SURE,why not? The SUbstitution-REciprocity method for measurement of odor quality discrimination thresholds: replication and extension to nonhuman primates

Recently, Olsson and Cain (2000, Chem. Senses, 25: 493) introduced a psychometric method which, for the first time, allows the standardized determination of odor quality discrimination (OQD) thresholds. The method defines a threshold value that is an average fraction by which one odorant has to be substituted with another to reach a criterion level of discrimination. This measure of discrimination is reciprocal in the sense that it is a result of two separate psychometric functions involving two different standards but the same comparison stimuli. Using the same odor stimuli as Olsson and Cain, with six human subjects but adopting a slightly different experimental design, we were able to replicate their finding that the proportion of correct discriminations changes monotonically with the proportion of adulterant in mixtures of eugenol and citral. As the SURE (SUbstitution-REciprocity) method is based on discriminative responses, it should also be applicable with nonhuman species which can be trained to give unequivocal discriminative responses at the behavioral level. Using an olfactory conditioning paradigm, we therefore trained four squirrel monkeys to discriminate between exactly the same pairs of odor stimuli as our human subjects. We found the psychometric functions of the monkeys to be similar to those of the human subjects. Our results show that the SURE method can successfully be employed with nonhuman primates and thus offers a new approach to study the odor spaces of nonhuman species. Future studies should elucidate whether the SURE method allows for direct comparisons of OQD thresholds and of similarities and differences between odor quality perception of different species.     

Methods to characterize the structure of food powders – a review

Food powders can exist in amorphous, crystalline or mixed structure depending on the order of molecular arrangement in the powder particle matrices. In food production, the structure of powders has a greatly effect on their stability, functionality, and applicability. The undesirable structure of powders can be accidentally formed during production. Therefore, characterization of powder structure as well as quantification of amorphous–crystalline proportions presenting in the powders are essential to control the quality of products during storage and further processing. For these purposes, many analytical techniques with large differences in the degree of selectivity and sensitivity have been developed. In this review, differences in the structure of food powders are described with a focus being placed on applications of amorphous powders. Essentially, applicability of common analytical techniques including X-ray, microscopic, vapor adsorption, thermal, and spectroscopic approaches for quantitative and qualitative structural characterization of food powders is also discussed.     

Odor Perception and Beliefs about Risk

Although the perceptual response to environmental odors canbe quite variable, such variation has often been attributedto differences in individual sensitivity. An information-processinganalysis of odor perception, however, treats both the receptionand the subsequent evaluation of odor information as determinantsof the perceptual response. Two experiments investigated whethera factor that influenced the evaluation stage affected the judgementof odor quality and the degree of adaptation to the odor. Peoplewere surveyed in order to measure their tacit perceptions ofthe healthfulness or hazardousness of nine common olfactorystimuli, and the instructional context influenced quality perception.In a second experiment subjects were exposed to an ambient odorunder one of three different conditions, and odorant characterizationinfluenced the degree of adaptation to the odor. Subjects whowere led to believe the odor was a natural, healthy extractshowed adaptation; those told that the odor was potentiallyhazardous showed apparent sensitization; while those told thatthe odor was a common olfactory test odorant showed a mixedpattern: some exhibited adaptation, whereas others showed sensitization.However, detection thresholds obtained before and after exposureshowed adaptation effects that are characteristic of continuousexposure. These findings raise the possibility that cognitivefactors may be modulating the overall sensory perception ofodor exposure (i) for some individuals who exhibit extreme sensitivityto odors and (ii) in situations where adaptation to environmentalodors is expected but does not occur. Chem. Senses 21: 447–458,1996.     

Coding of odors by a receptor repertoire

We provide a systematic analysis of how odor quality, quantity, and duration are encoded by the odorant receptor repertoire of the Drosophila antenna. We test the receptors with a panel of over 100 odors and find that strong responses are sparse, with response density dependent on chemical class. Individual receptors range along a continuum from narrowly tuned to broadly tuned. Broadly tuned receptors are most sensitive to structurally similar odorants. Strikingly, inhibitory responses are widespread among receptors. The temporal dynamics of the receptor repertoire provide a rich representation of odor quality, quantity, and duration. Receptors with similar odor sensitivity often map to widely dispersed glomeruli in the antennal lobe. We construct a multidimensional "odor space" based on the responses of each individual receptor and find that the positions of odors depend on their chemical class, concentration, and molecular complexity. The space provides a basis for predicting behavioral responses to odors.     

