Similar results were obtained with an odorant detector formed from four moth antennae Myrick et al. In a wind tunnel of 1. Interestingly, this experiment appears to have never been repeated. When two odorants are emitted from a single source, each with a given, constant concentration, the ratio of their concentrations is informative of the source identity as noted above , but very far from the source, due to diffusion and turbulent motion, the two odorants are spread out, their concentration ratio changes and the information about the identity can get lost.
The most pertinent question in this scenario is to what extent do odorants initially travel together in the same filaments maintaining the same ratio of concentrations? And if they do so, for how long? And are the mixing effects due to diffusion and advection in a turbulent flow synchronous or do they take place at different timescales and hence take effect at different distances from the source?
The answers to these questions will depend on the physical properties of the flow, on the chemical properties of the odorants and on the differences between them for an excellent review on this issue see Conchou et al. For example, compounds with lower adsorbing properties would travel over longer distances and faster than the other compounds see Beyaert and Hilker, , and references therein.
This effect can have a potential function as the ratio of the two components can inform the insect of the distance from the source. For example, Xiao et al. It is generally believed that the diffusive properties of odorants are not relevant for this particular issue Celani et al. Some indirect evidence supporting this hypothesis is presented in Duplat et al. They considered temperature and odorant concentration interchangeably as they are obeying the same type of advection-diffusion equations.
In spite of the large differences in Schmidt number, the differences in the profiles for these three cases diffusivity spans four orders of magnitude are quite subtle. Dusenbery, The goal of insects navigating an odor-landscape is to approach or escape the odorant source.
In a previous section, we saw how the statistical properties of plumes vary depending on the source position, sensor position, temperature, wind speed, etc. Which of these pieces of information about the plume structure could potentially help insects?
And which ones do they actually use? Do insects analyze and extract information from the complex structure of odor plumes as recently suggested in Boie et al. In this final section, we would like to show the relevance of these questions for the study of insect navigation. To this aim, we will use only a few illustrative examples from the literature. For an extensive review of odor-guided insect navigation see e. We saw that both downwind and crosswind distance from the source affect intermittency, average concentration, and frequency of bouts see e.
However, we also saw that their isolated local values of intermittency and average concentration prevent to unambiguously determine the distance to the source. In a recent experiment, Pang et al. Another example is the dependence of turbulence in the atmospheric boundary layer on the time of day: during the sunset there are less advection movements, and as a consequence plumes intermingle less Mylne and Mason, ; Yee et al. If insects wanted to use measures of turbulence for orientation, they would need to adjust this information for the time of day.
It has even been suggested that this effect influenced, via evolutionary selection, the circadian rhythm of the moth manduca sexta that exhibits nocturnal foraging Riffell et al. There are several strategies that motile organisms developed to locate an odorant source Belanger and Arbas, ; Vickers, ; Moore and Crimaldi, ; Gaudry et al.
A first classification of potential strategies can be performed based on the level of turbulence of the flow that the animals encounter. This strategy can range from simple biased random walks of bacteria Weissburg, to more sophisticated active sampling behaviors observed in Drosophila larvae Gomez-Marin et al.
We saw that close to the surface, the no-slip boundary condition generates a layer of low speed Connor et al. It has also been suggested that walking insects could take advantage of the diffusive distribution of odorants Baker et al. This is also reflected in the trajectories of desert ants which frequently change between upwind and crosswind directions, presumably because they constantly get into and out of the plume Buehlmann et al. In these conditions the patchiness of plumes prevents insects from using any gradient based information Wright, ; Gifford, ; Aylor et al.
Indeed, the behavioral relevance of stimulus intermittency has been repeatedly shown Kennedy et al. In a turbulent plume, insects go into and out of a plume frequently, so that in addition to locating the source, just re- locating the plume becomes challenging. Typically, their behavior can be described as alternating upwind surges when in a whiff and approximately crosswind casting during blanks David et al.
