At the heart of the researchers' side-by-side comparison is the quest to solve a fundamental mystery of vision: how rods and cones have such different sensitivities to light despite using very similar processes to detect it.
Rods function in near darkness, while rarer cones function in bright light, providing vibrant color vision. In each cell type, the process of forming vision begins when light activates a cell-specific molecule, called a visual pigment, and ends when the cell emits an electrical signal.
To set up the "taste test," the Hopkins researchers created frogs whose rods contained, in addition to their usual pigment, a pigment found only in cones. The researchers expected the rods to treat the two pigments differently -- picking up signals only from its native pigment and spurning the other -- or to behave a little like cones.
"Surprisingly, the cell's response to light was identical regardless of which pigment was activated," says King Wai Yau, Ph.D., professor of neuroscience in Johns Hopkins' Institute for Basic Biomedical Sciences. "It's as though the label of 'rod' pigment and 'cone' pigment is gone. The pigments alone do not explain the cells' functional differences."
Some scientists had speculated that the pigment defines a cell's role in vision, making a rod, a rod or a cone, a cone. Until now, however, no experiments have measured whether starting the process with the "wrong" pigment affects the cell's critical characteristics -- the size and shape of the electrical signals it produces.
Studying individual rods containing both the rod pigment, called rhodopsin, and a cone pigment (called human red cone pigment), the Johns Hopkins scientists discovered for the first time that ro
Contact: Joanna Downer
Johns Hopkins Medical Institutions