How visuals signals travel from eye to the brain

LONDON - Researchers at the Salk Institute for Biological Studies were able to trace for the first time the neuronal circuitry that connects individual photoreceptors with retinal ganglion cells, the neurons that carry visuals signals from the eye to the brain.

Their measurements also shed light on the neural code used by the retina to relay colour information to the brain.

“We think these data will allow us to more deeply understand neuronal computations in the visual system and ultimately may help us construct better retinal implants,” says E.J. Chichilnisky, an associate professor in the Systems Neurobiology Laboratories.

A unique neural recording system developed by an international team of high-energy physicists was able to record simultaneously the tiny electrical signals generated by hundreds of the retinal output neurons that transmit information about the outside visual world to the brain.

These recordings are made at high-speed and with fine spatial detail, sufficient to detect even a locally complete population of the tiny and densely spaced output cells known as “midget” retinal ganglion cells.

Visual processing begins when photons entering the eye strike one or more of the 125 million light-sensitive nerve cells in the retina. This first layer of cells, which are known as rods and cones, converts the information into electrical signals and sends them to an intermediate layer, which in turn relays signals to the 20 or so distinct types of retinal ganglion cells.

The Salk researchers simultaneously recorded hundreds of retinal ganglion cells, and based on density and light response properties, identified five cell types: ON and OFF midget cells, ON and OFF parasol cells, and small bistratified cells, which collectively account for approximately 75 percent of all retinal ganglion cells.

“Instead of a diffuse region of light sensitivity, we detected punctate islands of light sensitivity separated by regions of no light sensitivity,” said Chichilnisky.

When combined with information on spectral sensitivities of individual cones, maps of these punctate islands not only allowed the researchers to recreate the full cone mosaic found in the retina, but also to conclude which cone fed information to which retinal ganglion cell.

“Just by stimulating input cells and taking a high density recording from output cells, we can identify all individual input and output cells and find out who is connected to whom,” said Chichilnisky.

The study has been published in Nature. (ANI)