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

Abstract

Citation: Shivdasani MN, Sinclair NC, Gillespie LN, et al.; for the Bionic Vision Australia Consortium. Identification of characters and localization of images using direct multiple-electrode stimulation with a suprachoroidal retinal prosthesis. Invest Ophthalmol Vis Sci. 2017;58:3962-3974. DOI: 10.1167/iovs.16-21311 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.) Keywords: retinal prostheses, suprachoroidal, psychophysics, retinitis pigmentosa, electrical stimulation O ver the last decade, retinal prostheses have emerged as the only regulatory approved technology to provide artificial vision to patients with profound vision loss due to photoreceptor dystrophies such as retinitis pigmentosa. 1 These devices work by electrically stimulating surviving second-and thirdorder retinal neurons via an implanted array of electrodes to elicit the perception of light flashes termed phosphenes. Multiple electrodes can be stimulated to induce the perception of an image, usually captured by a video camera. While there have been over 20 groups worldwide trying to develop such a device, only three devices have achieved commercialization so far. The Argus II epiretinal (i.e., electrode array attached directly to the inner surface of the retina) prosthesis from Second Sight Medical Products, Inc. (Sy...

Gaze Compensation as a Technique for Improving Hand–Eye Coordination in Prosthetic Vision

“…We have previously found that suprachoroidal implant recipients made significant eye movements in response to stimuli during a static image localization task, despite being instructed not to. 16 A separate study in Argus II recipients found that camera-gaze misalignments occurred frequently during a visual search task, often due to the vestibulo-ocular reflexive movements that occur naturally during head scanning, and that patients rely on a series of complex head movements to properly localize objects in daily life. 8 Some have suggested that percept localization is so difficult that many patients simply use their devices as light detectors, ignoring any retinotopic information and instead relying solely on head and neck orientation.…”

Advances in retinal prosthesis systems

“…72,75 In a series of psychophysical tasks delivered by direct electrode stimulation, two of the subjects demonstrated better than chance character recognition and static object localization, while one was able to detect dynamic image trajectory. 76 At the time of explantation, electrical stimulation was still possible, although it was noted in all cases that a fibrous capsule had developed around the implant. 77…”

Oculomotor Responses to Dynamic Stimuli in a 44-Channel Suprachoroidal Retinal Prosthesis

“…9,10 Large electrodes (necessitated by safe charge density limits), current spread, and the incidental stimulation of axonal pathways lead to large and irregularly shaped phosphenes and limited spatial discriminability. [11][12][13] Additionally, non-selective stimulation that activates both "on" and "off" pathways indiscriminantly is likely to have unusual perceptual effects that are not well understood. 14 Novel electrode designs and advances in targeted stimulation strategies offer some promise for future visual prostheses.…”

“…9,13,19,20 In addition to spatial and form vision, visual prostheses should also ideally enable the perception of motion. Discrimination of direction of motion has previously been demonstrated in patients implanted with a 24-channel suprachoroidal retinal implant, 13 a subretinal implant, 9,20 and an epiretinal implant. 21 The subjective characteristics of the percepts experienced by participants during these tasks, as well as the particular perceptual cues used to identify motion, has received little attention.…”