Lack of Pattern Separation in Sensory Inputs to the Olfactory Bulb during Perceptual Learning

Abstract

Recent studies revealed changes in odor representations in the olfactory bulb during active olfactory learning (Chu et al., 2016; Yamada et al., 2017). Specifically, mitral cell ensemble responses to very similar odorant mixtures sparsened and became more distinguishable as mice learned to discriminate the odorants over days (Chu et al., 2016). In this study, we explored whether changes in the sensory inputs to the bulb underlie the observed changes in mitral cell responses. Using two-photon calcium imaging to monitor the odor responses of the olfactory sensory neuron (OSN) axon terminals in the glomeruli of the olfactory bulb during a discrimination task, we found that OSN inputs to the bulb are stable during discrimination learning. During one week of training to discriminate between very similar odorant mixtures in a Go/No-go task, OSN responses did not show significant sparsening, and the responses to the trained similar odorants did not diverge throughout training. These results suggest that the adaptive changes of mitral cell responses during perceptual learning are ensured by mechanisms downstream of OSN input, possibly in local circuits within olfactory bulb.

Significance Statement Odor representations in the rodent olfactory bulb have been demonstrated to undergo dynamic changes during a variety of olfactory experiences. In particular, odor representations in mitral cells, the primary projection neurons of the olfactory bulb, become more different (pattern separation) when mice are trained to distinguish between two similar odor mixtures. Here we address whether similar changes occur in olfactory sensory neurons, which provide input onto mitral cells. We find that olfactory sensory neuron odor representations do not exhibit pattern separation when mice are trained to discriminate between two similar odor mixtures, suggesting that the changes likely occur downstream of olfactory sensory neurons during this task.