Stimulation of the Retinal Network in Bionic Vision Devices: From Multi-Electrode Arrays to Pixelated Vision

Abstract

Bionic vision devices aiming to restore visual perception in the vision impaired rely on microelectrode arrays that can be implanted under the diseased retina. Arrays used today in first human trials are high density monopolar arrays comprising up to 1500 electrodes in a 3x3 mm a simple field calculations demonstrate that such high density arrays suffer from degradation of contrast between those 1500 stimulation sites when driven simultaneously. This effect can be described as electric crosstalk between the electrodes that strongly depends on the number of electrodes on such an array and proximity of electrodes to the target cells. The limit of spatial frequency of visual patterns that could be resolved by such arrays can be assessed to be 4.5; 1.2; and 0.7 cycles/mm, for an anticipated distance of target neurons of 20 (m, 200 (m, and 400 (m, respectively. This relates to a theoretically best achievable visual acuity of 2%, 0.6%, and 0.3% of normal vision, respectively (logMAR 1.7; 2.2; 2.5). These data suggest that novel strategies have to be pursued to either get closer to target structures, e.g. by the use of penetrating electrode arrays, or to create more confined stimulating fields within the retina, e.g. by the use of hexagonal arrays with multiple electrodes guarding one active electrode.