Surprising Codes for Innate Behavior
A baby mouse knows to run when it smells a fox, even if the animal has never encountered foxes before. But mice can also learn to respond to novel odors — they might figure out that the scent of their caretaker’s perfume signals that dinner is on the way. Two different parts of the brain mediate these different odor-induced behaviors. A brain area known as the posterolateral cortical amygdala is linked to innate behaviors, while a region called the piriform cortex drives odor learning.
These two regions have very different connectivity patterns. Inputs from olfactory bulb neurons are distributed across the piriform cortex, while connections to the cortical amygdala follow a more stereotyped pattern. “It looks like information is spread out all over piriform cortex,” says Sandeep Robert Datta, a neuroscientist at Harvard University and Simons Collaboration on the Global Brain investigator. “But in the cortical amygdala, it looks like anatomical hardwiring,” supporting a role in innate behavior.
Given these differences, researchers predicted that the piriform cortex and cortical amygdala would encode olfactory information differently. But new research published in Neuron in February shows that the two brain areas encode information in very similar ways. The findings suggest that innate behavior might be more flexible than scientists expected. “We hypothesize that wiring into the [cortical amygdala] might be hardwired at birth but still capable of plasticity as the animal learns to associate odors with outcomes,” Datta says.
Scientists have known for several years that the piriform cortex represents odors via population codes—groups of cells that fire together in response to a specific stimulus. But no one had done corresponding experiments in the cortical amygdala. Given its wiring patterns and role in innate behavior, researchers predicted that the cortical amygdala would be hardwired to trigger specific behaviors, instructing the animal to flee when it smelled foxes, for example. “That was an organizing assumption of the field,” Datta says. “But no one knew how information was organized in this part of the brain and how it generates innate behavior.”
In the new study, Datta and collaborator Giuliano Iurilli found that the cortical amygdala also uses population codes — different odors trigger activity in specific ensembles of neurons. “This was true regardless of whether it was a single odorant or a mixture,” Datta said. “Every odor, whether or not it has an innate behavioral meaning, activates a similarly sized population of neurons, and every one of these ensembles is equally de-correlated from other ensembles.”
Datta proposes that the cortical amygdala functions like a switchboard, with both hardwired and flexible components. Mice are born with the switches in certain positions — that fox odor is aversive, for example — but new information can flip those switches. Researchers are now looking for experimental proof of this idea. “We expect that activity in the cortical amygdala will change as the animal learns that fox odor is attractive under rewarding conditions in the lab,” he says.
The findings highlight the brain’s complexity in dealing with even apparently simple issues like innate behaviors. One of the attractions of studying innate behavior is based on the idea that hardwiring will reveal how signals in the environment get turned into behavior, Datta says. “I think there will be subtleties to this kind of wiring that reveals it’s not so simple. It will be an interesting challenge to figure out how to link information in the population codes to innate behaviors.”
