Distinct information conveyed to the olfactory bulb by feedforward input from the nose and feedback from the cortex

Abstract

Sensory systems are organized hierarchically, but feedback projections frequently disrupt this order. In the olfactory bulb (OB), cortical feedback projections numerically match sensory inputs. To unravel information carried by these two streams, we imaged the activity of olfactory sensory neurons (OSNs) and cortical axons in the mouse OB using calcium indicators, multiphoton microscopy, and diverse olfactory stimuli. Odorant mixtures of increasing complexity evoked progressively denser OSN activity, yet cortical feedback activity was of similar sparsity for all stimuli. Representations of complex mixtures were similar in OSNs but were decorrelated in cortical axons. While OSN responses to increasing odorant concentrations exhibited a sigmoidal relationship, cortical axonal responses were complex and non-monotonic, which could be explained by a model with activity-dependent feedback inhibition in the cortex. Our study indicates that early-stage olfactory circuits have access to both local feedforward signals and global, efficiently formatted information about odor scenes through cortical feedback.