Central Processing of Odor Concentration is a Temporal Phenomenon as Revealed by Chemosensory Event-Related Potentials (CSERP)

Chemosensory event-related potentials (CSERP) can be used toexamine central nervous odor processing. An important questionfor understanding odor perception is how different concentrationsare processed. In the present study two odors were chosen whichactivate either the olfactory (linalool) or the trigeminal (menthol)system. Both odors were presented to 11 subjects in four differentconcentrations. Four subjects had to attend actively to theodors while the others perceived the odors under passive attention.The results showed that increased concentrations of the olfactorystimulus resulted in shorter latencies of the N1 component butdid not affect the amplitudes of the CSERP. However, the amplitudesof the stimulus dependent, exogenous components (N1, P2) increasedwith higher concentrations of the trigeminal stimulus. The amplitudeof the late positive complex, which reflects endogenous processes,was usually larger when the odorous stimuli had to be attendedto actively. It is concluded that olfactory intensity codingresults in a qualitatively different but not in a stronger neuronalresponse of the human brain. Chem. Senses 22: 9–26, 1997.     

Bumble Bees Show Asymmetrical Discrimination Between Two Odors in a Classical Conditioning Procedure

Olfactory processing of two odorants and their mixture was investigated in bumble bees Bombus terrestris using classical conditioning of the proboscis extension. In a standard procedure, workers were able to learn linalool, phenylacetaldehyde, and the mixture of these two components, with a similar level of response to these three stimuli. Thereafter, when we applied a differential conditioning procedure where one rewarded odorant was presented alternately against an unrewarded one, an asymmetrical discrimination between the two pure odors was found. Bumble bees performed well in the discriminative task when linalool was the rewarded stimulus and phenylacetaldehyde the unrewarded one, but they had difficulty learning phenylacetaldehyde if it was the rewarded odor in the symmetrical procedure. Indeed, unrewarded stimulations with linalool appeared to disrupt the learning of the alternative odor, possibly due to an innate biological meaning of linalool.     

Different thresholds for detection and discrimination of odors in the honey bee (Apis mellifera)

Naturally occurring odors used by animals for mate recognition, food identification and other purposes must be detected at concentrations that vary across several orders of magnitude. Olfactory systems must therefore have the capacity to represent odors over a large range of concentrations regardless of dramatic changes in the salience, or perceived intensity, of a stimulus. The stability of the representation of an odor relative to other odors across concentration has not been extensively evaluated. We tested the ability of honey bees to discriminate pure odorants across a range of concentrations at and above their detection threshold. Our study showed that pure odorant compounds became progressively easier for honey bees to discriminate with increasing concentration. Discrimination is, therefore, a function of odorant concentration. We hypothesize that the recruitment of sensory cell populations across a range of concentrations may be important for odor coding, perhaps by changing its perceptual qualities or by increasing its salience against background stimuli, and that this mechanism is a general property of olfactory systems.     

Perceptual blending in odor mixtures depends on the nature of odorants and human olfactory expertise

Our olfactory system is confronted with complex mixtures of odorants, often recognized as single entities due to odor blending (e.g., coffee). In contrast, we are also able to discriminate odors from complex mixtures (e.g., off-odors). Therefore, the olfactory system is able to engage either configural or elemental processes when confronted with mixtures. However, the rules that govern the involvement of these processes during odor perception remain poorly understood. In our first experiment, we examined whether simple odorant mixtures (binary/ternary) could elicit configural perception. Twenty untrained subjects were asked to evaluate the odor typicality of mixtures and their constituents. The results revealed a significant increase in odor typicality in some but not all mixtures as compared with the single components, which suggest that perceptual odor blending can occur only in specific mixtures (configural processing). In our second experiment, we tested the hypothesis that general olfactory expertise can improve elemental perception of mixtures. Thirty-two trained subjects evaluated the odor typicality of the stimuli presented during the first experiment, and their responses were compared with those obtained from the untrained panelists. The results support the idea that general training with odors increases the elemental perception of binary and ternary blending mixtures.     

