Olfaction human

In 1986, the National Geographic Society, in cooperation with the MoneU Center, conducted a worldwide survey of the sense of smell. Over 10 million survey forms were sent to readers of the Society s journal, of which close to 1.5 million forms were completed and returned. With responses to 40 demographic and 42 odor-related questions, the results constitute the largest set of data on human olfaction (4). 

Here is a bit of a complication there is a lot of individual variation in the sense of human olfaction. Not everything smells the same to everyone. This holds both for the intensity of the perceived smeU as well as for its quality pleasant, floral, skunky, sweaty, or no odor at all. Andreas Keller has recently demonstrated that some significant part of this individual variation in the sense of smell derives from genetic variation in human odorant genes. Specifically, two single nucleotide polymorphisms (SNPs), leading to two amino acid substitutions in an odorant receptor, have dramatic affects on the perception of the odor of androstenone, a steroid derived from testosterone. 

In old age, humans experience a precipitous drop in olfactoiy ability (Doty et ah, 1984 Doty, 1986 see Section 5.7). Two excellent recent reviews of the ontogeny of human olfaction and olfactory communication are by Doty (1986) and Schaal (1988b). 

Figure 1 Organs involved in human olfaction (with permission from Givaudan).

All living creatures respond to chemicals in their environment. There can be little doubt that other mammals use their noses in much the same way as humans do. Drawing parallels between our sense of smell and that of another terrestrial species (which may have sensory modalities that we lack) requires a set of guidelines for assessing features of chemoreception that correspond to human olfaction. These guidelines seek to segregate olfaction from other chemical senses to discuss how other animals respond to chemical stimuli. Futime investigation may prove the impossibility of such a compartmentalization, so I shall attempt to circumscribe olfaction rather than to define it. 

To these species the structure of air currents is not invisible. A considerable portion of their brains has evolved to process information from their whiskers (or vibrissae), and one may plausibly suggest that their olfactory sensitivity derives, at least in part, from the ability to monitor the structure of air and to situate a scent within the currents that eddy about their snouts (Cain, et al., 1985). If, even in the absence of vibrissae, human olfaction can, nevertheless, sense the heterogeneity of odors, this adds another complexity to our experience of smell. As argued above, diffusion through mucus limits the temporal resolution of olfaction to about 0.1 second. If humans have the capacity... 

If human olfaction can discern the filamentous character of scent plumes in air, then a headspace analysis, no matter how complete, might not suffice to reconstruct the fragrance. The distribution of molecules would play a role, as would the rate with which they are replenished after they have been depleted by sniffing. If, on the other hand, olfaction (independent of other sensory inputs) cannot differentiate a heterogeneous stimulus from one that has been well mixed with air, then a complete chemical analysis could serve to archive odors. 

The editors have raised several good questions about this impressionistic claim, and at least three qualifications are required. First, it may be that vision furnishes not more total information but only more immediately usable information. Second, I am speaking of normal (20—20) human vision and normal human olfaction, unaided by instruments. (Suitably fantasized instruments, of course, could change any estimate of amount of information.) Third, smell has the power as sight does not to detect and possibly identify visually hidden sources. 

Because of the relative ease of measurement of many of nitrobenzene s properties and its ready detectability by both chemical analysis and human olfaction (sense of smell), its release, transport and fate, and the consequent exposure of human beings have been studied over a considerable period of time. Thus, the potential for human exposure to nitrobenzene is better understood than that of many other chemicals. 

HgS and NHg are not only poisonous but also offensive-odored so that the sensors for environmental use are required enough sensitivity comparable to the human olfaction. Many t3rpes of sensor have been proposed by many... 

In this framework, the state-of-the-art generally reports two complementary types of measurement methods human olfaction methods and analytical techniques (Van Harreveld 2003, Hammers et al. 2004, Stuetz et al. 2001). 

Human olfaction measurement considers the odour as a global concept and provides the true dimensions of the human perception. Yet, physiological differences in the smelling of various people often lead to subjective results with large uncertainties. Analytical techniques identify the various volatile compounds involved in the odour and give their chemical concentration. They have better scientific standing than sensory methods. However, the chemical composition of the gas mixture doesn t represent the odour perception. 

It has to be remarked that in spite of the widely accepted term electronic nose, current devices are still far from the structure and functions of natural olfaction sense. The unique common feature between artificial and natural system is that both are largely based on arrays of nonselective sensors. The concept underlying electronic nose systems has been demonstrated to be independent on the particular sensor mechanism indeed during the last two decades almost all the available sensor technologies have been utilized as electronic noses. Clearly, all these sensors are very different from the natural receptors. These dissimilarities make the perception of electronic nose very different from that of natural olfaction, so that the instrumental perception of the composition of air cannot be called odor measurement because odor is the sensation of smell as perceived by human olfaction. Nonetheless, the term odor analysis with electronic noses is now largely adopted, but it is important to keep in mind, especially in medical applications, that the electronic nose measurement may be very distant from the human perception. 

It is possible from these data to make a preliminary estimate of how much redundancy there is in human olfaction. 

Many odor theories have been proposed in the past, attempting to explain the multitude of often very complex phenomena observed in human olfaction. Most of them were only partially, if at sill, successful. Nevertheless, slowly a consensus developed and today it is generally assumed that the primary process of chemorecep-tion takes place at the cell membrane of a sensory neuron and involves physical contact of the stimulant with potentieil or actual receptor sites which could be either specialists - reacting only with one structural class - or generalists which would react with a multitude of structural classes. 

It has been observed that the discriminatory capabilities of human olfaction are tremendous It was estimated that an untrained person could differentiate up to ten million odors, perhaps even significantly more than that. Information theory then shows that in order to encode the qualities of ten million odors in a simple binary mode (Monoosmatic components on or off, their intensity, albeit important, is in this connection disregarded) only 2h to 27 specific profiles, disregarding possible and probable redundancies, and therefore the same number of complementary receptor sites would be required. Assuming furthermore that said redundancy, in which the informational modalities of two different specific receptor sites of two different olfactory neurons are confluent in one collector cell and therefore contribute to the expression of only one monoosmatic component is indeed operational it becomes necessary to increase the total number of types of specific receptor sites to 2k-30. This means that only 2U-30 specific detector proteins are required for structure recognition in the transduction process. This compares to about UOOO enzyme systems in different stages of activity estimated to be present in a cell any time. 

The olfactory stimulator can be controlled by digital electronics. Many different rates, intensities, and combinations of airborne materials can be presented by a mere keystroke. Moreover, because the system can easily dispense volumes as small as a few himdred pi of fluid, they can provide exquisitely fast and precise olfactory inputs near the threshold of human olfaction. A small version of the device can be used with laboratory animals (44). 

Lorig, T. S. 2000. The application of electroencephalographic techniques to the study of human olfaction A review and tutorial. International Journal of Psychonhvsioloey 36(2) 91 104. 

Wiesmann, M., Yousry, I., Heuberger, E. et al. 2001. Functional magnetic resonance imaging of human olfaction. Neuroimaging Clinics of North America ll(2) 237-250. 

Wysocki, C.J. G.K. Beauchamp. 1991. Individual differences in human olfaction. In Chemical Senses, Vol. 3, Genetics of Perception and Communication, eds. C.J. Wysocki M.R. Kare, pp. 353-374, Marcel Dekker, New York. 

NE Rawson, G Gomez. Cell and molecular biology of human olfaction. 

K Keyhani, PW Scherer, MM Mozell. A numerical model of nasal transport for the analysis of human olfaction. J Theor Biol 186 279-301, 1997. 

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