In the presence of wind, the plumes tend to be elongated along the wind direction. It can then appear intuitive to think of crosswind casting as a good approach to re-locating the plume once it has been lost. Unfortunately, we are still missing a complete representation of what different insects actually do when losing the plume. Odor plumes are too complex to be used in their full details when investigating animal physiology and behavior and hence need to be simplified. Odor steps are the most radical simplification, are very easy to generate and analyze, and are, therefore, the most common inputs used in insect neurophysiology until today — with some exceptions starting 10 years ago see e.
However, constant odor steps do not occur in natural plumes, whose most important characteristic is intermittency. In a recent study Jacob et al. The goals of this seminal work reflect the same thinking underlying this review: the complexity of olfactory stimuli has to be faced altogether because a reductionist approach might lead to misunderstanding the olfactory system.
The introduction of correct whiff and blank statistics is a great first step and there are several further improvements that can be made: 1 Removing the approximation of a single value for the concentration, for the obvious reason that otherwise ORNs or antennal lobe neurons cannot exhibit realistic responses; 2 Implementing simulated stimulation for crosswind distances different from zero, 3 Measuring the plume-structure for very small time-scales; as discussed earlier, the distributions are not known for the time-scales below tens of milliseconds.
It is reasonable to expect different distributions of whiffs and blanks for this situation. To solve the first and second issues, no further data are needed, one can implement the results from existing experimental studies Mylne and Mason, ; Yee et al. Weissburg, Biol. Wright, We have summarized some knowledge on the nature of odor stimuli touching on two specific aspects: 1 The concentration of odorants emanating from liquid dilutions and the temporal structure of odor stimuli produced by olfactory stimulators in the lab.
With respect to the odor concentration produced by using defined dilutions of odorants in a solvent, it is important to reiterate that the concentration in the air within the headspace of the liquid solution, be it on a filter paper or in a larger reservoir, is not a linear function of the dilution. What may be less appreciated are the regimes where these laws apply. In a sense, both regimes are very small if looked at on a linear scale see e.
Independently, it will always remain important to ascertain using direct measurements, e. With respect to the temporal structure of odorant stimuli from odor stimulation devices, we discussed recent results showing that the odor onset of an odor stimulus depends on the identity of the odorant. Combined with other results that indicate that olfactory systems are sensitive to the derivative of the odorant concentration as well as the odorant concentration itself, the difference in odor onset slope could have measurable effects on the response and this could lead to confusing results.
As with the odorant concentration, it should also become standard to ascertain the stimulus time course for any given experiment. Research in insect physiology is clearly moving toward more articulate stimuli — more odorants, more complex time courses.
Moreover, nowadays it is clear that a purely reductionist approach is insufficient to gain a full understanding of the neuroscience of insects — from molecules, to neurons and synapses, and to behavior. Future experiments may eventually all have to consider the entire environment-perception-action loop Wallach et al.
However, it is an immediate objective for the community to define protocols for more viable and precise spatio-temporal stimulus generation in the lab e. For the structure of natural odor plumes we surprisingly found that many of the most salient experimental results date back to the last century, only augmented by occasional more recent studies. Even in the quite simplified overview that we were able to include here it becomes clear that ultimately we still do not fully understand the nature of odor plumes.
A prime example is the measurement of intermittency, probably one of the most important plume descriptors, where theoretical results are in stark disagreement with the experimental evidence. Solving this issue is a well defined goal that should urgently be addressed by Neuroscientists and Physicists alike. When investigating insect behavior in natural plumes, in particular navigation, it will be important to better understand and experimentally characterize the particular plume conditions that insects face in any particular experiment.
To this aim, a big technological effort is needed to measure odor mixtures at a high spatio-temporal resolution Davies et al. This could then feed into lab experiments for which advanced stimulation devices for arbitrary time series are under development Kim et al.
MP and TN contributed to research and analysis of literature and writing the manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We are really grateful to M. Stopfer, A. Barron, and C. Martelli for helpful comments that greatly improved the manuscript. Andersson, M. Mechanisms of odor coding in coniferous bark beetles: from neuron to behavior and application.
Psyche , 1— Attraction modulated by spacing of pheromone components and anti-attractants in a bark beetle and a moth. What reaches the antenna? Senses 37, — Atema, J. Eddy chemotaxis and odor landscapes: exploration of nature with animal sensors. Auffarth, B. Understanding smell—the olfactory stimulus problem. Aylor, D. Turbulent dispersion of disparlure in the forest and male gypsy moth 1 response.
