This is best illustrated by the emotion of disgust. This emotion is frequently triggered by smell, probably because volatile chemicals are often a good cue for disease-related objects and events Oaten et al. A characteristic feature of disgust is that contact with an elicitor of this emotion feels contaminating e. While we might dislike looking at fake dog feces or plastic vomit, synthetic fecal or vomit odors still compel avoidance even if we know they are not real.
Such smells just feel bad. Functionally, it has been presumed that the primacy of affect reflects the need for rapid withdrawal or approach, without the need for presumably longer cognitive appraisal Yeshurun and Sobel, Perhaps there is some merit in this idea when it is applied in the context of ingestive behavior, where detection of microbial contaminants or natural poisons, may require rapid rejection of a food from the mouth before it is swallowed. However, this particular set of circumstances would be rare, because if an off-smell were detectable in the mouth, it would almost certainly be detectable by the nose prior to ingestion.
More generally, we seem well able to avoid dangerous situations when they are revealed to us by vision or audition e. That people can effectively avoid dangerous situations using non-affective means implies that affect-based processing is not uniquely effective in this regard. It could then be that the affect that routinely accompanies olfactory experience is actually just the product of economical cognition resulting from limited neocortical resources i. While it has been known for many years that people routinely describe certain food odors as smelling of particular tastes e.
Importantly, pure tastants do not trigger smell sensations noting that tastants are often contaminated by volatile chemicals; Mojet et al. A further issue is that taste is a discrete sensory system from smell. Taste receptors are located mainly on the tongue and they send information to the brain via a different route to that of smell Schiffman, Smell and taste information first converge in the brain, in secondary neocortical structures Rolls, While synesthesia has typically been explored in the context of the relatively rare individuals with grapheme-color synesthesia e.
Although there are similarities between these rare synesthesias and odor-induced tastes, especially in the stability of these experiences over time, their automaticity and involuntariness, there are also differences. The most important seems to be the largely idiosyncratic nature of many synesthetic inducer-concurrent mappings i. There are several possible mechanisms that could account for odor-induced tastes. One obvious one is that the use of taste-based terms to describe food odors could be metaphorical e.
This appears unlikely. The most obvious metaphor is for affect as with saying someone is sweet. Several studies have now shown that odor-induced taste experiences are dissociable from odor hedonics e. While metaphorical or verbal-semantic mediation accounts cannot be wholly excluded as explanations of odor-induced tastes, they seem unlikely.
A further explanation is that certain odors can activate brain regions also active during tasting, resulting in a taste-like experience that is highly perceptually similar to the experience induced by tastants on the tongue e. That is odor-induced tastes arise from odor-taste associations that are based upon a link between these two percepts, such that the odor percept comes to activate the taste percept. This conclusion has emerged from nearly 20 years of research. Key findings include the observation that: 1 odors that smell of a particular taste can enhance the intensity of that tastant when they are added to it e.
Together with many other supportive findings not summarized here see, Stevenson, , these findings suggest that odors can induce taste-like sensations. It has been suggested that the function of odor-induced taste is to assist in identifying prospective foods, so as to aid prediction of their likely taste in the mouth Stevenson and Tomiczek, Currently, and as in our evolutionary past, human food selection is heavily dependent upon color vision, as it is in our fruit-eating primate ancestors where it evolved Regan et al.
Detecting a food's likely taste via smell is an adaptation that may have had much greater functional significance for animals less reliant on vision although as in humans it may well augment visual decision making; Hiramatsu et al. Olfactory conscious experience appears to be mainly singular with one odor event perceived at a time. The large array of objects potentially available to visual attention during perception contrasts with the more limited range available to smell.
An odor will be redolent of other odors, it will be affectively toned, and if perceived before in a food, it will probably have taste-like qualities. The olfactory percept seems to directly encapsulate its meaning especially taste, affect , and it does so with minimal effort, notwithstanding the making of redolence ratings.
While visual and auditory percepts also contain considerable inherent information e. In addition, visual and auditory percepts are not usually accompanied by the visceral feel of affective contact a more qualitative difference. It might in theory be possible to explain these differences by reference to the demands and constraints of the olfactory system. However, there does not seem any compelling connection between the demands and constraints identified in Section The human olfactory system, and the unusual characteristics of human olfactory conscious content.
Instead, we suggest that most of the differences in conscious content may be explained by reference to olfaction's limited neocortical processing resources, the exception being odor-induced tastes, which may be a vestige but still useful of a once more adaptive food selection system. In the main, the processing differences examined in this section may reflect particularly economical forms of perception, cognition, and consciousness. Odorants access the olfactory receptors either orthonasally via the nostrils on the face, or retronasally via the internal nostrils at the back of the throat during eating and drinking.
One important distinction that has emerged here is that between content and modality awareness Rozin, ; Stevenson, b.
When an odor is smelled at the nose, a person knows both that an odorant is present and that the sensory system involved is smell. When an odor is sensed in the mouth as part of a food retronasally , alongside the anatomically discrete senses of taste and somatosensation mouth feel , the person is capable of perceiving retronasal odor quality, intensity and hedonic properties. However, this is not routinely accompanied by an awareness of the sense modality olfaction involved in its perception Stevenson, a. The evidence for a dissociation of content and modality awareness in the mouth is quite strong, although it has not received a lot of contemporary research attention.
Rozin asked participants which term they would use to describe a range of different foodstuffs, including several that had significant olfactory components. Another manifestation of the content modality dissociation is seen in people who have lost their sense of smell. The olfactory component of food is a major factor in our enjoyment of eating and drinking, as is made evident when one has a cold and retronasal olfaction becomes impaired.
