Trends in Neurosciences
ReviewFrom molecule to mind: an integrative perspective on odor intensity
Section snippets
Deciphering the neural code for odor intensity
‘I want to create a cologne that smells like a whisper. It’ll be for all the secret admirers out there.’ Jarod Kintz
Some odors whisper like a secret admirer, others blare like a megaphone. How does the brain encode such a broad spectrum of olfactory intensities? Most research in the field has focused on concentration, rather than intensity. Whereas the intensity of an odorant is clearly related to its concentration [1], odorant concentration also correlates with odor valence [2] and quality [3]
Intensity–concentration relation
Empirically, the perceived intensity of an odorant is a monotonic, sigmoidal function of the logarithm of odorant concentration, although most early observations captured only the linear portion over a middle range of concentrations. In fact, many odorants fail to attain sufficiently high vapor concentrations to saturate the psychophysical function [7], so linear [8] and exponential [9] functions can indeed approximate the intensity–concentration function. However, a comparison of models found
Predicting odor intensity from chemical structure
There is currently no general model predicting the psychophysical intensity function from chemical structure. Certain physicochemical properties of molecules tend to be associated with high-potency odorants, such as high volatility and intermediate water-lipid partitioning, but odor intensity still spans several orders of magnitude for compounds with similar volatility and hydrophobicity.
General models have emerged for a special case of the psychophysical function relating concentration to
Neural encoding: predicting odor intensity from spike patterns
Chemical signals detected by olfactory receptors (OR) in OSNs are transformed into sequences of action potentials or spikes relayed to the brain. Spikes are digital signals whose number, timing, and distribution across neural assemblies contain all of the stimulus information available to the nervous system. A variety of schemes have been proposed for spike encoding of odor intensity at different stages in the olfactory pathway, based on observing changes in neural activity as a function of
Odor intensity from cortical activation
To date, few studies have directly assessed the central neural network responsible for the processing of perceived odor intensity or odor concentration. A major reason for this dearth of knowledge is the aforementioned inherently confounding co-dependency between the three perceptual odor dimensions: intensity, valence, and quality. This interdependence among dimensions renders the task of isolating cognitive processing of intensity perception inherently difficult. Nonetheless, increased
How quickly is intensity computed?
In land-based vertebrates, olfactory perception begins with the inhalation (sniffing) of odorous air into the nasal cavity. A sniff begins with a stimulus-independent inhalation, but is rapidly modulated in response to sniff content. Sniffs of different odorant concentrations are uniform for the first 160–260 ms following sniff onset, but then diverge such that sniffs of a concentrated odorant will have a smaller volume than sniffs of a diluted odorant [73]. This relation is sufficiently robust
Concluding remarks
Most olfactory stimuli encountered in nature are complex mixtures of odorant molecules. When the intensities of the component odorants are known, what is the intensity of the mixture? The intensity of an odorant mixture is rarely a simple linear sum of the intensities of individual components [108], but as noted above, nonlinear models, such as the Hill equation, are often a better fit to the concentration–intensity relation. Thus, mixture interactions that deviate from a linear sum of
Acknowledgments
J.D.M. is supported by National Institutes of Health (NIH) grant DC013339. J.N.L. is supported by the Knut and Alice Wallenberg Foundation (KAW 2012.0141). J.R. is supported by NIH grant DC009613. G.L. is supported by NIH grant DC04208.
Glossary
- Amodal neural network
- a network that is not primarily dedicated to the processing of information from a specific sensory modality, such as vision, audition, or olfaction.
- GCaMP2 imaging
- GCaMP2 is a genetically encoded reporter protein whose fluorescence increases as intracellular calcium levels rise when neurons fire action potentials. In the olfactory system, mouse strains expressing GCaMP2 in OSNs, and in mitral/tufted cells, have been used to map patterns of glomerular activity encoding
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