Researchers at the Harvard University described how the olfactory system works in the brain and which neurons receive them, according to Medical Xpress report.
"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," Venkatesh Murthy, Professor of Molecular and Cellular Biology, explained, reported Medical Xpress. "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."
Feedback system was a known fact for over a century, however, no one knew which neurons of the olfactory bulb receive the feedback signals. "One of the challenges with this type of research is that these feedback neurons are not the only neurons that come back to the olfactory bulb," Murthy added. "The challenge has always been that there's no easy way to pick out just one type of neuron to activate."
A new method of using a virus which is genetically modified to produce a light sensitive protein was used and this method is known as "Optogenetics." Murthy was accompanied by three other scientists from his lab: Foivos Markopoulos, Dan Rokni and David Gire. The team marked the neurons which were activated with laser light. They then tracked the feedback mechanism from the brain to the olfactory bulb, reports Medical Xpress.
Murthy also noted the formatting process which is carried by the neurons which helps brain in identifying different range of signals. He said the olfactory bulb contains many principal neurons which send signals to the brain and also interneurons which help in formatting olfactory information which is carried out to the brain.
"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 [Peter: there can be both automatic as well as selective 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.
"When the cortical area decides to send these signals back to the olfactory bulb, it's effectively turning down the activity of these principal neurons," he added. "Why does the brain do this? Our theory is that the feedback is a way for the cortex to say, 'I heard you.' As the olfactory information is sent to higher regions of the brain, these signals come back and turn down the volume on the input."
Murthy's research shed light on some unknown facts such as the brain not only sends signals to the olfactory bulb, but also to the first layer of neurons.
"These weak connections help the principal neuron get over the top when it's listening to weak inputs," Murthy said in a report published in Medical Xpress. "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."
