2008 Woodie Awards
Photoreceptor "noise" affects quality of vision
Issue date: 5/1/08
A group of Hopkins scientists, led by King-Wai Yau of the School of Medicine, sought to find out why.
To do so, they capitalized on the fact that the brain can sometimes get confused. The inherent and necessary ability of opsins to change their shape has some unfortunate side effects, one of which is called "quantal noise."
This happens when a pigment changes shape spontaneously without having absorbed any light. This "gives rise to a false light signal that the animal cannot tell apart from real light," Yau said.
In other words, the brain reads "light" even though no light is actually present. Quantal noise thus puts a limit on the extent of our visual sensitivity since, in very low light the brain can't be trusted to distinguish between real light and quantal noise.
In rods - or more accurately, the protein rhodopsin - this noise is very low: about 0.01 false signals per second for each cell. Low quantal noise in rods is what allows us to pick up on incredibly small amounts of light.
For example, under the right conditions, humans can detect a single photon, the smallest possible amount of light energy in the universe. Cones, however, are a different, trickier story.
They are much less sensitive to low-intensity light. Hundreds of thousands of photons need to be absorbed in order to activate a single cone opsin. Based on the fact that rods have both low quantal noise and high sensitivity, the relative insensitivity in cones was thought to be a result of high quantal noise.
This, however, was just a theory. The quantal noise of a cone opsin had never been directly measured; individual false signals from cones are undetectable (with current technology, at least).
To get around this technical barrier, the Hopkins team availed themselves of some creative mouse breeding, creating a strain of mice whose rods, in addition to a full or half-size complement of rod pigments, also had genetically inserted red cone pigments.
To do so, they capitalized on the fact that the brain can sometimes get confused. The inherent and necessary ability of opsins to change their shape has some unfortunate side effects, one of which is called "quantal noise."
This happens when a pigment changes shape spontaneously without having absorbed any light. This "gives rise to a false light signal that the animal cannot tell apart from real light," Yau said.
In other words, the brain reads "light" even though no light is actually present. Quantal noise thus puts a limit on the extent of our visual sensitivity since, in very low light the brain can't be trusted to distinguish between real light and quantal noise.
In rods - or more accurately, the protein rhodopsin - this noise is very low: about 0.01 false signals per second for each cell. Low quantal noise in rods is what allows us to pick up on incredibly small amounts of light.
For example, under the right conditions, humans can detect a single photon, the smallest possible amount of light energy in the universe. Cones, however, are a different, trickier story.
They are much less sensitive to low-intensity light. Hundreds of thousands of photons need to be absorbed in order to activate a single cone opsin. Based on the fact that rods have both low quantal noise and high sensitivity, the relative insensitivity in cones was thought to be a result of high quantal noise.
This, however, was just a theory. The quantal noise of a cone opsin had never been directly measured; individual false signals from cones are undetectable (with current technology, at least).
To get around this technical barrier, the Hopkins team availed themselves of some creative mouse breeding, creating a strain of mice whose rods, in addition to a full or half-size complement of rod pigments, also had genetically inserted red cone pigments.
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