We all know how emotive smells can be; just the mere whiff of candy floss or plasticine can transport you back decades in a moment.
But how does our olfactory system recognise odours even at very low doses?
Image: Mitral and tufted cells. © Brain Institute and Department of Physiology, University of Utah School of Medicine
Now, IBEC’s Signal and Information Processing for Sensing Systems group has uncovered how we immediately and strongly identify smells even when their concentrations are very low, essentially segregating information about identification and concentration in order to be able to recognise the odour.
They did this by looking at the first stage of the olfactory bulb – the glomerular layer, which is the ‘input’ stage for smells – to see whether this segregation of information is performed there. Together with their collaborators in Rome, the group built a computational neural model of the area based on its known anatomical connections and tested it with two simulations. In the first experiment, the model was exposed to an odor stimulus dataset composed of six different smells, all at different concentrations; the second was an ‘odour morphing’ experiment – where one pure odour becomes a second, distinct odor via 19 gradual steps – was presented to the model.
“We found that the glomerular layer or input stage, where the identity and concentration information of an odour is mixed up when it enters, is indeed the origin of the olfactory system’s amazing ability to quickly identify smells while still preserving information about their concentration,” explains Agustin Gutierrez, senior researcher in the IBEC group and lead author on the paper, published recently in PlosONE. “It does this using two different types of neurons – the mitral cells (MCs) and the external tufted cells (ETs). The MCs are in charge of encoding the odour’s identity, while the ETs focus on its concentration – not because of their morphological differences, but because the two types of neurons have contrasting ways of interacting within the glomerular network. Both types then project all their information to the olfactory (piriform) cortex for decoding.”
It’s the first time that this important segregation of information in odour processing has been attributed to the glomerular layer, which was already known to perform several other tasks such as contrast enhancement.
Reference article: Davide Polese, Eugenio Martinelli, Santiago Marco, Corrado Di Natale & Agustin Gutierrez (2014). “Understanding Odor Information Segregation in the Olfactory Bulb by Means of Mitral and Tufted Cells”. PLoS ONE 9(10): e109716