Sound engineers know how to use gain control to avoid “redlining” or saturating the signal while amplifying weak but important signals. Your nose knows that trick, too.
The sense of smell is complex because of the tremendous variety of odorant molecules that must be interpreted. Molecules that trigger signals in the initial neurons trigger a cascade of responses that pass through a chain of olfactory organs and neurons before reaching the brain. Some of these organs “format” the signal for the brain, explained Harvard Professor Venkatesh Murthy in a Harvard press release, “Sniff mechanics,” posted also on Science Daily. Murthy’s team achieved a new level of precision to identify the functions in the olfactory pathway.
Reaching that level of precision was critical, Murthy explained, because while the olfactory bulb contains many “principal” neurons that are responsible for sending signals to other the parts of the brain, it is also packed with interneurons, which appear to play a role in formatting olfactory information as it comes into the brain.
But the complexity doesn’t stop there. There’s not only formatting, but feedback:
“The image of the brain as a linear processor is a convenient one, but almost all brains, and certainly mammalian brains, do not rely on that kind of pure feed-forward system,” Murthy said. “On the contrary, it now appears that the higher regions of the brain, which are responsible for interpreting olfactory information, are communicating with lower parts of the brain on a near-constant basis.”
Using advanced techniques, Murthy’s team identified that the interneurons receive messages back from the brain – feedback. This feedback serves as a method of gain control:
“If you make a system that is very good at detecting weak signals, it becomes saturated as the signal gets stronger, and eventually it’s impossible to differentiate between strong signals,” Murthy said. “To avoid that problem, brain circuits use a process called gain control. By inhibiting certain neurons, it ensures that you stay within the detection range, so you don’t miss the weak things, but you don’t miss the very strong things either.”
The brain can effectively turn down the gain knob on certain neurons, like a sound engineer at a mixing console subduing an input that is too loud, while simultaneously turning up the gain for inputs that are too weak. The brain can say, “I heard you” to one input while switching attention to weaker signals. Other senses use gain control, but “the extent of the olfactory feedback was surprising,” the press release said.
There’s also gain control at the receiving end. The olfactory bulb can turn up the gain for weak but important signals:
Even more surprising, Murthy said, was evidence that the olfactory bulb’s principal neurons were also receiving feedback signals — albeit weak ones — that appeared to prime them for incoming signals.
“These weak connections help the principal neuron get over the top when it’s listening to weak inputs,” Murthy said. “If there’s a weak smell coming in, but it’s not able to drive the principal neuron over the threshold to signal the rest of the brain, but say you’re in an environment where you’re primed to smell that weak smell — we believe this feedback from this higher area of the brain is sort of tickling these principal neurons, so when there’s a weak input you’re able to smell it.
Murthy recognized that the sense of smell is very important for many animals. A weak smell may signal danger, but it must not be overpowered by other strong odors in the environment. That’s where gain control is essential, Murthy explained: “We are hypothesizing that this mechanism, where the cortex is talking back to the olfactory bulb and suppressing neurons, through this feedback they may be able to detect that weak signal.”
Isn’t it wonderful what the blind, impersonal forces of nature evolved. Without any foresight, millions of mutations brought forth a finely-tuned mechanism, like a mixing console, and a brain to operate it. In the imagination of the evolutionist, all things are possible. Close your eyes and speculate.
Something about that mindset has a foul odor. It must be Darwin of the Gaps.
Fortunately, in this press release there was no mention of evolution. Hats off to Murthy for sparing us the foul odor and giving us another wonder of the human body that has the sweet smell of intelligent design, even without the sight of those words in print.