July 27, 2009

Research Sheds Light On Vision

Researchers from the John Hopkins School of Medicine have discovered a new type of cell used for vision in fish.

The report, which appears in the journal Nature, explains how researchers found a retinal horizontal cell that senses light and contributes to vision.

"This is mind-boggling," says King-Wai Yau, Ph.D., a professor of neuroscience at John Hopkins.

"For more than 100 years, it's been known that rod cells and cone cells are responsible for sensing light, and therefore, vision," says Yau.

"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."

The researchers focused their search on melanopsin, a light sensor found in some cells, and found the substance in some vertebrates and in the retinal horizontal cells in catfish and goldfish.  

Catfish contain two different types of retinal horizontal cells, one that connects to cone cells and responds to bright light, and one that connects to rod cells and responds to dim light.

The team isolated retinal horizontal cells and took electrical readings.  They found that light caused an electrical current change in cone horizontal cells but not in rod horizontal cells.
According to Yau, horizontal cells allow communication between nearby photoreceptor cells.  This communication allows the cells to compare the light they sense, and gives the brain information to see images.

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

The team also found that the horizontal cells found in fish were thousands of times less sensitive to light than 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," Yau said. "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."

According to Yau, research must now be conducted on the whole retina to fully understand how light sensitivity in horizontal cells contributes to vision.

The National Institutes of Health and the Ant³nio Champalimaud Vision Award funded the research.


Image Caption: This is a goldfish. Credit: Johns Hopkins Medicine


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