate whether a clinically relevant 2PD threshold exists that can predict symptomatic response after surgery.MethodsPatients who underwent CTR between 2014 and 2018 were retrospectively reviewed. Static 2PD scores in each digit, as well as Quick Disabilities of the Arm, Shoulder, and Hand (QuickDASH) and Levine-Katz scores, were collected from pre- and post-operative records. Pearson correlation coefficients assessed the relationship between pre-operative 2PD, early post-operative 2PD, and patient-reported outcome scores. Poor 2PD was defined as 2PD greater than 10 mm.ResultsEighty-nine hands in 73 patients with a mean follow-up of 1.8 years were analyzed. Mean pre- and post-operative 2PD was 7.2 mm and 6.4 mm, respectively, in the most affected digit when measurable. Twenty patients had poor 2PD scores pre-operatively and 14 post-operatively. There was a positive correlation between pre- and early post-operative 2PD scores but no correlation between pre-operative 2PD score and final post-operative functional scores. Only 30% of patients with poor pre-operative 2PD scores demonstrated improvement, compared with 69% of patients with measurable 2PD.ConclusionWe found that greater pre-operative 2PD scores predicted greater early post-operative 2PD scores but did not lead to worse functional outcome scores post-operatively. Patients with poor pre-operative 2PD scores were likely to demonstrate improvement in functional outcomes scores, while having less reliable improvement in 2PD after CTR. Patients with poor 2PD should be counseled that improvement of tactile perception is less predictable.

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Factors affecting two-point discrimination in Argus II patients.

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Factors affecting two-point discrimination in Argus II patients.

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