State-dependent representations of mixtures by the olfactory bulb

Abstract

Sensory systems are often tasked to analyse complex signals from the environment, separating relevant from irrelevant parts. This process of decomposing signals is challenging when a mixture of signals does not equal the sum of its parts, leading to an unpredictable corruption of signal patterns. In olfaction, nonlinear summation is prevalent at various stages of sensory processing. Here, we investigate how the olfactory system deals with binary mixtures of odours under different brain states, using two-photon imaging of olfactory bulb (OB) output neurons. Unlike previous studies using anaesthetised animals, we found that mixture summation is more linear in the early phase of evoked responses in awake, head-fixed mice performing an odour detection task, due to dampened responses. Despite this, and responses being more variable, decoding analyses indicated that the data from behaving mice was well discriminable. Curiously, the time course of decoding accuracy did not correlate strictly with the linearity of summation. Further, a comparison with naïve mice indicated that learning to accurately perform the mixture detection task is not accompanied by more linear mixture summation. Finally, using a simulation, we demonstrate that, while saturating sublinearity tends to degrade the discriminability, the extent of the impairment may depend on other factors, including pattern decorrelation. Altogether, our results demonstrate that the mixture representation in the primary olfactory area is state-dependent, but the analytical perception may not strictly correlate with linearity in summation.

Reward contingency modulates olfactory bulb output via pathway-dependent peri-somatic inhibition

“…4B ). These two paradigms decouple the odour-reward association, while inducing different levels of engagement in the head-fixed mice 31 . Imaging sessions took place after the mice, previously trained on the fine discrimination task, were switched to, and experienced at least one session of the new paradigm ( Fig.…”

Value-related learning in the olfactory bulb occurs through pathway-dependent perisomatic inhibition of mitral cells

“…In the second case (“Random association”), we delivered the water on randomly selected trials, so that both 60/40 and 40/60 odour mixtures were followed by water 50% of the time ( Fig 4B ). These 2 paradigms decouple the odour-reward association, while inducing different levels of engagement in the head-fixed mice [ 40 ]. Imaging sessions took place after the mice, previously trained on the difficult discrimination task, were switched to, and experienced at least 1 session of the new paradigm ( Fig 4C ).…”

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

“…While previous studies point to normalization of olfactory responses in the PC 38 , several key features remain unknown. For instance, even though the olfactory environment consists of a complex mixture of chemicals, we currently lack an understanding of how cortical neurons represent realistic odorant mixtures, which elicit complex interactions even in the OSNs 24 , 39 – 43 . Importantly, in the context of feedback, it is unclear which of the different computations ascribed to the PC, related to odorant identity 25 , 44 , quality 45 , attention 46 , and predictive coding 47 , are conveyed back to the OB.…”