Retinal Bypass Prosthetic Vision--Lateral Geniculate Nucleus

Researchers at Harvard Medical Schools' Neurobiology Department are attempting to develop a prosthesis to restore vision in patients without a functioning retina or optic nerve.

Because the new technique completely bypasses the retina, it suggests the possibility of developing neural prostheses that can restore vision to patients with extensive retinal damage due to congenital conditions such as glaucoma, macular degeneration and retinitis pigmentosa. In such patients, conventional retinal implants, which are already in the clinical phase of development, are of little or no use, because they require at least some properly functioning retinal cells.

The LGN is a structure found deep in the brain with the thalamus. It can be thought of as a “relay station” in the visual pathway, because it receives electrical signals about visual stimuli from the eye, via the optic nerve, and then relays them to the primary visual cortex in the occipital lobe of the brain. In the LGN, the visual fields from both eyes are represented somatotopically; that is, the visual scene impinging on the retina is mapped in a straightforward manner onto the LGN tissue, such that adjacent points in the visual field stimulate adjacent LGN neurons. The receptive fields of LGN neurons are similar to those of retinal cells; they are simple and well characterized. Structurally and functionally, the LGN is subdivided into a number of “streams” - the parvocellular and magnocellular pathways. Cells in the former pathway have small cell bodies, and process colour information slowly, while cells in the latter have large cell bodies, process information slowly and do not carry colour information.

Because previous studies have shown that electrical stimulation of the visual cortex in blind people can elicit visual sensations, and because the structure of the LGN is so well characterized, John Pezaris and R. Clay Reid sought to determine whether or not similar sensations could be elicited by stimulation of the LGN. They first trained two adult macaque monkeys to quickly direct their gaze towards points of light presented to them on a computer screen. Individual microelectrodes were then embedded into the LGN through small craniotomies. In response to electrical stimulation applied to specific regions of the LGN, the animals shifted their gaze to the corresponding part of the computer screen. This suggested that the monkeys’ visual systems registered spots of light, despite the absence of any external visual stimuli. The researchers then implanted two electrodes in different parts of the LGN, and stimulating one and then the other in quick succession; in response to this, the monkeys quickly turned their heads from one direction to the other. Thus, the electrodes were successfully used to generate artificial visual percepts; from the observed responses of the monkeys, these percepts were indistinguishable from spots of light entering the eye.

The researchers now aim to build a visual prosthetic device consisting of a small digital camera mounted in the lens of a special pair of glasses (left). The camera would gather images and send them to an external signal processor, which would translate the images into electrical impulses and then send them wirelessly to a device that stimulates an electrode array embedded in the LGN. However, the research is still in the early stages, and much work remains to be done before such a device can be developed.

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