2008 Woodie Awards
Photoreceptor "noise" affects quality of vision
Issue date: 5/1/08
Humans are overwhelmingly visual animals. While most other mammals are sniffing around their respective worlds, we rely on the sharpness of our eyes for information about our environment. (Try sniffing around for a staircase, and you'll likely break a few bones.)
Much of how the visual system works has been elucidated (pun intended) in the last few decades, but a lot is still not well understood.
We know, for example, that the gift of sight boils down to about 100 million specialized cells in the retina, the thin layer of tissue at the back of the eye.
These cells, called rods and cones, are responsible for translating variations in light - different wavelengths, different intensities and so on - into an electrical signal the rest of the brain can read and ultimately use to construct a picture of the world.
How rods and cones make their electrical signals is a wonder of evolution. Each rod or cone contains a light-absorbing pigment; rods have rhodopsin and cones have cone opsin. When an opsin absorbs light, its shape changes, allowing it to bind to a certain protein.
This sets off an intricate, multistep pathway of protein activation and deactivation whose eventual outcome is the closing of millions of ion channels and the alteration of an electric signal to the brain.
When these channels are open (in other words, in the dark), positively charged ions, such as sodium, flow freely into the cell, creating what scientists call a "dark current."
When they close, however, the influx is blocked, and with time, the cell's charge becomes more and more negative. This hyperpolarization, as it's called, is the electrical signal that tells the brain that light is present.
This process is pretty well described, but one of the field's irksome mysteries is why rods and cones are differentially sensitive to light. Rods are mainly active when light is low, around dusk or in poorly lit rooms, while cones respond to higher intensities of light such as those present throughout the day.
Much of how the visual system works has been elucidated (pun intended) in the last few decades, but a lot is still not well understood.
We know, for example, that the gift of sight boils down to about 100 million specialized cells in the retina, the thin layer of tissue at the back of the eye.
These cells, called rods and cones, are responsible for translating variations in light - different wavelengths, different intensities and so on - into an electrical signal the rest of the brain can read and ultimately use to construct a picture of the world.
How rods and cones make their electrical signals is a wonder of evolution. Each rod or cone contains a light-absorbing pigment; rods have rhodopsin and cones have cone opsin. When an opsin absorbs light, its shape changes, allowing it to bind to a certain protein.
This sets off an intricate, multistep pathway of protein activation and deactivation whose eventual outcome is the closing of millions of ion channels and the alteration of an electric signal to the brain.
When these channels are open (in other words, in the dark), positively charged ions, such as sodium, flow freely into the cell, creating what scientists call a "dark current."
When they close, however, the influx is blocked, and with time, the cell's charge becomes more and more negative. This hyperpolarization, as it's called, is the electrical signal that tells the brain that light is present.
This process is pretty well described, but one of the field's irksome mysteries is why rods and cones are differentially sensitive to light. Rods are mainly active when light is low, around dusk or in poorly lit rooms, while cones respond to higher intensities of light such as those present throughout the day.
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