A New Look at Vision
Another Light-Sensitive Cell Discovered

Nearly all species have some ability to detect light. At least three types of cells in the human retina allow us to see images or distinguish  between night and day. Now, researchers at the Johns Hopkins School of Medicine have discovered yet another type of cell that can sense light and contribute to vision.

In the journal Nature, the team of Johns Hopkins neuroscientists report their discovery that retinal horizontal  cells - nerve cells once thought only to talk to neighboring nerve cells - are light sensitive themselves.

"This is mind-boggling," said neuroscientist King-Wai Yau. "For more than 100 years, it's been known that rod cells and cone cells are responsible for  sensing light, and therefore, vision.

"Then, about seven years ago, another light sensor was discovered in the retina, revealing a  third type of light-sensitive cells in mammals, so we set out to look at whether this was true in other vertebrates as well."

Focusing their efforts on the melanopsin light sensor, which is responsible for sensing day and night but barely involved -- in mammals, at least -- in seeing images, Yau's team looked for  melanopsin-containing cells in other vertebrates,  and found some in the retinal horizontal cells in goldfish and catfish.

Catfish contain two types of retinal horizontal  cells: those that connect to cone cells, which  respond to bright light, and those that connect to rod cells, which respond to dim light. The  team took electrical readings from single  isolated retinal horizontal cells. They found  that light caused a change in electrical current in cone horizontal cells but not in rod horizontal cells.

Horizontal cells, says Yau, allow cross-talk between neighboring photoreceptor cells, allowing these cells to compare the light they sense, a process necessary for the brain to see images.

"The brain processes what it sees in context to the surroundings. This allows our  brain to see borders and contours; horizontal cells are the reason why we can recognize and see a face, for example."

Testing light at different wavelengths, the team found that the fish horizontal cells are thousands of times less light sensitive than their partner cone cells.

"The bottom line is that the light effect on the  horizontal cells is subtle, perhaps to allow the  eyes of these animals to fine-tune their
functions to different ambient light conditions. But that these horizontal cells are light sensitive at all is a very surprising
finding and changes how we think about retinas as a whole."

Source: The Solomon H. Snyder Department of Neuroscience
at Johns Hopkins School of Medicine

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