Artificial vision

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Artificial vision

Artificial vision refers to the technology (usually, visual implants) that allows blind people to see. The main aim of visual implants is to relay pictures to the brain using either cameras or photoreceptor arrays. There are different types of implants used to stimulate vision: retinal, cortical, optic nerve, and biohybrid. None of the currently available technologies restores full vision, but they are often able to improve one’s ability to recognize shapes and movements.

Cortical implants

A number of scientific groups are working on cortical implants, which directly stimulate the visual cortex and can be used by a majority of blind patients. The main differences between the implants are the way in which they interact with the brain. Surface-type implants, such as the Dobelle implant, are placed directly on the brain surface. Others, such as penetrating implants, are designed to be introduced into brain tissue for direct contact with the neurons responsible for relaying visual information. As of 2006, penetrating implants were under study at several universities and laboratories affiliated with the National Institutes of Health.

The first surface visual implant, an electrode array, was surgically inserted by a team led by American ophthalmologist William H. Dobelle in 1978. With the Dobelle implant, surface electrodes are connected to a camera installed on one side of special eyeglasses. During the treatment, the patient’s cortex is systematically and slowly stimulated to re-learn how to see. Camera and distance sensors in the sunglasses send the signals to a computer, and it in

turn sends pulses to an electrode array. Vision is black and white with light appearing as phosphenes, which are visual sensations resulting from mechanical stimulation of the eye (similar to the visual sensations created when pressing upon the eyeball with closed eyes). Quality of vision is dependent not only on the technology, but also on the patient, as different people see varying numbers of phosphenes. The necessity to carry a battery and a computer on the belt can be considered a disadvantage, but the Dobelle implant allows easy upgrades to newer technology. The main drawback of this implant is the price tag (around $100,000) for the treatment. Research and scientific scrutiny of the Dobelle implant continues into the twenty-first century, with limited clinical trials showing positive results.

Retinal implants

Retinal implants were designed for people with retinal diseases such as retinitis pigmentosa or age-related macular degeneration. These implants rely on intact retinal neurons to transmit the stimuli to the brain. A number of varying designs are being used in clinical trails; the main difference between them is the localization of the implant in the retina. Epiretinal implants are placed on the retinal surface and subretinal implants are placed under the retina.

The first retinal implant to move into clinical trials (in 2000) was a subretinal artificial silicon retina

KEY TERMS

Phosphenes— Visual sensations or impressions resulting from a mechanical pressure on an eyeball or excitation of the retina by stimulus other than light.

Retina— An extremely light-sensitive layer of cells at the back part of the eyeball. The image formed by the lens on the retina is carried to the brain by the optic nerve.

Visual acuity— Keenness of sight and the ability to focus sharply on small objects.

Visual cortex— Area of the brain (occipital lobe) concerned with vision.

(ARS). The ARS is a very small silicon microchip that is 0.08 in (2 mm) in diameter and 0.001 in (25 µm) thick, composed of miniature solar cells. The arrays of these cells respond to light similar to natural photoreceptors.

The artificial retina component chip (ARCC) is an example of an epiretinal implant. The ARCC is composed of light sensors and electrode arrays that send signals to the retinal neurons. However, the system requires additional power and is dependent upon an external camera to detect a picture and induce a laser pulse. Incoming laser light is detected by the photo sensors in the implant to create a picture. The camera and the laser are built into special sunglasses.

The vOICe system was developed as an alternative to surgical implants. Users claim that they are receiving visual sensations from using the system, which involves changing images from a video camera into corresponding sound patterns that in turn, stimulate the visual cortex. The vOICe technology, therefore, can be classified as artificial vision, although only its effectiveness as a sensory substitution has been established. The main advantages are that there is no surgical intervention and the cost of the device is only $2,500.00.

Quality of artificial vision

Resolution of the artificial systems is an important consideration in their design and usefulness. A pixel resolution of 5x5, such as with the ARCC, allows reading of some individual letters; a 10x10 pixel vision (ASR) can allow further distinction of form, but a pixel resolution of 32x32 is usually necessary to allow a person freedom of movement. Better resolutions of 64x64 pixels and up to 250x250 pixels are considered to be a matter of time. Resolution of the cortical implants is measured mostly in vision acuity because their aim is not only freedom of movement, but also ability to read.

As of 2006, significant restoration of vision through direct stimulation of the brain remained a topic of research rather than a practical option for the blind.