The Brain Knows What the Nose Smells, but How?
The sense of smell is a strange one, and its ties with memory even stranger. How does a single sniff of a scent alert even the tiniest animal immediately to which predator is nearby? How does a human know with a single whiff of an ardour, precisely what to expect when they walk into the next room?
The sense of smell is a very complex process, and one which has been largely ignored by virtual reality efforts, except those that focus on stimulating the nose directly, such as the scent collar and scent dome, devices that release physical scents into the atmosphere below the nose, and hope it does its work.
The problem with these methods as interfaces into the virtual, is of course that the physical residue is out of the control of the virtual environment, and any actions taken within won't affect what is left lingering on the breeze. Worse, if the user has a stuffy nose, or no sense of smell, that translates into the VR experience, and the sense of immersion is broken.
The problem is, no one had really successfully disentangled the processes that lay behind the sense of smell. We knew it started with the scent being picked up by specific receptors in the nose, but this February it wasn't clear exactly how these scent signals proceeded from nose to noggin for neural processing.
In a study to be published in Nature, Stanford researchers describe a new technique that makes it possible to map long-distance nerve connections in the brain. The scientists used the technique to map for the first time the path that the scent signals take from the olfactory bulb, the part of the brain that first receives signals from odour receptors in the nose, to higher centres of the mouse brain where the processing is done.
"No one could trace signals across neural connections to a specific type of neuron at a specific location before," said biology Professor Liqun Luo. This is Luo's first study of the mouse olfactory system, but his lab has spent 10 years studying olfactory pathways in the fruit fly. Because mouse brains are so much larger and more complex that those of flies, Luo and postdoctoral researcher Kazunari Miyamichi had to develop an entirely new experimental technique.
Further of course, these techniques can be used for far more than just studying the sense of smell. Yet this was their initial use, with promise of much more to come.
During the work, the researchers discovered that most of the nerve pathways heading to the higher processing centres that direct the mice's innate like or dislike of certain odours, and trigger a response to them, originated from one region -- the top part of the olfactory bulb. In other words, the bulbs are more than simple scent detectors, and actually perform much of the preprocessing necessary to deal with the input, before sending it on to the brain.
This is a positive development for scent research as it strongly suggests that an artificial signal sent into the olfactory nerves would itself control much of the brain's responses, and could be utilised to bring a fuller immersion experience than just transmitting scent alone. Of course, there is still much work still to be done to determine if this is truly the case.
This is especially good news when looked at in partner to the other aspect of the study. The neurons heading to the brain areas which process an individual's responses to a given odour are scattered all over the olfactory bulb, with no detectable central organisation. This again suggests that the olfactory nerve is going to be the point best suited to targeting, especially since the researchers also found that each neuron in the brain's higher centres receives signals from at least four neurons in the olfactory bulb, each of which receives input from a large number of like odour receptors.
This progressive funnelling and processing helps explain how the brain integrates the information from many different odours, Luo said.
Luo said he will use the techniques in this study to take a more detailed look at other parts of the mouse olfactory bulb and brain, with the eventual goal of understanding how the brain processes specific odours.