Visual function and the visual pathway Implants from the eye to the brain

According to the World Health Organization, eye diseases affect nearly 2.2 billion people worldwide. They have a major impact on quality of life. Although the majority of vision disorders can be treated by medical consultation or surgery, 20% are linked to hereditary and/or incurable pathologies, such as retinal diseases or glaucoma. Retinal diseases, such as retinopathy pigmentosa (RP) and age-related macular degeneration (AMD), affect the photoreceptors in the retina, while glaucoma and diabetic retinopathy affect the ganglion cells, and therefore by extension the optic nerve connected to the brain. While degeneration of the photoreceptors leaves the other layers of the retina relatively intact, which can then be electrically stimulated to restore sight, atrophy of the optic nerve removes any eye-brain connection. Visual restoration must therefore take place in the higher visual areas of the brain.

Despite the development of alternative strategies (transplantation, optogenetic therapy), electrical stimulation remains the only clinically validated approach to restoring vision in people suffering from severe blindness as a result of RP. Over 40 years of research have been devoted to understanding the role of electrical stimulation, and to improving its effectiveness in visual prostheses.

Phosphenes are visual sensations that occur following electrical stimulation. They were first mentioned in connection with a visual prosthesis when Brindley and Lewin placed electrodes on the visual cortex of a blind subject. Over the years, human trials have tested the effects of electrical stimulation in all areas of the optic pathway, demonstrating its restorative properties on vision.

This article seeks to present the different strategies that have been explored to restore visual function in blind patients using electrical stimulation. It includes a presentation of the anatomy of the eye, visual diseases and the electrophysiological interaction between neuronal tissue and the electrode. Since the 2000s, these strategies have evolved in terms of the type of stimulation delivered, its location and the structure of the implants. This article presents the implants currently on the market, as well as those currently in clinical trials. We also look at some of the innovations that have contributed to their development, as well as future challenges in this field of brain-machine interface applications.

A glossary of terms is provided at the end of the article.