Odor similarity does not influence the time needed for odor processing

The brain's link between perception and action involves several steps, which include stimulus transduction, neuronal coding of the stimulus, comparison to a memory template and choice of an appropriate behavioral response. All of these need time, and many studies report that the time needed to compare two stimuli correlates inversely with the perceived distance between them. We developed a behavioral assay in which we tested the time that a honeybee needs to discriminate between odors consisting of mixtures of two components, and included both very similar and very different stimuli spanning four log-concentration ranges. Bees learned to discriminate all odors, including very similar odors and the same odor at different concentrations. Even though discriminating two very similar odors appears to be a more difficult task than discriminating two very distinct substances, we found that the time needed to make a choice for or against an odor was independent of odor similarity. Our data suggest that, irrespective of the nature of the olfactory code, the bee olfactory system evaluates odor quality after a constant interval. This may ensure that odors are only assessed after the olfactory network has optimized its representation.     

Effect of repeated presentation on sweetness intensity of binary and ternary mixtures of sweeteners

The purpose of the present study was to determine the effect of repeated presentation of the same sweet stimulus on sweetness intensity ratings. The sweet stimuli tested in this study were binary and ternary blends of 14 sweeteners that varied widely in chemical structure. A trained panel evaluated the sweetness intensity over four sips of a given mixture presented at 30 s intervals. The individual components in the binary sweetener combinations were intensity-anchored with 5% sucrose, while the individual sweeteners in the ternary mixtures were intensity-anchored with 3% sucrose (according to formulae developed previously). Each self-mixture was also evaluated (e.g. acesulfame-K-acesulfame-K). The main finding of this study was that mixtures consisting of two or three different sweeteners exhibited less reduction in sweetness intensity over four repeated sips than a single sweetener at an equivalent sweetness level. Furthermore, ternary combinations tended to be slightly more effective than binary combinations at lessening the effect of repeated exposure to a given sweet stimulus. These findings suggest that the decline in sweetness intensity experienced over repeated exposure to a sweet stimulus could be reduced by the blending of sweeteners.     

Human odor detectability: new methodology used to determine threshold and variation

Current ambiguity concerning the related issues of optimal means for measurement of odor sensitivity and the functional properties of the olfactory system hinders progress in basic and applied research on the human sense of smell. To address these needs, we selected n-amyl acetate (nAA) as a test odorant and developed a methodology in which participants (Ps) receive multiple presentations each session of several concentrations. Yes-no responses as to whether odor was detected are analyzed using binomial statistics, with the probability that a given proportion of yes responses (or greater) would occur by chance alone being treated as the inverse of detectability. Over the course of multiple sessions, this information is also used to maximize the collection of data in the peri-threshold region. Surprisingly, data collected over as many as 14 sessions were fit well by a single logistic regression model relating probability and concentration. Threshold concentrations, defined as those corresponding to a probability of 0.05, varied from 7.11 to 167.53 p.p.b. (v/v) for 11 Ps. Our approach and findings, if shown to be representative of other combinations of Ps and odorants, could accelerate the pace of research in human olfaction by providing a comprehensive operational definition of the limit of the olfactory system to detect odorant molecules.     

Intranasal localizability of odorants: influence of stimulus volume

When an odorant is presented to one side of the nose and air to the other, the ability to localize which side received the odorant depends upon trigeminal nerve stimulation. It has been shown that performance on this lateralization task increases as stimulus concentration increases. In this study, we determined the influences of stimulus volume and sex on the ability to localize each of 8 odorants presented at neat concentrations: anethole, geraniol, limonene, linalool, menthol, methyl salicylate, phenyl ethanol, and vanillin. At a low stimulus volume (11 mL), only menthol was localized at an above-chance level. At a high stimulus volume (21 mL), above-chance localization occurred for all odorants except vanillin. Women were significantly better than men in localizing menthol. Stimuli rated as most intense were those that were most readily localized. The detection performance measures, as well as rated intensity values, significantly correlated with earlier findings of the trigeminal detectability of odorants presented to anosmic and normosmic subjects. This study suggests that differences in stimulus volume may explain some discrepant findings within the trigeminal chemosensory literature and supports the concept that vanillin may be a "relatively pure" olfactory stimulus.     

