Improved visual performance in letter perception through edge orientation encoding in a retinal prosthesis simulation.

Objective. Stimulation strategies for retinal prostheses predominately seek to directly encode image brightness values rather than edge orientations. Recent work suggests that the generation of oriented elliptical phosphenes may be possible by controlling interactions between neighboring electrodes. Based on this, we propose a novel stimulation strategy for prosthetic vision that extracts edge orientation information from the intensity image and encodes it as oriented elliptical phosphenes. We test the hypothesis that encoding edge orientation via oriented elliptical phosphenes leads to better alphabetic letter recognition than standard intensity-based encoding. Approach. We conduct a psychophysical study with simulated phosphene vision with 12 normal-sighted volunteers. The two stimulation strategies were compared with variations of letter size, electrode drop-out and spatial offsets of phosphenes. Main results. Mean letter recognition accuracy was significantly better with the new proposed stimulation strategy (65%) compared to direct grayscale encoding (47%). All examined parameters-stimulus size, phosphene dropout, and location shift-were found to influence the performance, with significant two-way interactions between phosphene dropout and stimulus size as well as between phosphene dropout and phosphene location shift. The analysis delivers a model of perception performance. Significance. Displaying available directional information to an implant user may improve their visual performance. We present a model for designing a stimulation strategy under the constraints of existing retinal prostheses that can be exploited by retinal implant developers to strategically employ oriented phosphenes.

Embracing the irregular: a patient-specific image processing strategy for visual prostheses.

We propose a stimulation strategy for retinal prostheses that makes use of irregular shapes of elicited phosphenes. It is patient specific and thus relies on prior psychophysical measurements. Visual perceptions are stored in a phosphene map that relates stimulation parameters to the visual stimulus elicited. Based on this map, stimulation parameters are chosen in such a way that the edges of the target image are optimally represented through the shape of the phosphene. In a psychophysical pilot study, we compare this approach to one in which we choose phosphenes to match the brightness of the target image. We find that participants perform similarly well with both strategies overall. However, the results indicate that each strategy may have advantages for different stimulus sizes. Both of the proposed strategies are novel in using only previously recorded phosphenes rather than a model based on idealized assumptions about the relationship between stimulation parameters and phosphene properties.

Minimisation of required charge for desired neuronal spike rate.

Retinal implants restore limited visual perception to blind implantees by electrical stimulation of surviving neurons. We consider the efficacy of two electrical stimulation parameters, frequency of stimulation and interphase gap between cathodic and anodic phases, on the required charge to reach a desired neuronal spike rate. Using a Hodgkin-Huxley model of a neuron, we find the most efficient means of achieving a desired spike rate for neurons by electrical stimulation is to use a stimulation frequency identical to the desired spike rate, as well as a long interphase gap.

Can electric current steering be used to control perception of a retinal prosthesis patient.

We consider a form of current steering to elicit desired perceptions in users of a retinal prosthesis. While it is common to use a single, remote return electrode to balance electrical stimulation, advances in chip design and electrical switching have enabled more flexibility in stimulation paradigms. We have created a finite-element model of a retina and a ten electrode prosthesis in COMSOL. Different configurations of stimulating and return electrodes are considered and employed to predict possible user perception. We investigate charge balance on electrodes in our varying geometries and consider the impact of inhomogeneous resistance between electrodes and the tissue.

Feature accentuation in phosphenated images.

We present and evaluate different approaches to feature accentuation in phosphenated images for different image resolutions. The goal of this study is to find methods to attract an implantee's visual attention to important image content like faces, obstacles or road signs. We do this by defining an important region in the image and accentuating it by either increasing the brightness of outlining phosphenes or by using elliptical phosphenes to circumscribe the feature. While we only see limited benefit of ellipse phosphenes for a high-resolution prosthesis, the use of elliptical phosphenes of different orientations is a promising way to highlight features in a low-resolution phosphene representation of an image.

Predicting phosphene elicitation in patients with retinal implants: a mathematical study.

Single pulse waveforms were considered in a recent model for phosphene elicitation in patients with a retinal prosthesis. Waveforms are constrained to charge-balanced stimuli consisting of a single cathodic and anodic pulse pair. Mathematical models of such stimuli have been constructed and presented based upon patient testimonials. In this work, we derive analytic expressions that may be employed to determine equibrightness levels for different waveforms. We provide an example calculation to show quantitative improvements in stimulation efficiency that are consistent with qualitative findings on waveform effects.

Eye position prediction in the case of nystagmus and refixations.

Nystagmus is a condition in which the eyes move in an uncontrolled fashion. These eye movements confound attempts to identify patient fixation, as desired eye positions cannot be maintained. We aim to estimate the eye positions in the case of refixations and superimposed uncontrolled motion. By incorporating saccade detection and resetting our estimation algorithms, we are able to track the nystagmus motion independently of fixation direction. We employ our algorithm on data collected from patients with latent/manifest-latent and pendular nystagmus conditions.