Chapter 14 Summary

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1.   Olfaction is one of the two chemical senses; the other is taste (discussed in Chapter 15). To be perceived as a scent, a chemical must possess certain physical properties; however, even some molecules that have these characteristics cannot be smelled. Olfaction has some unique physiological properties, one of which is that only between 35%–40% of the genes that code for olfactory receptors in humans are functional. Another unusual feature is that most odorants also stimulate the ----- olfactory somatosensory visual system via the trigeminal nerve, and it is often impossible to distinguish the contribution of olfactory sensation from trigeminal stimulation.
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2.   Anosmia is the complete absence of a sense of smell. It is most frequently caused by sinus disease, which can usually be treated and enable the return of normal olfactory function. However if anosmia is caused by head trauma, it is likely to be permanent. Anosmia can lead to severe disturbances in an individual’s quality of life. Gradual loss of olfaction is a normal consequence of aging; however, sudden olfactory loss can be the first sign of Alzheimer’s or Parkinson’s disease and should be investigated.

3.   The dominant biochemical theory of odor perception— -pattern theory—contends that the fit between a molecule and an olfactory receptor (OR) determine which molecules are detected as scents, and that specific odorants activate a combinatorial code of ORs, producing specific patterns of spatial and temporal neural activation for each perceived scent. However, this theory is not universally accepted, and alternate explanations exist (e.g., vibration theory).
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4.   Recently, researchers demonstrated a closer connection between the ----- auditory touch visual system and olfaction than has ever before been thought to exist. Two examples are binaral rivalry and the discovery of “olfactory white.” There is also a difference between active sniffing and passive inhalation of odors at both neurological and functional levels. Active sniffing may even have therapeutic applications for patients suffering from extreme physical disabilities.
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5.   Although we can potentially detect over a trillion odors, almost all odors that we encounter in the real world are mixtures, and we appear ----- not to be to be very good at analyzing the discrete chemical components of scent mixtures. Olfaction is thus primarily a synthetic, as opposed to analytical, sense. However, analytical olfactory ability can be developed with training. True odor imagery is also weak (or nonexistent) for most people, but training, as in the case of odor experts (e.g., perfumers), appears to facilitate this ability.
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6.   The psychophysical study of smell has shown that various odorant intensity levels and various cognitive functions are required for odor detection, discrimination, and recognition. Identification differs from odor recognition in that, in the former, one must come up with a name for the olfactory sensation. It is very difficult to name even very familiar odors, and as we age this becomes even harder. This state is known as the -of-the-nose phenomenon—one of several indications that linguistic processing is highly disconnected from olfactory experience. However, new research has shown that at least one culture may possess an enhanced verbal connection with odors. Regardless, unlike the case with other sensory experiences, we do not need to access any semantic information about an odor in order to respond to it appropriately, as long as it is familiar.
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7.   Another important discrepancy between the physical experience and the psychological experience of odors is the difference between receptor adaptation and cognitive habituation. Receptor adaptation occurs after ----- continual sporatic odorant exposure over a number of minutes, can be undone after a few minutes away from the odorant, and is explained by a basic biochemical mechanism. By contrast, cognitive habituation occurs after long-term exposure (e.g., in a living or work environment) to an odor and takes ----- hours weeks years away from the odor to undo. Psychological influences can have strong effects on both perceived odor adaptation and habituation also.
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8.   The most immediate and basic response we have to an odor is whether we like it or not; this is evaluation. Odor hedonics are measured by pleasantness, familiarity, and intensity ratings. Pleasantness and familiarity are linearly related to odor liking; odor intensity has a more complex relationship with hedonic perception. Substantial evidence suggests that our hedonic responses to odors are learned and not innate, even for so-called stenches. That we have learned to like or dislike various odors rather than being born with hardwired responses is evolutionarily adaptive for ----- generalist specialist species like humans. The caveats to the learned-response proposition are odors that are highly trigeminally irritating (pain inducing) and the genetic variability in the number and types of receptors expressed across individuals, which may influence olfactory sensitivity and hence odor hedonic perception.
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9.   The key to olfactory learning is the emotional value of the context in which the odor is first encountered. If the emotional context is good, the odor will be liked; if it is bad, the odor will be disliked. Previously acquired emotional associations with odors also underlie validated aromatherapy effects. Emotional potency further distinguishes odor-evoked memories from memories triggered by other sensory cues. The neuroanatomy of the olfactory and limbic systems and their neuroevolutionary development illustrate how emotional processing and olfactory processing are uniquely and intimately interrelated.
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10. Pheromones are chemicals emitted by individuals that affect the physiology and/or behavior of other members of the same species and may or may not have any smell. In all mammals that have been shown to use pheromones for communication, detection is mediated through the organ (VNO) and processed by the accessory olfactory bulb (AOB). Humans do not possess a functional VNO or AOB, and evidence for human pheromones is controversial. However, human chemosignals that are processed through the olfactory system appear to have some influence on hormonal status and sexual arousal.
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