<P> After binding the odorant, the receptor is activated and will send a signal to the glomeruli . Each glomerulus receives signals from multiple receptors that detect similar odorant features . Because several receptor types are activated due to the different chemical features of the odorant, several glomeruli are activated as well . All of the signals from the glomeruli are then sent to the brain, where the combination of glomeruli activation encodes the different chemical features of the odorant . The brain then essentially puts the pieces of the activation pattern back together in order to identify and perceive the odorant . This distributed code allows the brain to detect specific odors in mixtures of many background odors . </P> <P> It is a general idea that the layout of brain structures corresponds to physical features of stimuli (called topographic coding), and similar analogies have been made in olfaction with concepts such as a layout corresponding to chemical features (called chemotopy) or perceptual features . While chemotopy remains a highly controversial concept, evidence exists for perceptual information implemented in the spatial dimensions of olfactory networks . </P> <P> Although conventional wisdom and lay literature, based on impressionistic findings in the 1920s, have long presented human olfaction as capable of distinguishing between roughly 10,000 unique odors, recent research has suggested that the average individual is capable of distinguishing over one trillion unique odors . Researchers in the most recent study, which tested the psychophysical responses to combinations of over 128 unique odor molecules with combinations composed of up to 30 different component molecules, noted that this estimate is "conservative" and that some subjects of their research might be capable of deciphering between a thousand trillion odorants, adding that their worst performer could probably still distinguish between 80 million scents . Authors of the study concluded, "This is far more than previous estimates of distinguishable olfactory stimuli . It demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far out performs the other senses in the number of physically different stimuli it can discriminate ." However, it was also noted by the authors that the ability to distinguish between smells is not analogous to being able to consistently identify them, and that subjects were not typically capable of identifying individual odor stimulants from within the odors the researchers had prepared from multiple odor molecules . In November 2014 the study was strongly criticized by Caltech scientist Markus Meister, who wrote that the study's "extravagant claims are based on errors of mathematical logic". The logic of his paper has in turn been criticized by the authors of the original paper . </P> <P> Different people smell different odors, and most of these differences are caused by genetic differences . Although odorant receptor genes make up one of the largest gene families in the human genome, only a handful of genes have been linked conclusively to particular smells . For instance, the odorant receptor OR5A1 and its genetic variants (alleles) are responsible for our ability (or failure) to smell β - ionone, a key aroma in foods and beverages . Similarly, the odorant receptor OR2J3 is associated with the ability to detect the "grassy" odor, cis - 3 - hexen - 1 - ol . The preference (or dislike) of cilantro (coriander) has been linked to the olfactory receptor OR6A2 . </P>

The olfactory region can sense approximately how many odors
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