Identification of Characters and Localization of Images Using Direct Multiple-Electrode Stimulation With a Suprachoroidal Retinal Prosthesis.

Purpose: Retinal prostheses provide vision to blind patients by eliciting phosphenes through electrical stimulation. This study explored whether character identification and image localization could be achieved through direct multiple-electrode stimulation with a suprachoroidal retinal prosthesis. Methods: Two of three retinitis pigmentosa patients implanted with a suprachoroidal electrode array were tested on three psychophysical tasks. Electrode patterns were stimulated to elicit perception of simple characters, following which percept localization was tested using either static or dynamic images. Eye tracking was used to assess the association between accuracy and eye movements. Results: In the character identification task, accuracy ranged from 2.7% to 93.3%, depending on the patient and character. In the static image localization task, accuracy decreased from near perfect to <20% with decreasing contrast (patient 1). Patient 2 scored up to 70% at 100% contrast. In the dynamic image localization task, patient 1 recognized the trajectory of the image up to speeds of 64 deg/s, whereas patient 2 scored just above chance. The degree of eye movement in both patients was related to accuracy and, to some extent, stimulus direction. Conclusions: The ability to identify characters and localize percepts demonstrates the capacity of the suprachoroidal device to provide meaningful information to blind patients. The variation in scores across all tasks highlights the importance of using spatial cues from phosphenes, which becomes more difficult at low contrast. The use of spatial information from multiple electrodes and eye-movement compensation is expected to improve performance outcomes during real-world prosthesis use in a camera-based system. (ClinicalTrials.gov number, NCT01603576.).

Determining the Contribution of Retinotopic Discrimination to Localization Performance With a Suprachoroidal Retinal Prosthesis.

Purpose: With a retinal prosthesis connected to a head-mounted camera, subjects can perform low vision tasks using a combination of electrode discrimination and head-directed localization. The objective of the present study was to investigate the contribution of retinotopic electrode discrimination (perception corresponding to the arrangement of the implanted electrodes with respect to their position beneath the retina) to visual performance for three recipients of a 24-channel suprachoroidal retinal implant. Proficiency in retinotopic discrimination may allow good performance with smaller head movements, and identification of this ability would be useful for targeted rehabilitation. Methods: Three participants with retinitis pigmentosa performed localization and grating acuity assessments using a suprachoroidal retinal prosthesis. We compared retinotopic and nonretinotopic electrode mapping and hypothesized that participants with measurable acuity in a normal retinotopic condition would be negatively impacted by the nonretinotopic condition. We also expected that participants without measurable acuity would preferentially use head movement over retinotopic information. Results: Only one participant was able to complete the grating acuity task. In the localization task, this participant exhibited significantly greater head movements and significantly lower localization scores when using the nonretinotopic electrode mapping. There was no significant difference in localization performance or head movement for the remaining two subjects when comparing retinotopic to nonretinotopic electrode mapping. Conclusions: Successful discrimination of retinotopic information is possible with a suprachoroidal retinal prosthesis. Head movement behavior during a localization task can be modified using a nonretinotopic mapping. Behavioral comparisons using retinotopic and nonretinotopic electrode mapping may be able to highlight deficiencies in retinotopic discrimination, with a view to address these deficiencies in a rehabilitation environment. (ClinicalTrials.gov number, NCT01603576).