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1. The Function of the Olfactory System in Animals and Humans
- Johns Hopkins University Press
- Chapter
- Additional Information
1 The Function of the Olfactory System in Animals and Humans Smell this book. Seriously, close your eyes, bring the pages of the book to your nose and inhale.* Volatile molecules from the pages, the binding glue, the cover, and, depending on the age of the book at this reading, the accumulated dust and debris of storage enter your nose (via the external nares) and pass over your olfactory receptor sheet on their way to your trachea and lungs (via the internal nares). What happens at the olfactory receptor sheet is nothing short of a remarkable bit of analytical chemistry. Current views of peripheral odorant transduction suggest that individual molecular and submolecular features of the myriad different molecular species you just inhaled are each recognized by a small subset out of hundreds of different olfactory receptors. Thus, no single olfactory receptor for book odor exists ; rather, the olfactory receptor sheet performs an analysis to identify the scores or hundreds of individual volatile components over the book. At a conscious , perceptual level, however, we have no direct access to the results of * Did you inhale with a single deep fill of the lungs or did you use a series of short, fast sniffs? Either process draws odorant molecules over the olfactory receptor sheet, although short, fast sniffs may allow greater, or at least differential, access of odorants to the recesses within the olfactory epithelium . How you inhale could thus impact the nature of odor perception—a topic we will not address here, but that is receiving increasing attention both in terms of receptor sheet stimulation and central olfactory pathway processing. this phenomenal peripheral analysis. Instead, we perceive a wholistic, unitary percept of book odor—largely a single perceptual odor object. The perception may have, at most, two to three identifiable major components, but the vast majority of the exquisite analysis occurring at the periphery is beyond our conscious reach. The focus of this book is the nature and consequences of odor object perception . How odor objects are formed, how they are shaped by experience, and how the process of synthetic object perception results in both unique capabilities and distinct limitations for olfactory perception are some of the questions to be addressed. Exciting new findings in the psychophysics, neurophysiology , and functional anatomy of the olfactory system are brought together to support our thesis (Wilson and Stevenson 2003). Along the way, we will attempt to highlight new avenues of research to which this view of olfaction draws attention. A fundamental premise here is that odor objects are learned through experience. Odorants and odorant features that co-occur are synthesized through plasticity within central circuits to form single perceptual outcomes that are resistant to background interference, intensity fluctuations, or partial degradation. Learned odor objects may include multimodal components, and recognition of familiar odor objects can be shaped by context, attention, and expectation. Experience-dependent odor object perception and synthetic odor coding are certainly not new ideas. In 1890, William James, a founding father of modern psychology noted that “every perception is an acquired perception” (James 1890, 78). Specifically addressing olfaction, he made the following observation : “We know that a weak smell or taste may be very diversely interpreted by us, and that the same sensation will now be named as one thing and the next moment as another . . . In this wise one may make a person taste or smell what one will, if one only makes sure that he shall conceive of beforehand as we wish by saying to him ‘Doesn’t it smell just like, etc.?’ (James 1890, 97–98).* Together, these two quotes from 1890 suggest that simply knowing what molecular features are present at the olfactory receptor sheet is not sufficient to allow accurate prediction of the resulting olfactory percept. Rather, how the activity within the sensory afferent is read and formed into a percept depends on past experience and current expectations, among other things. Learning to Smell 2 * James’s use of the term taste here actually implies “flavor,” which is intimately dependent on olfaction . [44.221.46.132] Project MUSE (2024-03-28 20:45 GMT) More recently, in 1973 Ernest Polak outlined a model of olfaction wherein he hypothesized a large set of diversely tuned receptor neurons signaling the unique pattern of odor features present in a stimulus. Once the unique pattern of features is extracted, “the brain attempts to recognize this odor image by scanning and resolving it into previously stored patterns” (Polak 1973...