Emotional stimulation alters olfactory sensitivity and odor judgment

Emotions have a strong influence on the perception of visual and auditory stimuli. Only little is known about the relation between emotional stimulation and olfactory functions. The present study investigated the relationship between the presentation of affective pictures, olfactory functions, and sex. Olfactory performance was assessed in 32 subjects (16 male). Olfactory sensitivity was significantly reduced following unpleasant picture presentation for all subjects and following pleasant picture presentation for male subjects only. Pleasantness and intensity ratings of a neutral suprathreshold odor were related to the valence of the pictures: After unpleasant picture presentation, the odor was rated as less pleasant and more intense, whereas viewing positive pictures induced a significant increase in reported odor pleasantness. We conclude that inducing a negative emotional state reduces olfactory sensitivity. A relation to functional deviations within the primary olfactory cortices is discussed.     

CROSS-MODAL ADDITIVITY OF TASTE AND SMELL

Subjects were simultaneously given subthreshold levels of taste and odor stimuli, delivered orally, for both a commonly paired and an uncommonly paired taste–odor combination. Results indicate cross‐modal summation of subthreshold concentrations of both taste–odor pairs when the olfactory stimulus is delivered orally. Results of control studies suggest that the summation was indeed across modalities, and not due to the taste of the odor compound or the smell of the taste compounds. Furthermore, results indicate that regardless of taste–odor pair commonness, taste and smell can combine in a completely additive fashion (i.e., at threshold detectability when both stimuli are presented simultaneously at 50% threshold level) if the taste–odor pair is presented orally. In several instances, but not all, measured probabilities exceeded those predicted by probability summation, indicating that hyperadditive mixing often occurs, but there do seem to be individual differences. Cross‐modal summation, regardless of taste–odor pair commonness, has broader implications for the development of foods, beverages and pharmaceuticals, especially in masking undesirable tastes and smells.     

Determination of Photoreceptor Cell Spectral Sensitivity in an Insect Model from In Vivo Intracellular Recordings

Intracellular recording is a powerful technique used to determine how a single cell may respond to a given stimulus. In vision research, intracellular recording has historically been a common technique used to study sensitivities of individual photoreceptor cells to different light stimuli that is still being used today. However, there remains a dearth of detailed methodology in the literature for researchers wishing to replicate intracellular recording experiments in the eye. Here we present the insect as a model for examining eye physiology more generally. Insect photoreceptor cells are located near the surface of the eye and are therefore easy to reach, and many of the mechanisms involved in vision are conserved across animal phyla. We describe the basic procedure for in vivo intracellular recording of photoreceptor cells in the eye of a butterfly, with the goal of making this technique more accessible to researchers with little prior experience in electrophysiology. We introduce the basic equipment needed, how to prepare a live butterfly for recording, how to insert a glass microelectrode into a single cell, and finally the recording procedure itself. We also explain the basic analysis of raw response data for determining spectral sensitivity of individual cell types. Although our protocol focuses on determining spectral sensitivity, other stimuli (e.g., polarized light) and variations of the method are applicable to this setup.     

Olfactory ecology and the processing of complex mixtures

Natural olfactory stimuli typically are mixtures of which the identities, concentrations, and ratios of chemical constituents are important for many odor-mediated behaviors. Despite abundant behavioral examples, links between odor-evoked behavior and the processing and discrimination of complex olfactory stimuli remains an area of active study. Coupling electrophysiological and behavioral experiments, recent studies in a variety of different insect models have provided new insights into the perceptual and neural mechanisms about how natural olfactory stimuli are processed, and how plasticity and internal state of the insect may influence the odor representation. These studies show that complex stimuli are represented in unique percepts that are different from their individual constituents, and that the representation may be modulated by experience and influenced by other sensory modalities.     

