There's more to sniffing than meets the eye.
The world is a complicated place, which is why our skulls house a particularly complex piece of equipment to make sense of it.
Because our brains do such a wonderful job of unraveling the information from our senses and stitching it into a seamless narrative, it is fairly easy to forget what an incredible job it is doing.
From a cacophony of signals, our senses are able to pick out the important information and bring it to our attention.
Of all the senses, olfaction often gets the least attention. Despite being our oldest sense, we rely on it much less than vision and hearing. However, it is an intriguing area of study and can offer insight into the way that our brains code and relay information in general.
The sniffing conundrum
Imagine, if you will, walking into a chocolate shop and sniffing the air. Each aromatic component of the chocolatey aroma activates specific neurons in your nose. These will converge on glomeruli, which are structures on the olfactory bulb. In this way, they create a specific "chocolate" pattern.
At the same time, as we sniff, the mechanical pressure that is caused by the air entering our nose also triggers glomeruli. But the air triggers both chocolate and non-chocolate receptors. Until recently, it was unclear how the brain could differentiate between the two signals.
When nerves fire, they essentially send out an identical message each time, which are called action potentials. So, it was difficult to see how the brain could determine whether the action potential was coming from the airflow or the sweet, sweet chocolate.
Researchers from the RIKEN Center for Developmental Biology, led by Takeshi Imai, recently attacked this question head-on. They discovered that the answer lies in the timing.
"Surprisingly, we found that temporal firing patterns of neurons can distinguish between airflow-driven mechanical signals and those generated by odors. Not only that, we discovered that the mechanosensation actually improves olfaction by acting as a pacemaker for temporal patterning."
Their results were recently published in the journal Neuron.
Visualizing neuronal responses to a sniff
To gain this new insight, they researchers designed a way to control rhythmic sniffing in mice. They found that when the mice were presented with deodorized (aroma-free) air, lots of glomeruli were activated by the airflow alone. The activity ebbed and flowed in cycles that matched the rate of sniffing.
They identified that although the rate of activity was the same, the glomeruli were all out of phase with each other. So, one might be triggered 200 milliseconds after each sniff, another at 230 milliseconds, and another at 400 milliseconds. The pulses were of the same duration, but the activation was staggered.
The following video illustrates the activity rate of glomeruli as the mice sniffed:
As the scientists increased the airspeed, glomeruli activity also hotted up. However, the so-called phase coding remained similar, in that the firing patterns were more intense but still staggered in the same way.
But when an odor was presented to the mice, the timing of glomerulus activity shifted substantially within the sniff cycle. And interestingly, regardless of how strong the aroma was, the phase was shifted the same amount.
The following video shows the glomerular firing pattern with and without an aroma:
The importance of phase code
The next question to answer was why the receptors in the nose are sensitive to airflow at all. In order to investigate this, the team maintained a constant airflow (no pulsing air as you would get when sniffing) and found that the precision of the phase code was diminished, especially if there were only traces of the odor. This would make it much more difficult to tell odors apart.
Phase coding is a relatively poorly understood phenomenon in the field of neural coding — that is, how neurons respond and translate stimuli into signals.
"Although it has also been found in the hippocampus in relation to memory formation," Imai says, "we still do not know much about it.
"Hopefully," he continues, "our finding will facilitate a better understanding of how neurons communicate with each other and how meaning can be derived from their signals."
There is still much to learn about phase coding. Next, Imai wants to "understand how the precise temporal patterns are generated in the olfactory bulb, and why they are affected by odors but not mechanically originating signals."
Olfaction still holds a great many mysteries, but this work provides a small piece of the puzzle. It also helps to lift the lid on how neurons transmit detailed information with such a simple language.