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Types of Visual Prostheses

Visual prostheses are being studied worldwide.
There are many types of visual prostheses being proposed.

"Electrically stimulating nerve tissues associated with vision (such as the retina) to help transmit electrical signals with visual information to the brain through intact neural networks." This is the basic concept of the visual prosthesis. This idea is rational and feasible, and therefore has sparked R&D throughout the world.

There is more than one specification for the prosthesis. Many types of prostheses are being proposed because of its potential for the development of various types of devices with existing technologies. Such devices take into consideration patient condition, nerve tissue subject to stimulation, and their role in the visual network. Let us introduce a representative prosthesis being researched and developed. These devices can be categorized in many ways; however, a common categorization would be the relationship between electrodes and tissues targeted for electrical stimulation.

Totally implantable system with all functions required for artificial vision is not possible with the present technology for any type of prosthesis. The current system is designed with electrodes implanted in the body working together with several devices worn outside the body.

Major targeted tissues for electrical stimulation
with the visual prosthesis

1. Retinal Implant

The retina is the first nerve tissue in visual network that generates electrical signals. A retinal implant is a type that stimulates the retina with electrodes. This type of visual prosthesis is also called an artificial retina. The retina is a membrane with a 3D layer structure. There are many options regarding how to stimulate retina and the direction from which to stimulate. There are 3 major methods.

Epiretinal Implant
This type stimulates the retina from the top side with electrodes. Multielectrode array is set up on the retinal surface. The difficulty in positioning the electrodes is how to fit the planar array on the curve of the eye with all electrodes maintaining the same distance from the retina. A challenge is array processing and a surgical method to fix the arrays. Most of the research for this type of implant is conducted in the USA.

Subretinal Implant
An electrode array is implanted under the retina. The fundamental concept is to develop a chip with the same function as a photoreceptor and implant it to replace damaged photpreceptor. Intraocular pressure is used to fix the electrode array, and innate eye movement is usable for scanning the scene; however, surgery would be complicated and there is the issue of power supply after the surgery. Most of the research for this type of implant is conducted in the USA and Germany.

Suprachoroidal Transretinal Stimulation (STS)
This is proposed as an alternative to reduce the issues seen in epiretinal and subretinal implants. It was proposed and developed in Japan under Japan's Artificial Vision Project, which NIDEK is a part of. Stimulation electrode array is set up inside the sclera, a tissue covering the outermost part of the eye, and a return electrode is set up intravitreously. Using these two electrode parts, electrical stimulation passes inwardly through the retina. The method is advantageous in that it is safer for the retina, more reliable with regard to electrical stimulation and the
implant can be performed on a more durable part of the eye than other methods. Other groups follow Japan in the research of this transretinal stimulation strategy like our STS.

Of the many types of visual prostheses, the retina stimulating type is the subject of most research. The reason being that it is easier to access by surgery than any other target tissue for electrical stimulation. It is also known that there is a corresponding relation between the location in the retina and the visual field. The center of the retina corresponds to the center of the visual field, and the peripheral retina corresponds to the peripheral visual field. For example, if the center of retina is damaged, difficulty seeing the central area of the visual field will be experienced. Yet if this correlation can be used, stimulation of the retina with a certain pattern (the letter N in the example on the diagram below) would result in seeing the same pattern in the brain.

With retinal implants, this type of 2D information transmission can be obtained through pattern stimulation.

2. Optic Nerve Implant

The optic nerve is composed of a bundle of nerve fibers from the retinal output cells called retinal ganglion cells (RGCs). This bundle exits from the eye and is connected to nerve cells in the brain. Visual prostheses with a cuff electrode array having embedded electrodes in a film substrate is wrapped around optic nerves from outside as well as a type stimulating optic nerve head with wire electrodes are being proposed.

It has been found that development of system software that controls resultant phosphenes would enable replication of images. This is considered a feasible method.

Optic Nerve Implant

3. Cortical Implant

This visual prosthesis stimulates the visual cortex in the brain responsible for vision with electrode arrays. The visual cortex is located at the lower back section of the head that bulges out slightly. The brain is the final destination of the visual information network. Stimulating this final destination is thought to be another way of reconstructuring images, even if the start of the visual network, the eye, or transmit point, the nerve cells, are not functioning.

However, implanting electrodes in the brain would require much more extensive surgery than other methods, and risks involved during and after surgery and other safety issues are the main challenges for this stimulation approach.

Other types of artificial vision are also being studied, including one that electrically stimulates the lateral geniculate nucleus in the brain, one that chemically stimulates the visual network with a neurotransmitter, and others that use light‒activated protein.