Scientists have long believed that the path from smell to perception was scattered randomly throughout the brain. However, a recent study from Cold Spring Harbor University, USA found that smell signals were sent across the brain in an organised manner through specific pathways, much like sight and hearing mechanisms.
“The sense of smell has been looked at for a long time. It so happened that those tools that people have used in the past few decades were limited by throughput,” says Dr Dinu Florin Albeanu, associate professor at the Cold Spring Harbor University, USA and one of the study’s investigators.
Advanced screening methods previously available were limited in their capacity to scan neurons simultaneously in various brain regions. Hence, scientists had difficulty obtaining a complete map of the relevant smell pathways, leading them to believe that smell perception was dispersed throughout the brain.
A three-point network
In the present study, the researchers overcame this limitation by developing a unique DNA-based mapping technology that could trace over 1000 neurons simultaneously. Their experiments in mice revealed that smell traverses through a triangular closed circuit involving three areas: the olfactory bulb, the piriform cortex and a third unit comprising three other regions associated with smell memory and perception.
The olfactory bulbs, just above the nasal cavity, is the first point of contact in the olfaction (smell) process. The piriform cortex is the master processing centre of smell, followed by the three regions that respectively process, locate and store odours as memories, and complete the smell circuit.
In the present study researchers made the mice inhale an odour, which activated the olfactory bulb. Neurons then passed the message to the piriform cortex before relaying them to the final smell processing regions. That was not all. The researchers observed that smell processing also happened via a direct pathway from the olfactory bulb to these same regions (the third zone), completing the triangular circuit.
“The piriform cortex is able to process the information in parallel, allowing for some crosstalk within the different parts of the brain,” says Dr Albeanu. This may facilitate keeping information separate and combining it when needed, he adds.
The researchers also studied the effect of the piriform cortex’s structure on smell transmission to the other regions. They observed that the neurons in the piriform cortex were distributed in a way that enabled a specific array of them to relay information to each of the regions (in the third area) involved in recollection of smell. “This in turn determines how information of smell is broadcast to other brain regions,” explains Dr Albeanu.
These findings could aid in understanding how the brain perceives smell. “It could also help us understand what happens when we lose our sense of smell, and why is it that our perception of the world is so affected by it,” says Dr Albeanu.