Scientists used new techniques to trace the process from smell to brain

Our sense of smell has a great influence on
our behavior and mood.
Scents can evoke vivid memories of the past and can also remind us of a smouldering fire
.
For neuroscientists, however, the sense of smell remains the most mysterious of our five senses
.

Once the nose detects something, how does the brain judge its meaning? Scientists don't know for sure
.
To help them find out, researchers at Cold Spring Harbor Laboratory (CSHL), Florin Albeanu, Alexei Koulakov, and Anthony Zador have created a new map
of the brain's olfactory circuitry.

"At this point, how do we deal with odors, that's an open question," Albeanu said
.
"What features does the brain extract and use in the smell space to create the perception of olfactory objects?" What are the mechanisms in the brain?"

Because previous studies of the olfactory cortex failed to uncover any logical organization between neurons, many neuroscientists suspect that information about odors is randomly transmitted through the
brain.
But these studies only examined the connectivity patterns
of a few dozen neurons.

Using new DNA-based brain localization techniques developed in Zador's lab, called MAPseq and BARseq, the CSHL team can trace the paths of more individual olfactory processing neurons than ever before — one mouse has thousands of neurons
in its brain.
Thanks to their work, scientists may be able to understand olfactory circuits and their underlying logic
.

The new map depicts how
sensory information is transmitted between olfactory processing parts of the brain.
These areas include the olfactory bulb (responsible for receiving sensory information from the nose), the main olfactory processing center known as the piriform cortex, and several other brain regions
that receive input from the olfactory bulb.

The team found that in the piriform cortex, neurons located in the front of the brain have different connection patterns
than neurons located in the back of the brain.
"As you move along this axis, you see a gradual change
in the neurons' projection patterns in how they propagate information to other areas of the brain," Koulakov said.
"It's
synchronized with the way the olfactory bulb projects into these brain regions as well as the same location in the piriform cortex," he explains.

Each parallel "circuit pattern" may process a different aspect of
odor information.
This is reminiscent of what neuroscientists have found in other parts of the brain, where the connections and locations of neurons correspond
to specific features of sensory input such as sight and sound.
In the auditory system, the position of neurons along the axis is related to
the frequency of the sound we hear.
Similarly, in the visual system, the position of neurons transmits information about the position of the object seen, among other features, and different neural circuits are tuned to the position and identity of the object ("where" vs.
"what" path).

Researchers say olfactory maps could provide a breakthrough for "the last frontier of sensory neuroscience
.
" This, they argue, suggests the existence of different neural circuits dedicated to assessing the properties of the smell, how pleasant it is, or where it comes from, and how to act
on it.
Albeanu said: "It puts us in a very different mental state
from the field in a way.
This is a step
towards understanding the nature of olfactory processing.
”

High-throughput sequencing of single neuron projections reveals spatial organization in the olfactory cortex