To evaluate the efficacy of a suprachoroidal retinal prosthesis, a highly configurable external neurostimulator is required. In order to meet functional and safety specifications, it was necessary to develop a custom device. A system is presented which can deliver charge-balanced, constant-current biphasic pulses, with widely adjustable parameters, to arbitrary configurations of output electrodes. This system is shown to be effective in eliciting visual percepts in a patient with approximately 20 years of light perception vision only due to retinitis pigmentosa, using an electrode array implanted in the suprachoroidal space of the eye. The flexibility of the system also makes it suitable for use in a number of other emerging clinical neurostimulation applications, including epileptic seizure suppression and closed-loop deep brain stimulation. Clinical trial registration number NCT01603576 (www.clinicaltrials.gov).
The rapid extraction of variations in evoked potentials (EPs) is of great clinical importance. Parametric modeling using autoregression with an exogenous input (ARX) and robust evoked potential estimator (REPE) are commonly used methods for extracting EPs over the conventional moving time average. However, a systematic study of the efficacy of these methods, using known synthetic EPs, has not been performed. Therefore, the current study evaluates the restrictions of these methods in the presence of known and systematic variations in EP component latency and signal-to-noise ratios (SNR). In the context of rapid extraction, variations of wave V of the auditory brainstem in response to stimulus intensity were considered. While the REPE methods were better able to recover the simulated model of the EP, morphology and the latency of the ARX-estimated EPs was a closer match to the actual EP than than that of the REPE-estimated EPs. We, therefore, concluded that ARX rapid extraction would perform better with regards to the rapid tracking of latency variations. By tracking simulated and empirically induced latency variations, we conclude that rapid EP extraction using ARX modeling is only capable of extracting latency variations of an EP in relatively high SNRs and, therefore, should be used with caution in low-noise environments. In particular, it is not a suitable method for the rapid extraction of early EP components such as the auditory brainstem potential.
Purpose.: The suprachoroidal location for a retinal prosthesis provides advantages over other locations in terms of a simplified surgical procedure and a potentially more stable electrode–neural interface. The aim of this study was to assess the factors affecting perceptual thresholds, and to optimize stimulus parameters to achieve the lowest thresholds in patients implanted with a suprachoroidal retinal prosthesis. Methods.: Three patients with profound vision loss from retinitis pigmentosa were implanted with a suprachoroidal array. Perceptual thresholds measured on individual electrodes were analyzed as a function of stimulus (return configuration, pulse polarity, pulse width, interphase gap, and rate), electrode (area and number of ganged electrodes), and clinical (retinal thickness and electrode–retina distance) parameters. Results.: A total of 92.8% of 904 measurements made up to 680 days post implantation yielded thresholds (range, 44–436 nanocoulombs [nC]) below the safe charge limit. Thresholds were found to vary between individuals and to depend significantly on electrode–retina distance, negligibly on retinal thickness, and not on electrode area or the number of ganged electrodes. Lowest thresholds were achieved when using a monopolar return, anodic-first polarity, short pulse widths (100 μs) combined with long interphase gaps (500 μs), and high stimulation rates (≥400 pulses per second [pps]). Conclusions.: With suprachoroidal stimulation, anodic-first pulses with a monopolar return are most efficacious. To enable high rates, an appropriate combination of pulse width and interphase gap must be chosen to ensure low thresholds and electrode voltages. Electrode–retina distance needs to be monitored carefully owing to its influence on thresholds. These results inform implantable stimulator specifications for a suprachoroidal retinal prosthesis. (ClinicalTrials.gov number, NCT01603576.)
Phosphenes are the fundamental building blocks for presenting meaningful visual information to the visually impaired using a bionic eye device. The aim of this study was to characterize the size, shape, and location of phosphenes elicited using a suprachoroidal retinal prosthesis.Three patients with profound vision loss due to retinitis pigmentosa were implanted with a suprachoroidal electrode array, which was used to deliver charge-balanced biphasic constant-current pulses at various rates, amplitudes, and durations to produce phosphenes. Tasks assessing phosphene appearance, location, overlap, and the patients' ability to recognize phosphenes were performed using a custom psychophysics setup.Phosphenes were reliably elicited in all three patients, with marked differences in the reported appearances between patients and between electrodes. Phosphene shapes ranged from simple blobs to complex forms with multiple components in both space and time. Phosphene locations within the visual field generally corresponded to the retinotopic position of the stimulating electrodes. Overlap between phosphenes elicited from adjacent electrodes was observed with one patient, which reduced with increasing electrode separation. In a randomized recognition task, two patients correctly identified the electrode being stimulated for 57.2% and 23% of trials, respectively.Phosphenes of varying complexity were successfully elicited in all three patients, indicating that the suprachoroidal space is an efficacious site for electrically stimulating the retina. The recognition scores obtained with two patients suggest that a suprachoroidal implant can elicit phosphenes containing unique information. This information may be useful when combining phosphenes into more complex and meaningful images that provide functional vision.
One strategy to improve the effectiveness of prosthetic vision devices is to process incoming images to ensure that key information can be perceived by the user. This paper presents the first comprehensive results of vision function testing for a suprachoroidal retinal prosthetic device utilizing of 20 stimulating electrodes. Further, we investigate whether using image filtering can improve results on a light localization task for implanted participants compared to minimal vision processing. No controlled implanted participant studies have yet investigated whether vision processing methods that are not task-specific can lead to improved results.Three participants with profound vision loss from retinitis pigmentosa were implanted with a suprachoroidal retinal prosthesis. All three completed multiple trials of a light localization test, and one participant completed multiple trials of acuity tests. The visual representations used were: Lanczos2 (a high quality Nyquist bandlimited downsampling filter); minimal vision processing (MVP); wide view regional averaging filtering (WV); scrambled; and, system off.Using Lanczos2, all three participants successfully completed a light localization task and obtained a significantly higher percentage of correct responses than using MVP ([Formula: see text]) or with system off ([Formula: see text]). Further, in a preliminary result using Lanczos2, one participant successfully completed grating acuity and Landolt C tasks, and showed significantly better performance ([Formula: see text]) compared to WV, scrambled and system off on the grating acuity task.Participants successfully completed vision tasks using a 20 electrode suprachoroidal retinal prosthesis. Vision processing with a Nyquist bandlimited image filter has shown an advantage for a light localization task. This result suggests that this and targeted, more advanced vision processing schemes may become important components of retinal prostheses to enhance performance. ClinicalTrials.gov Identifier: NCT01503576.
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.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.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.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.).
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.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.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.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).
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