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Moth uses fine tuning for odour resolution. Nature , — A pulsed cloud of sex pheromone elicits upwind flight in male moths. Belanger, J. Behavioral strategies underlying pheromone-modulated flight in moths: lessons from simulation studies.
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Erskine, A. London: UCL. While the value of a sense of smell is obvious, what is the value of a sense of taste? Different tasting foods have different attributes, both helpful and harmful. For example, sweet-tasting substances tend to be highly caloric, which could be necessary for survival in lean times.
Bitterness is associated with toxicity, and sourness is associated with spoiled food. Salty foods are valuable in maintaining homeostasis by helping the body retain water and by providing ions necessary for cells to function.
Both taste and odor stimuli are molecules taken in from the environment. The primary tastes detected by humans are sweet, sour, bitter, salty and umami.
The first four tastes need little explanation. The identification of umami as a fundamental taste occurred fairly recently—it was identified in by Japanese scientist Kikunae Ikeda while he worked with seaweed broth, but it was not widely accepted as a taste that could be physiologically distinguished until many years later. The taste of umami, also known as savoriness, is attributable to the taste of the amino acid L-glutamate. In fact, monosodium glutamate, or MSG, is often used in cooking to enhance the savory taste of certain foods.
What is the adaptive value of being able to distinguish umami? Savory substances tend to be high in protein. All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules into the air from its surface, it has no smell. And if a human or other animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about olfactory receptor subtypes that work in various combinations to allow us to sense about 10, different odors.
Compare that to mice, for example, which have about 1, olfactory receptor types, and therefore probably sense more odors. Both odors and tastes involve molecules that stimulate specific chemoreceptors.
Although humans commonly distinguish taste as one sense and smell as another, they work together to create the perception of flavor. Odorants odor molecules enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity as illustrated in Figure The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area about 5 cm 2 in humans.
Recall that sensory cells are neurons. An olfactory receptor , which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors, distributed among hundreds of different receptor types that respond to different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about million, most dogs have about 1 billion, and bloodhounds—dogs selectively bred for their sense of smell—have about 4 billion.
The overall size of the olfactory epithelium also differs between species, with that of bloodhounds, for example, being many times larger than that of humans. Olfactory neurons are bipolar neurons neurons with two processes from the cell body. Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules.
This coupling of smell and taste explains why foods seem lackluster with a head cold. This happens because the thalamus sends smell information to the hippocampus and amygdala , key brain regions involved in learning and memory.
Although scientists used to think that the human nose could identify about 10, different smells, Vosshall and her colleagues have recently shown that people can identify far more scents. Starting with different odor molecules, they made random mixtures of 10, 20, and 30 odor molecules, so many that the smell produced was unrecognizable to participants. Predictably, the more overlap there was between two types of mixtures, the harder they were to tell apart.
After calculating how many of the mixtures the majority of people could tell apart, the researchers were able to predict how people would fare if presented with every possible mixture that could be created from the different odor molecules. They used this data to estimate that the average person can detect at least one trillion different smells, a far cry from the previous estimate of 10, The one trillion is probably an underestimation of the true number of smells we can detect, said Vosshall, because there are far more than different types of odor molecules in the world.
No longer should humans be considered poor smellers. In fact, new research suggests that your nose can outperform your eyes and ears, which can discriminate between several million colors and about half a million tones. A beginner's guide to the brain and nervous system. See how discoveries in the lab have improved human health. Read More. For Educators Log in. This image may look like a carnival mask, but it actually shows the key structures mammals use every time they smell.
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It is mandatory to procure user consent prior to running these cookies on your website. Back to Parent Page. Share This Page. Smell The molecules that activate the sense of smell the technical name is olfaction are airborne; they enter the body via the nose and mouth and attach to receptor cells that line the mucus membranes far back in the nose. Taste The other primary chemical sense, taste technically, the gustatory system , responds to molecules dissolved in liquid.
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