More formal psychophysical tests of peoples ability to identify particular odors in the mouth reveal a capacity to do so that is similar but somewhat poorer than the capacity at the nose e. The food industry clearly believes that people can experience smells in their mouth. They spend large sums of money on sensory evaluation panels that reliably judge many purely olfactory attributes of food, which result from the release of volatiles during eating and drinking Moskowitz and Hartmann, One way to consider these findings is to regard taste and smell in the mouth as one perceptual system—taste but one could equally use the term flavor —as originally suggested by James Gibson Gibson's idea can be operationalized by considering taste and smell as having a shared attentional channel in the mouth Stevenson, b.
Although this dual-channel account has not been well investigated, it is consistent with two important findings. First, it does not seem possible to attend to a smell in the mouth without also attending to a co-present taste and vice versa, suggesting that both these senses are entwined e.
Second, if an odor is sniffed at the nose and a taste is placed in the mouth, the presence of the taste can generate an illusory transfer of the location of the odor from nose to mouth Stevenson et al. That is the orthonasal smell now appears to be part of a flavor in the mouth. This phenomenon does not occur if an odor is sniffed at the nose and a tasteless somatosensory stimulus is placed in the mouth instead e. Together, these findings suggest a special connection between taste and smell that is not shared between smell and oral somatosensation. Functionally, a case can be made for the idea that the brain needs to connect olfactory information arising in the mouth with olfactory events in the environment so as to aid smell-based food selection now or at least in the past.
Irrespective of whether this view is correct, it still leaves the problem as to the benefit, if any, served by not knowing that smells in the mouth are smells. We suggest two possibilities. One is that it may be more efficient to learn the relationship between a food and its immediate and delayed consequences, if this information is automatically associated i.
One consequence of this may be evidence of learning even in the face of contradictory explicit knowledge e. A second possibility is that there may have been no evolutionary pressure for awareness of smells in the mouth, and so we just retain an information processing system that predates a conscious reflective component, which is usually deemed necessary for human associative learning e.
In many respects, olfaction shares with vision and audition basic aspects of attentional processing Keller, Strong, unpleasant or novel odors may involuntarily attract our attention, and we can selectively attend to the olfactory modality, enhancing our reaction time to events in this channel e. One reason to suspect that attentional processing differences do exist comes from the unusual neural architecture of olfaction Smythies, Unlike the major senses, which route all incoming information via the thalamus, the olfactory system is unique in having two routes to neocortex, a thalamic relay and a direct link Tham et al.
Thus, the olfactory system may be able to transmit information to the neocortex independently of the thalamus.
This is important because the thalamus has been presumed to play a key role in attentional processing in the major senses e. Recent work has suggested that at least one particular aspect of attentional processing may be different for smell. As described earlier, the olfactory primary cortex undergoes a rapid reduction in neural response to continued chemical stimulation e.
The presumed reason for this is so that the system is ready and able to detect new odorants as they arise. Importantly, this process of adjustment is principally a cortical change or more properly a paleocortical one , and not a loss of sensitivity at the receptor level. In fact animal work shows convincingly that olfactory receptors retain sensitivity to an odorant that no longer generates any neural response in primary olfactory cortex Wilson and Linster, The cortical locus of this reduced responsivity, combined with retained receptor sensitivity, suggests that it may be termed habituation i.
In the major senses it is relatively easy to voluntarily attend to stimuli that are habituated. In the classic example of the ticking clock, one can voluntarily attend to the sound, but as attention is drawn to other stimuli the ticking again appears to pass out of consciousness James, This does not seem to be the case for the olfactory system as an experiment recently conducted in our laboratory suggests Mahmut and Stevenson, submitted.
Participants were placed in an odorized room and asked to describe its smell using redolence and certainty ratings.
One group was then continuously exposed to the smell, but only in one nostril this being counterbalanced across participants , the other nostril being blocked recall that each side of the nose has its own discrete olfactory epithelium. Performance, in this group of subjects of their open nostril reflects the effects of peripheral adaptation and central habituation, while performance in their blocked nostril just reflects central habituation, which is bilateral Cain, The other group of participants had both nostrils blocked, so as to equate exposure created when participants removed the nose plugs to make ratings of the room's odor.
After a period of around 20 min exposure, we asked both groups to again describe the room's smell using redolence and certainty ratings. The key result is in the group that had just one nostril blocked, with the other open throughout exposure. When we tested the nostril that had been blocked throughout exposure, they were unable to describe the room's smell when their attention was directed toward it, relative to the way they had at the start of the experiment. This reflects the effect of centrally based habituation, as this nostril and its associated receptors had minimal prior exposure to the odor, and so no sensory adaptation should have occurred.
Participants in the other group, who previously had both nostrils blocked, were still able to describe the odor in the same way as they had at the start of the experiment i.
Symbolic Olfactory Display
In sum, participants asked to attend to a centrally habituated odor seemed unable to voluntarily recover its conscious representation. As we noted earlier, the olfactory system has to detect new odorants against the background of currently present odorants, and habituation may play a significant role in this process noting that the persistence of one odorant will not necessarily block perception of another. While it would be tempting to describe failure to re-attend to a habituated odor as a consequence of keeping the olfactory system optimized to detect the advent of new odorants, this explanation seems inadequate.
- Tasting and Smelling. Handbook of Perception and Cognition, Second Edition!
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