Global activity shaping strategies for a retinal implant

Abstract

Objective: Retinal prostheses provide visual perception via electrical stimulation of the retina using an implanted array of electrodes. The retinal activation resulting from each electrode is not point-like; instead each electrode introduces a spread of retinal activation that may overlap with activations from other electrodes. With most conventional stimulation strategies this overlap leads to image blur. Here we propose a 'shaping' algorithm that uses multiple electrodes to manipulate the current between electrodes in a desired way. Approach: We assume a forward model for the conversion of electrode strengths to retinal activation. Three alternative global shaping algorithms are developed by calculating reverse models under different assumptions: linear inversion using singular value decomposition to 2 produce the pseudoinverse, a linearly constrained quadratic program, and a binary quadratic program to partition the target pattern. The algorithms were assessed using both the mean squared error between the resulting images and desired images, as well as their adherence to the maximum allowed electrode currents. Main Results: Under wide activation spreads the linear inversion algorithm gave improved solutions but faced two limitations: under low-noise conditions the electrode amplitudes exceeded their set limit; the set of solutions did not include the possibility of using negative local currents to induce retinal activation. The linearly constrained quadratic program and binary quadratic program respectively addressed these problems, but required much greater computation time. Significance: This provides a framework for improving the resolution of future retinal implants, especially those with high density electrode arrays.

Stimulation Strategies for Improving the Resolution of Retinal Prostheses

“…The hexapolar and multipolar approaches described above use combinations of electrodes as current sources and sinks to steer or focus current. An alternative approach, proposed by Spencer et al (2019), is to shape retinal activity directly, rather than through current, by utilizing a model that predicts the pattern of retinal activity resulting from multielectrode stimulation, estimated from recordings made with the implant. The proposed stimulation strategy effectively inverts the model to find the pattern of electrical stimulation on the electrode array that optimally matches a target pattern of retinal activity.…”

“…A key component of the strategy proposed by Spencer et al (2019) is that it incorporates methods to determine the limitations on spatial resolution imposed by noise in the measurement of the RGC electrical receptive fields. Without noise, the strategy can in principle achieve a spatial resolution limited only by the spacing between electrodes.…”

In-Vivo Microsystems: A Review

“…Although this does not give a high-quality image, it does allow a blind person sufficient image quality to function and navigate around objects. This work has shown the feasibility of the approach, and work is continuing to improve the image quality [ 76 , 77 ], including increasing the area of the electrode array and increasing the density of electrodes [ 78 , 79 ]. Another approach is to implant arrays of the probe directly into the optical cortex at the back of the brain [ 80 ].…”

Neural activity shaping utilizing a partitioned target pattern

“…Figure 1(a) details the forward linear-nonlinear (LNL) model that calculates the neural activity across the retina or visual cortex for a given pattern of electrode settings [11][12][13]. The LNL model is based on experimental results that demonstrate that the generator signal can be estimated from recordings of neural responses to electrical stimulation [12,13].…”

“…Rather than targeting control of the current in neural tissue, the neural activity shaping strategy [11] instead calculates electrode settings that aim to shape neural activity explicitly. This neural activity shaping strategy can be implemented with subsets of electrodes to induce a local pattern of neural activation or with all electrodes to induce an entire field of neural activation.…”