Genetic variability and robustness of host odor preference in Drosophila melanogaster

Chemosensory stimuli play a crucial role for host selection in insects, including the fruit fly Drosophila melanogaster. Drosophila has been instrumental in unraveling the neurological basis of olfactory processing in insects. Basic knowledge regarding chemical ecology and thorough studies of olfactory preferences are still lacking to a great extent in D. melanogaster, however. We have characterized repeatable variation in olfactory preference between five classical D. melanogaster wild-type strains toward a large array of natural host odors and synthetic compounds. By recording the rate of attraction over up to 24 hr, we could compare stimuli varying in attractiveness and characterize phenotypic parameters on the basis of individual stimuli and the whole stimulus array. Behavioral differences between strains were predominantly due to variation in a single phenotypic parameter: their overall responsiveness toward optimal and suboptimal olfactory stimuli. These differences were not explained by variation in olfactory sensitivity, locomotory activity, or general vigor monitored by survival. Comparisons with three recently established wild-type strains indicated that a high behavioral threshold against accepting suboptimal olfactory stimuli is the characteristic phenotype of wild D. melanogaster.     

Sensitivity to reinforcement in successive and delayed discriminations

Pigeons' responses were recorded in successive 15-s subintervals of 60-s components of several multiple variable-interval schedules of food reinforcement. In the standard multiple schedule or successive discrimination, discriminative stimuli were present throughout each component. In the delayed discrimination or memory procedure, red or green stimuli were present in the first 15 s of components and were followed by a white stimulus for the remainder of both components. Ratios of responses in the first 15 s of the two components, where discriminative stimuli were present, were sensitive to ratios of reinforcers obtained in the two components, to the same extent in both multiple and memory procedures. In both procedures, sensitivity to reinforcement decreased systematically over component subintervals, but to a greater extent in the memory procedure where discriminative stimuli were absent. The reduction in sensitivity with time since presentation of prior discriminative stimuli in the memory procedure was therefore influenced by two main factors: delayed stimulus control by the discriminative stimuli presented earlier in the component, and a decrease in sensitivity to reinforcement with increasing time since component alternation.     

A Single-fly Assay for Foraging Behavior in Drosophila

For many animals, hunger promotes changes in the olfactory system in a manner that facilitates the search for appropriate food sources. In this video article, we describe an automated assay to measure the effect of hunger or satiety on olfactory dependent food search behavior in the adult fruit fly Drosophila melanogaster. In a light-tight box illuminated by red light that is invisible to fruit flies, a camera linked to custom data acquisition software monitors the position of six flies simultaneously. Each fly is confined to walk in individual arenas containing a food odor at the center. The testing arenas rest on a porous floor that functions to prevent odor accumulation. Latency to locate the odor source, a metric that reflects olfactory sensitivity under different physiological states, is determined by software analysis. Here, we discuss the critical mechanics of running this behavioral paradigm and cover specific issues regarding fly loading, odor contamination, assay temperature, data quality, and statistical analysis.     

High hunger state increases olfactory sensitivity to neutral but not food odors

Understanding how hunger state relates to olfactory sensitivity has become more urgent due to their possible role in obesity. In 2 studies (within-subjects: n = 24, between-subjects: n = 40), participants were provided with lunch before (satiated state) or after (nonsatiated state) testing and completed a standardized olfactory threshold test to a neutral odor (Experiments 1 and 2) and discrimination test to a food odor (Experiment 2). Experiment 1 revealed that olfactory sensitivity was greater in the nonsatiated versus satiated state, with additionally increased sensitivity for the low body mass index (BMI) compared with high BMI group. Experiment 2 replicated this effect for neutral odors, but in the case of food odors, those in a satiated state had greater acuity. Additionally, whereas the high BMI group had higher acuity to food odors in the satiated versus nonsatiated state, no such differences were found for the low BMI group. The research here is the first to demonstrate how olfactory acuity changes as a function of hunger state and relatedness of odor to food and that BMI can predict differences in olfactory sensitivity.     

