Simulation of a phosphene-based visual field: Visual acuity in a pixelized vision system

Abstract

A visual prosthesis for the blind using electrical stimulation of the visual cortex will require the development of an array of electrodes. Passage of current through these electrodes is expected to create a visual image made up of a matrix of discrete phosphenes. The quality of the visual sense thus provided will be a function of many parameters, particularly the number of electrodes and their spacing. We are conducting a series of psychophysical experiments with a portable "phosphene" simulator to obtain estimates of suitable values for electrode number and spacing. The simulator consists of a small video camera and monitor worn by a normally sighted human subject. To simulate a discrete phosphene field, the monitor is masked by an opaque perforated film. The visual angle subtended by images from the masked monitor is 1.7 degrees or less, depending on the mask, and falls within the fovea of the subject. In the study presented here, we measured visual acuity as a function of the number of pixels and their spacing in the mask. Visual acuity was inversely proportional to pixel density, and trained subjects could achieve about 20/26 visual acuity with a 1024 pixel image. We conclude that 625 electrodes implanted in a 1 cm by 1 cm area near the foveal representation of the visual cortex should produce a phosphene image with a visual acuity of approximately 20/30. Such an acuity could provide useful restoration of functional vision for the profoundly blind.

Electronic restoration of vision in those with photoreceptor degenerations

“…Cha and colleagues 70 and Cha, Horch and Normann 71 used psychophysical studies to determine the minimum number of electrodes required to be able to read letters. They showed that 625 electrodes in a 25 by 25 array with a centre-to-centre spacing of 400 mm all contained within a 1.0 cm by 1.0 cm grid 71 allowed resolution of images in a tiny central field.…”

“…They showed that 625 electrodes in a 25 by 25 array with a centre-to-centre spacing of 400 mm all contained within a 1.0 cm by 1.0 cm grid 71 allowed resolution of images in a tiny central field. For navigational tasks, a lower resolution but larger field of view was necessary.…”

Implantes eletrônicos para restabelecimento da visão em cegos

“…Os pesquisadores devem ainda definir os parâmetros elétricos para a segurança e efetividade de um pixel. Alguns estudos mostraram que 625 pixels podem ser a estimulação mínima para criação de uma imagem simples (41) . A eficácia da estimulação elétrica nos tecidos pode ser influenciada pela propriedade elétrica da célula alvo, pela distância dos eletrodos até a célula alvo, impedância dos tecidos, relação entre os limiares de corrente elétrica, pela duração da ativação neuronal e pela influência de estímulos repetitivos na via visual (42)(43)(44) .…”

An Implant for a Visual Cortical Stimulator

“…On the other hand, the number of stimulation sites/channels is very high. Indeed, for a meaningful image to be perceived by the user, hundreds of stimulation sites are expected to be required [10]. This puts severe constraints on the external to internal (downlink) data rate and the total power efficiency of the device.…”