CAN A CONCUSSION

Chemosensory disorders have diagnostic significance for prevalent human illnesses. Primarily olfactory, they are frequently considered gustatory due to the oral retro-nasal origin of some odorous volatiles.1 However, the multitude of possible smells, recently exaggerated from thousands to more than a trillion,2 makes measuring individual odor sensing appear daunting.3, 4 Furthermore, this conundrum has led to attempts to segregate roles of relevant functional groups,5 or use of crowd-sourcing methods,6 to count the number of recognizable multi-component odor objects that an individual can recognize.7

Odor component recognition
As in other biological chemical signaling systems, including taste-sensing8, 9, 10; distinct odors likely activate dedicated receptors (OR).11, 12 However, given the rapid odor adaptation with the passage of time, odor recognition is momentary.4 The ortho-nasal odor-stimulus is limited to the inhalation phase by the act of sniffing,13 and on the same time scale as sniffing, the odor rapidly adapts. To further complicate the stimulus situation, odor sensing occurs in dynamic natural sensing settings in which odors of many individual odor chemicals are simultaneously either waxing or waning. The olfactory system needs to be able to identify chemicals of harm or value under these conditions. Surprisingly, this may be accomplished by a combination of ‘mixture suppression and selective adaptation.’

In experimentally controlled simulations, mixture-component selective adaptation shows individual or shared prominent characteristic odors (‘odor notes’) are detected by humans.4 However, individual molecular stimulus features are not. The perceptual saliencies of two individually presented components, A and B ‘odor notes’, are represented by bright colors in Fig. 1. The saliencies are reduced when both components are within a mixture (below), the feature known as ‘mixture suppression’ in olfaction,14, 15, 16 and taste.17 However, when component A alone is adapted for 10 s (center), its ambient salience in the following mixture (below) is further weakened but un-adapted component B is ‘released’ from mixture inhibition. This startling outcome resulting by combining ‘mixture suppression’ and ‘selective adaptation’ of independent stimuli (defined as those that do not cross-adapt) explains how the olfactory system is able to identify individual mixture components in few-component mixtures.3, 18, 4 Thus, the odor mixtures are analyzed; they are not synthesized by a combinatorial process into a distinctly different quality. Gustatory mixtures of independent taste stimuli are also analyzed via the combination of ‘mixture suppression’ and ‘selective adaptation’.19