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Odor representations in olfactory cortex

Abstract

Events in our external world are transformed into internal percepts and experiences. How can we begin to understand this transformation? It starts with the stimulation of peripheral sensory organs and ultimately requires the holistic synthesis of ensemble neural activity in the cortex. The cortex is composed of circuits connecting diverse types of neurons across various functional brain regions. In order to fully understand the cortical representation of an external stimulus, we must dissect out basic components of the circuit and characterize their stimulus response properties. We study neural circuits in primary olfactory (piriform) cortex of rodents using electrophysiological recordings in brain slices and < italic>in vivo. We examine the underlying synaptic mechanisms of odor representations, and how it can be modulated. First, we demonstrate an early developmental critical period for plasticity and structural changes in principal neurons of the olfactory cortex in response to sensory afferent activity. Next, we use in vivo recordings to demonstrate that synaptic inhibition is widely recruited by odor stimuli, whereas synaptic excitation is more selective in principal neurons. We show that the recruitment of both local interneurons as well as intracortical excitatory connections serve to shape odor-evoked synaptic activity. Synchronous beta (15-30 Hz) oscillations between the olfactory cortex and bulb are thought to be important for odor discrimination. We find that odor-evoked synaptic currents in principle cells of olfactory cortex are couple to beta frequency oscillations in the local field potential. A time window between oscillatory synaptic excitation and inhibition restricts the spike timing of odor-evoked spikes. Beta frequency oscillations have been shown to require a centrifugal feedback loop from olfactory cortex back to the bulb. We use an optogenetic approach to reveal that cortical feedback projections provide inhibition onto mitral cells in the olfactory bulb via activation of granule cells. This work establishes a framework to understand basic components of odor representations in olfactory cortex, and a role for cortico-bulbar feedback loop. In combination with work done in the visual, auditory, somatosensory and gustatory cortices, this study contributes to a more complete understanding of sensory information processing by cortical circuits

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