Who Nose? -- Nothing to Sniff at: Mammalian Brains and Distinguishing Odors

     "Neuroscientists have discovered that at least six types of mammals -- from mice to cats -- distinguish odors in roughly the same way, using circuitry in the brain that's evolutionarily preserved across species.

       The world is filled with millions distinct smells, but how mammals' brains evolved to tell them apart has been something of a mystery.

       Now, in a study published today, two neuroscientists from the Salk Institute and UC -- San Diego have discovered that at least six types of mammals distinguish odors in roughly the same way, using circuitry in the brain that's evolutionarily preserved across species.

      "The study yields insights into organizational principles underpinning brain circuitry for olfaction in mammals that may be applied to other parts of the brain and other species," says Dr. Charles Stevens, distinguished professor emeritus in the Salk Lab.

     In brief, the study reveals that the size of each of the three components of the neural network for olfaction scales about the same for each species, starting with receptors in the nose that transmit signals to a cluster of neurons in the front of the brain called the olfactory bulb which, in turn, relays the signals to a "higher functioning" region for odor identification called the piriform cortex.

     "These three stages scale with each other, with the relationship of the number of neurons in each stage the same across species," says Dr. Shyam Srinivasan, assistant project scientist with UC San Diego's Kavli Institute for Brain and Mind, and the paper's coauthor. "So, if you told me the number of neurons in the nose, I could predict the number in the piriform cortex or the bulb."

      The current study extends research by the same authors, published last year in which described how mouse brains process and distinguish odors using what's known as "distributed circuits." Unlike the visual system, for example, where information is transmitted in an orderly manner to specific parts of the visual cortex, the researchers discovered that the olfactory system in mice relies on a combination of connections distributed across the piriform cortex.

     Following that paper, the authors sought to determine if the distributed neural circuitry revealed in mice is similar in other mammals. For the current work, the researchers analyzed mammal brains of varying sizes and types. Their calculations, plus previous studies over the past few years, were used to estimate brain volumes. 

      The new study revealed that the average number of synapses connecting each functional unit of the olfactory bulb to neurons in the piriform cortex is invariant across species.

      "It was remarkable to see how these were conserved," says Dr. Stevens.

     Specifically, identification of individual odors is linked to the strength and combination of firing neurons in the circuit that can be likened to music from a piano whose notes spring from the depression of multiple keys to create chords, or the arrangement of letters that form the words on a page.

       "The discrimination of odors is based on the firing rate, the electric pulse that travels down the neuron's axon," says Srinivasan. "One odor, say for coffee, may elicit a slow response in a neuron while the same neuron may respond to chocolate at a faster rate."

      This code used for olfaction is different than other parts of the brain.

     "We showed that the connectivity parameters and the relationship between different stages of the olfactory circuit are conserved across mammals, suggesting that evolution has used the same design for the circuit across species, but just changed the size to fit the animals' environmental niche," says Dr. Stevens.

     In the future, Dr. Stevens plans to examine other regions of the brain in search of other distributed circuits whose function is based on similar coding found in this study.

     Dr. Srinivasan says he will focus on how noise or variability in odor coding determines the balance between discrimination and learning, explaining that the variability the duo is finding in their work might be a mechanism for distinguishing odors, which could be applied to making better machine learning or AI systems."

     Smell-o-vision, anyone?

Stay cool and not stinky,
Steph