Salk Institute for Biological Studies



"Visual information is transmitted from the eye to the brain in just 1.25 million optic nerve fibers—about as many fibers as there are pixels in a cheap digital camera. Somehow we exploit this information to perform all sorts of critical tasks all the time, such as recognizing objects and navigating busy freeways, that even the most advanced robots are only starting to do. I want to understand how the retina, the neural tissue lining the back of the eye, encodes visual information so the brain can use it to produce visual experience."

Visual processing begins when photons entering the eye strike a layer of light-sensitive nerve cells in the retina, known as rods and cones. These cells convert light into electrical signals and send them to an intermediate layer, which in turn relays signals to a collection of 1.25 million neurons known as retinal ganglion cells. These cells carry visual signals from the eye to the brain.

To understand how this neural circuitry in the retina produces high-resolution vision, Chichilnisky's lab uses a state-of-the-art 512-electrode recording system, developed in collaboration with an international group of high-energy physicists. This system is capable of recording simultaneously the tiny electrical signals generated by hundreds of retinal ganglion cells that transmit information about the outside visual world to the brain. These recordings are made at high density and with fine spatial detail, sufficient to detect complete populations of the tiny and densely spaced output cells known as "midget" retinal ganglion cells.

This has allowed the researchers to map the full cone mosaic found in a region of the retina and to trace for the first time the neuronal circuitry that connects individual photoreceptors to retinal ganglion cells. It also has shed light on the neural code used by the retina to relay color information to the brain.

Based on this research, Chichilnisky's team is designing approaches to making artificial retinas to restore vision in people who are blinded by retinal degenerative diseases, such as macular degeneration or retinitis pigmentosa. In principle, retinal implants could bypass the damaged retina with the help of tiny electrode arrays that mimic the electrical signals sent to the brain in response to light. In order to engineer these prosthetics, however, scientists need to understand how neurons in the retina function as a network to produce an image, and how to electrically stimulate the retina in a manner that can reproduce important aspects of normal vision.

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