Emerging principles of molecular signal processing by mitral/tufted cells in the olfactory bulb

The olfactory system shares many principles of functional organization with other sensory systems, but differs in that the sensory input is in the form of molecular information carried in odor molecules. Current studies are providing new insights into how this information is processed. In analogy with the spatial receptive fields of visual neurons, the molecular receptive range of olfactory cells is defined as the range of odor molecules that will affect the firing of that cell. Olfactory receptor molecules belong to a large gene family; it is hypothesized that individual receptor molecule may have relatively broad molecular receptive ranges, and that an individual receptor cell need therefore express only one or a few different types of receptors to cover a broad range. Mitral/tufted cells have narrower molecular receptive ranges, comprising molecules with related structures (odotopes). This is believed to reflect processing through the olfactory glomeruli, each glomerulus acting as a convergence center for related inputs. Varying overlapping specificities of receptor cells, glomeruli and mitral/tufted cells appear to provide the basis for discrimination of odor molecules, in analogy with discrimination of color in the visual systems.     

Measures of individual differences in taste and creaminess perception

Previous reports that the sensitivity to the bitter tasting substance 6-n-propylthiouracil (PROP) is related to the sensitivity to other tastes, to chemical irritants, and to fats and oils have led to adoption of PROP as a measure of general oral sensitivity and as a predictor of dietary habits that could impact health. The results, however, have not been consistent. It was recently discovered that the ability to perceive "thermal taste" (i.e., sweetness from thermal stimulation alone) was associated with higher responsiveness to 4 prototypical taste stimuli but not to PROP. This finding implied that individual differences in taste perception are determined in large part by factors other than those related to genetic expression of the PROP receptor. The present study followed up this observation by comparing individual differences in perception of 4 prototypical taste stimuli (sucrose, NaCl, citric acid, and quinine) and PROP under conditions that also enabled assessment of the reliability of individual intensity ratings of taste. Creaminess ratings of 3 milk products that had different fat contents were also collected to investigate further the relationship between taste and oral somatosensory perception. The results showed that intensity ratings across 2 trials were significantly correlated for all 5 taste stimuli and that averaging across replicates led to significant correlations among the 4 prototypical stimuli. In contrast, the bitterness of PROP was correlated only with the bitterness of quinine. None of the taste stimuli, including PROP, was significantly correlated with ratings of creaminess. These results imply 1) that with the exception of PROP, as few as 2 intensity ratings of common taste stimuli can reveal individual differences in overall taste perception and 2) that any relationship between taste and oral sensation is too weak to be detected under the same conditions. Accordingly, the results support other evidence that the genetic factors which determine the ability to perceive PROP do not play a major role in overall taste and oral somatosensory perception.     

Individual differences in sensitivity to the odor of 4,16-androstadien-3-one

Individual differences in sensitivity to the putative human pheromone androstadienone were investigated in three experiments. In experiment 1, the absolute detection threshold for androstadienone was determined to be 211 micro M using the method of constant stimuli. Detection for the related compound estratetraenol was also investigated but a threshold could not be determined. In experiment 2, using an adaptive threshold test on 100 participants, the sensitivity distribution for androstadienone, but not for the reference odor phenylethyl alcohol, was bimodal, with a smaller group of individuals with a high sensitivity to androstadienone (supersmellers). A lack of correlation between thresholds for androstadienone and phenylethyl alcohol further suggested that the bimodality for androstadienone was not due to individuals with a high general olfactory sensitivity. In line with an earlier observation, there was a statistical tendency for women to be more sensitive to androstadienone than men. Results of experiment 3 preclude the possibility that the bimodal sensitivity distribution for androstadienone would depend on individual differences in trigeminal activation. Altogether, the current study suggests that olfactory sensitivity to androstadienone is bimodally distributed in the population with a subgroup consisting of highly sensitive people.     

Individual differences in the chemical senses: is there a common sensitivity?

