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Updated: Jan 9, 2026
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Quadruple Immunostaining of the Olfactory Bulb for Visualization of Olfactory Sensory Axon Molecular Identity Codes
Published on: June 5, 2017
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Order code in the olfactory system
Khristina Samoilova1,2, Joshua S Harvey3, Hirofumi Nakayama3
1Cold Spring Harbor Laboratory, Cold Spring Harbor, NY.
Biorxiv : the Preprint Server for Biology
|December 3, 2025
Summary
Odor identity recognition relies on two brain coding strategies: primacy and order codes. These olfactory bulb mechanisms help identify smells across varying concentrations and animals.
Area of Science:
- Neuroscience
- Olfactory System Research
- Sensory Coding
Background:
- Recognizing odor identity across concentrations is crucial for olfactory behaviors.
- The representation of odor identity in the early olfactory system is not fully understood.
- Two theories, order coding and primacy coding, propose mechanisms for odor identity representation.
Purpose of the Study:
- To investigate how odor identity is represented in the mouse olfactory bulb.
- To compare the predictive power of the order code and primacy coding theories.
- To elucidate the coding strategies employed by the olfactory system.
Main Methods:
- Calcium imaging was used to measure glomerular responses to odorants in the mouse olfactory bulb.
- Receptor affinities were analyzed in a low-dimensional space.
- The order code and primacy models were compared for predicting odor identity across concentrations and animals.
Main Results:
- Two distinct clusters of glomeruli with independent odor representations were identified.
- Odorants evoked orderly activation waves, supporting the order coding model.
- The primacy model showed comparable performance to the order code in predicting odor identity.
- Odor mixture responses lay along geodesic lines connecting component odors.
Conclusions:
- Odor information in the olfactory bulb is likely represented by two complementary strategies: primacy and order codes.
- The findings suggest a sophisticated coding system within the olfactory bulb.
- Both theories contribute to understanding how the brain processes olfactory information.
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