Sequential mechanisms underlying concentration invariance in biological olfaction

Abstract

Concentration invariance—the capacity to recognize a given odorant (analyte) across a range of concentrations—is an unusually difficult problem in the olfactory modality. Nevertheless, humans and other animals are able to recognize known odors across substantial concentration ranges, and this concentration invariance is a highly desirable property for artificial systems as well. Several properties of olfactory systems have been proposed to contribute to concentration invariance, but none of these alone can plausibly achieve full concentration invariance. We here propose that the mammalian olfactory system uses at least six computational mechanisms in series to reduce the concentration-dependent variance in odor representations to a level at which different concentrations of odors evoke reasonably similar representations, while preserving variance arising from differences in odor quality. We suggest that the residual variance then is treated like any other source of stimulus variance, and categorized appropriately into “odors” via perceptual learning. We further show that naïve mice respond to different concentrations of an odorant just as if they were differences in quality, suggesting that, prior to odor categorization, the learning-independent compensatory mechanisms are limited in their capacity to achieve concentration invariance.

Weaker Ligands Can Dominate an Odor Blend due to Syntopic Interactions

“…For example, a fruit fly that smells a banana should be able to identify this banana also when the scent gets stronger as it approaches the source. Odor-concentration invariance has been shown in behavior and physiology for different species and sequential ORN recruitment, and network effects in the brain are thought to underlie this capacity (Sachse and Galizia 2003; Uchida and Mainen 2007; Root et al 2008; Asahina et al 2009; Cleland et al 2012). Generally, the dose–response curve to an odor follows a sigmoidal shape.…”

Keeping their distance? Odor response patterns along the concentration range

“…The latter is an aspect often overlooked in the literature that seems to be focused on the problem of concentration-invariant perception (Uchida and Mainen, 2007; Asahina et al, 2009; Schmuker et al, 2011; Cleland et al, 2012). Interestingly, though, Duchamp-Viret et al (1990) noted that odor response patterns in the frog olfactory bulb broaden with rising concentration and that pattern separation improves at higher odor concentrations.…”

Gain Control Network Conditions in Early Sensory Coding

“…This is the role of gain control, which is ubiquitous for sensory processing in the brain [1]. It allows us to recognize a melody independent of how loud the music plays, identify objects in a wide range of light conditions or recognize an odorant irrespective of its concentration or our distance from the source [2].…”

“…We are investigating these conditions in the framework of a model network of excitatory and inhibitory neurons inspired by the structure of the insect AL (note similar studies in the olfactory bulb [2], [42]). Since this work is of broad relevance to brain microcircuits we will refer to the PNs as the excitatory population and the inhibitory LNs as the inhibitory population in the remainder of the paper.…”