Summary: Centrifugal fibers which carry impulses from parts of the central nervous system to early sensory regions of the brain play a critical role in olfactory processing.
Source: University of Rochester
What happens when we smell a rose? How does our brain process the essence of its fragrance? Is it like a painting—a snapshot of the flickering activity of cells—captured in a moment in time? Or like a symphony, an evolving ensemble of different cells working together to capture the scent? New research suggests that our brain does both.
“These findings reveal a core principle of the nervous system, flexibility in the kinds of calculations the brain makes to represent aspects of the sensory world,” said Krishnan Padmanabhan, Ph.D., an associate professor of Neuroscience and senior author of the study recently published in Cell Reports.
“Our work provides scientists with new tools to quantify and interpret the patterns of activity of the brain.”
Researchers developed a model to simulate the workings of the early olfactory system—the network the brain relies on for smelling.
Employing computer simulations, they found a specific set of connections, called centrifugal fibers, which carry impulses from other parts of the central nervous system to the early sensory regions of the brain, played a critical role. These centrifugal fibers act as a switch, toggling between different strategies to efficiently represent smells.
When the centrifugal fibers were in one state, the cells in the piriform cortex—where the perception of an odor forms—relied on the pattern of activity within a given instant in time.
When the centrifugal fibers were in the other state, the cells in the piriform cortex improved both the accuracy and the speed with which cells detected and classified the smell by relying on the patterns of brain activity across time.
These processes suggest the brain has multiple responses to representing a smell. In one strategy, the brain uses a snapshot, like a painting or a photograph, at a given moment to capture the essential features of the odor. In the other strategy, the brain keeps track of the evolving patterns. It is attuned to which cells turn on and off and when—like a symphony.
The mathematical models the researchers developed highlight the critical feature of the nervous system—not only diversity in terms of the components that make up the brain but also how these components work together to help the brain experience the world of smell.
“These mathematical models reveal critical aspects of how the olfactory system in the brain might work and could help build brain-inspired artificial computing systems,” Padmanabhan said.
“Computational approaches inspired by the circuits of the brain such as this have the potential to improve the safety of self-driving cars, or help computer vision algorithms more accurately identify and classify objects in an image.”
Top-down feedback enables flexible coding strategies in the olfactory cortex
Feedback shapes temporal structure of piriform cortical cell activity
Feedback controls information to piriform cortex by restructuring bulb output
Feedback reshapes cortical representation of glomerular activation pattern
Feedback improves behavioral performance in odor discrimination task
In chemical sensation, multiple models have been proposed to explain how odors are represented in the olfactory cortex. One hypothesis is that the combinatorial identity of active neurons within sniff-related time windows is critical, whereas another model proposes that it is the temporal structure of neural activity that is essential for encoding odor information.
We find that top-down feedback to the main olfactory bulb dictates the information transmitted to the piriform cortex and switches between these coding strategies.
Using a detailed network model, we demonstrate that feedback control of inhibition influences the excitation-inhibition balance in mitral cells, restructuring the dynamics of piriform cortical cells. This results in performance improvement in odor discrimination tasks.
These findings present a framework for early olfactory computation, where top-down feedback to the bulb flexibly shapes the temporal structure of neural activity in the piriform cortex, allowing the early olfactory system to dynamically switch between two distinct coding models.