Taste, smell, and chemical irritation (so-called trigeminal sensation) combine in our daily experience to produce the supramodal sensation of flavor, are processed by partly overlapping neural mechanisms, and show functional interconnectivity in experiments. Given their collaboration in flavor formation and the well-established connections between these senses, it is plausible that polymodal detection mechanisms might contribute to individual differences in measured sensitivity. One would expect the existence of a general chemosensory sensitivity factor to result in associations among taste, smell, and trigeminal stimulation thresholds. Measures of 5 detection thresholds from all the chemical senses were assessed in the same group of young healthy subjects (n=57). An unbiased principal components analysis (PCA) yielded a 2-component solution. Component 1, on which taste thresholds loaded strongly, accounted for 29.4% of the total variance. Component 2, on which the odor and trigeminal lateralization thresholds loaded strongly, accounted for 26.9% of the total variance. A subsequent PCA restricted to a 3-component solution cleanly separated the 3 sensory modalities and accounted for 75% of the total variance. Thus, though there may be a common underlying factor that determines some individual differences in odor and trigeminal lateralization thresholds, a general chemical sensitivity that spans chemosensory modalities seems unlikely.     

The molecular basis of odor coding in the Drosophila larva

We have analyzed the molecular basis of odor coding in the Drosophila larva. A subset of Or genes is found to be expressed in larval olfactory receptor neurons (ORNs). Using an in vivo expression system and electrophysiology, we demonstrate that these genes encode functional odor receptors and determine their response spectra with 27 odors. The receptors vary in their breadth of tuning, exhibit both excitation and inhibition, and show different onset and termination kinetics. An individual receptor appears to transmit signals via a single ORN to a single glomerulus in the larval antennal lobe. We provide a spatial map of odor information in the larval brain and find that ORNs with related functional specificity map to related spatial positions. The results show how one family of receptors underlies odor coding in two markedly different olfactory systems; they also provide a molecular mechanism to explain longstanding observations of larval odor discrimination.     

A plethora of painful molecules

Pain is a fundamental experience with a complex and multi-layered neurobiological basis. In recent years a powerful battery of techniques has been brought to bear to unravel the mechanisms by which painful stimuli are transduced and processed. There have been several recent discoveries regarding the molecular transduction mechanisms in nociceptors and novel molecular and cellular mechanisms underlying the spinal processing of painful stimuli. The mechanisms by which sensory neurons initiate hyperalgesia and touch evoked pain (allodynia) have been addressed particularly successfully in recent studies. The rich variety of key molecular players that have emerged in physiological and pathophysiological pain states reflects the sophistication and uniqueness of this vitally important sense.     

A physicochemical model of odor sampling

We present a general physicochemical sampling model for olfaction, based on established pharmacological laws, in which arbitrary combinations of odorant ligands and receptors can be generated and their individual and collective effects on odor representations and olfactory performance measured. Individual odor ligands exhibit receptor-specific affinities and efficacies; that is, they may bind strongly or weakly to a given receptor, and can act as strong agonists, weak agonists, partial agonists, or antagonists. Ligands interacting with common receptors compete with one another for dwell time; these competitive interactions appropriately simulate the degeneracy that fundamentally defines the capacities and limitations of odorant sampling. The outcome of these competing ligand-receptor interactions yields a pattern of receptor activation levels, thereafter mapped to glomerular presynaptic activation levels based on the convergence of sensory neuron axons. The metric of greatest interest is the mean discrimination sensitivity, a measure of how effectively the olfactory system at this level is able to recognize a small change in the physicochemical quality of a stimulus. This model presents several significant outcomes, both expected and surprising. First, adding additional receptors reliably improves the system’s discrimination sensitivity. Second, in contrast, adding additional ligands to an odorscene initially can improve discrimination sensitivity, but eventually will reduce it as the number of ligands increases. Third, the presence of antagonistic ligand-receptor interactions produced clear benefits for sensory system performance, generating higher absolute discrimination sensitivities and increasing the numbers of competing ligands that could be present before discrimination sensitivity began to be impaired. Finally, the model correctly reflects and explains the modest reduction in odor discrimination sensitivity exhibited by transgenic mice in which the specificity of glomerular targeting by primary olfactory neurons is partially disrupted.