Representation of Plume Dynamics by Mitral and Tufted Cells in the First Relay of Mouse Olfactory Processing

Abstract

In ethologically relevant environments odor molecules travel in plumes. As a result, an odor does not diffuse slowly through the air, but rather is pulled and pushed stochastically away from its source in discrete filaments. From the perspective of an olfactory searcher, filaments are experienced as discrete whiffs of odor. The frequency, strength, and duration of whiffs contain information regarding an odor and the location of its source. Thus, the spatiotemporal dynamics of plumes are theoretically informative tools for olfactory search, but it is unclear how these complex cues are used by mice to create internal representations of their odor environments. In the past, recording from plumes without disruption their dynamics has been difficult. Thus, simultaneously obtaining neural recordings of olfactory processing in the first olfactory relay, the olfactory bulb (OB), and odor recordings of the concentration of natural stimulus as it changes over time has been difficult. This dissertation investigates methods of exploring the response of the OB to natural odor scenes. We used wide-field imaging to examine the collective responses of mitral and tufted cell (MTC) activity in the dorsal OB. We segmented MTC activity into glomerular complexes and found that some complexes were significantly correlated with odor concentration across plume encounters. The magnitude of this correlation with plume dynamics, or plume following ability, was positively associated with more reliable responses and stronger response dynamics. Thus, these recordings determine that plume dynamics play an important role in structuring information processing in the OB. We also used new high-density Neuropixel 2.0 probes to perform electrophysiological recordings of OB activity at the cellular level during plume presentation, establishing a method for examining how this plume following behavior of glomeruli related to the individual cells that make up glomeruli. This approach could help determine if observed plume following in wide-field recordings is due to similarly tuned MTCs or is the collective result of MTCs heterogeneously tuned to dynamic features of plumes. For the Neuropixels recordings we also propose the use of correlated changes between different LFP frequency domains to locate the position of the mitral cell layer (MCL) along the electrode array. Theta oscillations, gamma oscillations, and spiking activity all have characteristic changes observable near the mitral cell layer (MCL). Thus, the MCL was located by identifying coincident locations where theta and gamma power dipped and spiking power began to rise. The switch from isolated features to natural odor scenes has been important in the study of visual processing. In this dissertation we study neural responses in the first olfactory relay in mice as they process the full complexity of natural odor scenes, plumes.