By Jonny Lupsha, Wondrium Staff Writer
Optic nerves send visual info from the eye to the brain. It’s a collection of more than one million nerve fibers and, if damaged, impairs sight. A brain implant for those with optic nerve damage could restore sight.
Toxic optic neuropathy is an uncommon condition in which the optic nerves at the back of the eyes are damaged by a toxin. After developing the condition and being blind for more than a decade, a patient underwent neurosurgery and received an implant the size of a penny that directly stimulates the region of the brain that involves visual perception. While her sight wasn’t fully restored, she recently was able to detect two-dimensional shapes and letters.
How do the optic nerves work, ordinarily? In her video series Understanding the Brain, Dr. Jeanette Norden, Professor of Cell and Developmental Biology in the School of Medicine at Vanderbilt University, said that the optic nerve is formed by the axons of the retina’s ganglion cells.
A Retinal Refresher
The retina is a layer of tissue on the back of the eye. It senses light and it sends signals to your brain, telling you what it’s seeing. These signals are sent along several parts of the eye, including ganglion cells. Every neuron in the brain has an axon, which is a tube-shaped portion of the neuron much thinner than a human hair.
The axon is essentially an electric cable, carrying electrical impulses from one place to another.
“In our eye, we have axons which leave at the optic disc; these are retinal ganglion cell axons, which will form the optic nerve and track,” Dr. Norden said. “These axons, when they come out of one eye—obviously the same thing is happening on the other side—but when these axons come out of the eye, they are called the optic nerve, and when they join with axons from the other eye, it’s called the optic tract.”
The next stop is the lateral geniculate nucleus, or LGN. The LGN is part of the section of the brain called the thalamus, and it serves as a relay center between the optic nerve and the rest of the thalamus.
How We Know Where Things Are
“The lateral geniculate nucleus receives information from parts of both eyes, but information information from each eye is going to synapse in a different layer,” Dr. Norden said. “The lateral geniculate has one nucleus in the thalamus and it has multiple layers. It turns out that each eye will synapse in only one of those layers, and it means that the brain is keeping information from each eye separate.”
Furthermore, some layers of the LGN process information about color, while others process information about motion, and still others process other kinds of visual information. This means information is processing in parallel pathways and kept separate. There is also an orderly representation of the projection.
“Parts of each eye project to each side of the brain, but basically that means that each side of the brain is getting a whole map of one retina—or one half of the visual field—and it turns out that projection is point to point,” Dr. Norden said. “That means that each place on the lateral geniculate is receiving retinal ganglion cells in a specific place on the retina, and that orderly representation means that there is a map on the visual structures of every point in visual space.
“So, our brain knows exactly where in visual space something we are looking at is located.”
The journey from the optic nerve to the thalamus is impossibly complex. Damaged optic nerves can cause blindness, but as technology has just shown, that blindness may be partially reversible.