Summary: Centrifugal fibers that send signals from other parts of the central nervous system to early sensory regions of the brain play a pivotal role in how odors are processed.
Source: University of Rochester
What happens in the brain when you smell a rose? Does the olfactory system create a single instantaneous picture of the scent, or does it assemble a dynamic sequence of neural activity over time? New modeling work indicates the brain can use both approaches, switching between them depending on top-down input.
“These results point to a general principle of nervous system function: flexibility in the computations the brain uses to represent sensory information,” said Krishnan Padmanabhan, Ph.D., associate professor of Neuroscience and senior author of the study published in Cell Reports.
The research team built a detailed computational model of the early olfactory system—the circuitry that links incoming odor signals to cortical representations—and used large-scale simulations to explore how different circuit elements shape odor coding.
The simulations identified a set of centrifugal fibers, which convey signals from higher brain regions back to early sensory areas, as a crucial control mechanism. These feedback fibers act like a switch, enabling the olfactory network to adopt distinct coding strategies for the same odor input.
When the centrifugal fibers favored one configuration, neurons in the piriform cortex—the cortical region where odor perception is formed—relied primarily on the instantaneous pattern of activity: a spatial snapshot of which cells were active during a sniff.
In the alternate configuration, feedback changed the temporal dynamics of the network so piriform cortex neurons made use of evolving patterns of activity across time. In that mode, the cortex improved both the speed and accuracy of odor detection and discrimination by integrating how activity rose and fell across multiple moments.
Put simply, the early olfactory system can operate like a still photograph, capturing a single combination of active cells, or like a musical performance, tracking the order and timing of cells turning on and off. The centrifugal feedback determines which strategy is emphasized at a given time.
The models show that top-down feedback reshapes activity early in the system by controlling inhibition in the main olfactory bulb. By influencing the balance of excitation and inhibition in mitral cells, feedback alters the temporal structure of signals reaching the piriform cortex and therefore the form of the cortical representation.
This flexible mechanism helps explain why different experiments have emphasized either snapshot-like combinatorial representations or temporally structured coding in olfaction: both strategies can be valid, and the circuit can switch between them through feedback control.
“Mathematical modeling gives us a framework to quantify and interpret the patterns of neural activity we see experimentally,” Padmanabhan said. “Understanding how feedback flexibly sculpts sensory representations may also inform engineered systems that take inspiration from the brain.”
The authors note that computational ideas derived from brain circuits—like adaptable coding strategies that emphasize spatial or temporal features—could inspire applications in artificial systems, for instance improving object recognition or robustness in machine perception.
About this olfaction research news
Author: Press Office
Source: University of Rochester
Contact: Press Office – University of Rochester
Image: The image is in the public domain
Original Research: Open access.
“Top-down feedback enables flexible coding strategies in the olfactory cortex” by Zhen Chen et al. Cell Reports
Abstract
Top-down feedback enables flexible coding strategies in the olfactory cortex
Highlights
- Feedback alters the temporal structure of piriform cortical activity
- Top-down signals reshape information delivered to piriform cortex by modifying olfactory bulb output
- Feedback changes how glomerular activation patterns are represented in cortex
- Feedback-mediated changes improve performance in odor discrimination tasks
Summary
Research on odor representation has proposed competing models: one emphasizes the combinatorial identity of neurons active within a brief sniff-related window, while another stresses the importance of the temporal sequence of neural activity. The current study reconciles these perspectives by showing that top-down feedback to the olfactory bulb can switch the system between these coding regimes.
Using a detailed network model, the authors demonstrate that feedback control of inhibition in the bulb changes the excitation–inhibition balance of mitral cells, which in turn alters the dynamics of piriform cortical neurons. By restructuring these dynamics, feedback can enhance the brain’s ability to detect and distinguish odors.
These findings provide a coherent framework for early olfactory computation: top-down feedback flexibly sculpts the timing of cortical activity, allowing the olfactory system to dynamically toggle between spatial snapshot coding and temporally structured coding depending on task demands or behavioral state.