Photoreceptor "noise" affects quality of vision
Rods, of course, are much more sensitive to light; thus the quantal noise of the inserted red cone pigments became more easily detectable. Indeed, "the frequency of spontaneous events was low enough to be individually counted," Yau said.
That wasn't enough, however. To further boost the false signals, the red cone opsin-insertion mice were bred with another group of mutant mice that lacked a protein known to dampen the hyperpolarization signal.
Ultimately then, the researchers had mice with red cone pigments in their rods and rods, in turn, that produced amplified signals.
Several measurements and calculations later, the team had some surprising results. Spontaneous changes in the shape of the red cone opsin accounted for only about nine false events per second.
This is significantly more than the 0.01 in rods, but previous work had observed a total of 6,400 false events per second from each red cone. In other words, and contrary to the prevailing theory, the lion's share of false signals coming from red cones was not a result of quantal noise.
This revelation has produced something of a mystery: What's causing those other 6,391 false events every second? "Besides the quantal noise coming from the pigment, there is other noise originating from the steps in the phototransduction process downstream from the pigment," Yau said.
"This other noise is sometimes called the 'continuous noise' because it is not quantized like the quantal noise."
It's not easy to distinguish the two types of noise, so most scientists have in the past just lumped them into an equivalent measure of noise (the 6,400 false events). "Sometimes it's OK not to treat the two types of noise as separate," Yau said, "but other times it's important to separate them, as we have done."
That wasn't enough, however. To further boost the false signals, the red cone opsin-insertion mice were bred with another group of mutant mice that lacked a protein known to dampen the hyperpolarization signal.
Ultimately then, the researchers had mice with red cone pigments in their rods and rods, in turn, that produced amplified signals.
Several measurements and calculations later, the team had some surprising results. Spontaneous changes in the shape of the red cone opsin accounted for only about nine false events per second.
This is significantly more than the 0.01 in rods, but previous work had observed a total of 6,400 false events per second from each red cone. In other words, and contrary to the prevailing theory, the lion's share of false signals coming from red cones was not a result of quantal noise.
This revelation has produced something of a mystery: What's causing those other 6,391 false events every second? "Besides the quantal noise coming from the pigment, there is other noise originating from the steps in the phototransduction process downstream from the pigment," Yau said.
"This other noise is sometimes called the 'continuous noise' because it is not quantized like the quantal noise."
It's not easy to distinguish the two types of noise, so most scientists have in the past just lumped them into an equivalent measure of noise (the 6,400 false events). "Sometimes it's OK not to treat the two types of noise as separate," Yau said, "but other times it's important to separate them, as we have done."
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