title: Diverse Representations of Olfactory Information in Centrifugal Feedback Projections.
abstract: Although feedback or centrifugal projections from higher processing centers of the brain to peripheral regions have long been known to play essential functional roles, the anatomical organization of these connections remains largely unknown. Using a virus-based retrograde labeling strategy and 3D whole-brain reconstruction methods, we mapped the spatial organization of centrifugal projections from two olfactory cortical areas, the anterior olfactory nucleus (AON) and the piriform cortex, to the granule cell layer of the main olfactory bulb in the mouse. Both regions are major recipients of information from the bulb and are the largest sources of feedback to the bulb, collectively constituting circuits essential for olfactory coding and olfactory behavior. We found that, although ipsilateral inputs from the AON were uniformly distributed, feedback from the contralateral AON had a strong ventral bias. In addition, we observed that centrifugally projecting neurons were spatially clustered in the piriform cortex, in contrast to the distributed feedforward axonal inputs that these cells receive from the principal neurons of the bulb. Therefore, information carried from the bulb to higher processing structures by anatomically stereotypic projections is likely relayed back to the bulb by organizationally distinct feedback projections that may reflect different coding strategies and therefore different functional roles.
  
  
  
  Principles of anatomical organization, sometimes instantiated as "maps" in the mammalian brain, have provided key insights into the structure and function of circuits in sensory systems. Generally, these characterizations focus on projections from early sensory processing areas to higher processing structures despite considerable evidence that feedback or centrifugal projections often constitute major conduits of information flow. Our results identify structure in the organization of centrifugal feedback projections to the olfactory bulb that is fundamentally different from the organization of feedforward circuits. Our study suggests that understanding computations performed in the olfactory bulb, and more generally in the olfactory system, requires understanding interactions between feedforward and feedback "maps" both structurally and functionally.
authors.affiliation: Crick-Jacobs Center for Theoretical and Computational Biology, Computational Neurobiology Laboratory, Department of Neuroscience, University of Rochester School of Medicine, Rochester, New York 14642 krishnan_padmanabhan@urmc.rochester.edu.
authors.forename: Krishnan
authors.initials: K
authors.lastname: Padmanabhan
authors.affiliation: Systems Neurobiology Laboratories, Laboratory of Cellular Pharmacology, Graduate School of Pharmaceutical Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan, and.
authors.forename: Fumitaka
authors.initials: F
authors.lastname: Osakada
authors.affiliation: Laboratory of Genetics, and.
authors.forename: Anna
authors.initials: A
authors.lastname: Tarabrina
authors.affiliation: Laboratory of Genetics, and.
authors.forename: Erin
authors.initials: E
authors.lastname: Kizer
authors.affiliation: Crick-Jacobs Center for Theoretical and Computational Biology, Systems Neurobiology Laboratories.
authors.forename: Edward M
authors.identifier: 0000-0002-6366-5267
authors.initials: EM
authors.lastname: Callaway
authors.affiliation: Laboratory of Genetics, and.
authors.forename: Fred H
authors.identifier: 0000-0002-0938-4106
authors.initials: FH
authors.lastname: Gage
authors.affiliation: Crick-Jacobs Center for Theoretical and Computational Biology, Computational Neurobiology Laboratory, Howard Hughes Medical Institute, Salk Institute for Biological Studies, La Jolla, California 92037.
authors.forename: Terrence J
authors.initials: TJ
authors.lastname: Sejnowski
mesh_terms: D000818:Animals; D001931:Brain Mapping; D016000:Cluster Analysis; D007839:Functional Laterality; D006023:Glycoproteins / Q000378:metabolism; D021621:Imaging, Three-Dimensional; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D009830:Olfactory Bulb / Q000166:cytology* / Q000000981:diagnostic imaging; D066194:Olfactory Cortex / Q000000981:diagnostic imaging / Q000502:physiology*; D009833:Olfactory Pathways / Q000000981:diagnostic imaging / Q000502:physiology*; D011984:Sensory Receptor Cells / Q000502:physiology*; D012903:Smell; D014161:Transduction, Genetic
chemical_list: D006023:Glycoproteins; D008164:Luminescent Proteins
keywords: centrifugal; circuit maps; feedback; olfaction; olfactory bulb; piriform cortex