The association between smells and places appears to be a deeply embedded aspect of human cognition. A new study published in the journal Nature has attempted to provide a potential explanation of how the connection between smell and space is expressed at the neuronal level.
“Odor molecules do not inherently carry spatial information. However, animals in the wild use odors for spatial navigation and memory, which allow them to locate valuable resources such as food,” said study first author Dr. Cindy Poo, a postdoctoral researcher in Neuroscience at the Champalimaud Research, Portugal.
“We wanted to understand the neural basis of these behaviors, and so we decided to study how the brain combines olfactory and spatial information.”
According to study senior author Dr. Zachary Mainen, the Director of the Champalimaud Neuroscience Program, olfaction is the only sense that has direct reciprocal connections to the hippocampus, a region of the brain that structures memory and spatial navigation.
Hippocampal neurons are functioning as “place cells,” each of them becoming active at different locations in the environment. “We know that the hippocampal system sends signals to the primary olfactory cortex,” said Dr. Poo. “So we suspected that this brain region might do more than just identify different smells.”
To test the relation between smell and place, Dr. Poo and her colleagues developed a puzzle, making rats sample odors at the four ends of a plus-shaped maze, and then finding hidden rewards. “In this task, the rats had to learn and remember exact associations of odors and locations,” Dr. Poo explained.
By monitoring the rats’ neuronal activity in the olfactory cortex, the scientists found that olfactory neurons can encode spatial maps. “Our results exceeded our expectations,” said Dr. Poo. “We had predicted that some neurons here might care about location to a certain degree. However, by carefully studying the activity of olfactory cortex neurons while the animal was navigating in the maze, we found that these neurons had learned an entire map of the environment.”
“This study also opens up a new window to understand how the senses are used for navigation and memory. Humans rely on visual landmarks more than odors, but it’s likely that the principles of how we remember where we’ve been and get to where we’re going are very similar,” concluded Dr. Mainen.