Temporal integration in nasal lateralization and nasal detection of carbon dioxide

Two experiments examined time/concentration trading for the detection of carbon dioxide, an irritant with little or no odor. Experiment 1 employed the nasal lateralization method: subjects attempted to determine which nostril received carbon dioxide and which received pure air when presented simultaneously. Experiment 2 employed a temporal, two-alternative, forced-choice, detection paradigm with monorhinal stimulation. In both experiments, stimulus duration was varied at a number of fixed concentrations to determine the shortest, detectable pulse. Under both conditions, threshold pulse duration decreased as stimulus concentration increased. Power functions with exponents of less than negative one described the data quite well: More than a twofold increase in duration was needed to compensate for a twofold decrease in concentration. Thus, for carbon dioxide, the nasal trigeminal system functions as an imperfect integrator at threshold-level.     

High-resolution Quantification of Odor-guided Behavior in Drosophila melanogaster Using the Flywalk Paradigm

In their natural environment, insects such as the vinegar fly Drosophila melanogaster are bombarded with a huge amount of chemically distinct odorants. To complicate matters even further, the odors detected by the insect nervous system usually are not single compounds but mixtures whose composition and concentration ratios vary. This leads to an almost infinite amount of different olfactory stimuli which have to be evaluated by the nervous system.To understand which aspects of an odor stimulus determine its evaluation by the fly, it is therefore desirable to efficiently examine odor-guided behavior towards many odorants and odor mixtures. To directly correlate behavior to neuronal activity, behavior should be quantified in a comparable time frame and under identical stimulus conditions as in neurophysiological experiments. However, many currently used olfactory bioassays in Drosophila neuroethology are rather specialized either towards efficiency or towards resolution.Flywalk, an automated odor delivery and tracking system, bridges the gap between efficiency and resolution. It allows the determination of exactly when an odor packet stimulated a freely walking fly, and to determine the animal´s dynamic behavioral reaction.     

Gender differences in chemosensory perception and event-related potentials

The present study investigated chemosensory gender differences by means of ratings of total nasal chemosensory intensity, unpleasantness and sensory irritation and simultaneous recordings of chemosensory event-related potentials (CSERPs) for three concentrations of the olfactory/trigeminal stimulus pyridine in 19 women and 17 men, all young adults. Results show that, compared to men, women gave higher intensity and unpleasantness ratings, in particular for the highest stimulus concentration. The gender differences in perceived intensity are reflected in the signal-to-noise ratio of the individual CSERP averages, revealing more identifiable early components (P1, N1) in women than in men. The late positive component, labeled P2/P3, displayed larger amplitudes at all electrode sites and shorter latencies at Cz, in women compared to men. The effects of increased pyridine concentration on perception (larger in women) and CSERPs (similar across gender) imply that the two measures involve partially different neural processing. CSERP component identifiability is proposed here as a general means of assessing signal-to-noise ratio of the CSERPs.     

Color enhances orthonasal olfactory intensity and reduces retronasal olfactory intensity

The effect of color on orthonasal and retronasal odor intensities was investigated. When odorants were smelled orthonasally (i.e., through the nostrils), color enhanced odor intensity ratings, consistent with previous reports. However, when odorants were smelled retronasally (i.e., the odorous solution was put in the mouth), color reduced odor intensity ratings. These different effects of color on odor intensity (i.e., enhancement orthonasally and suppression retronasally) appear to be the result of route of olfactory stimulation rather than of any procedural artifact. This supports previous reports that retronasal and orthonasal odors are perceived differently.     

Perceptual interactions in odour mixtures: odour quality in binary mixtures of woody and fruity wine odorants

The qualitative perceptual interactions in three binary mixtures of wine odorants were studied: isoamyl acetate (fruity note)/whisky lactone (woody note), ethyl butyrate (fruity note)/whisky lactone (woody note) and ethyl butyrate (fruity note)/guaiacol (woody note). For each binary mixture, the perceived quality and intensity of 24 stimuli (four supra-threshold concentration levels of each of the two compounds and their 16 possible combinations) were evaluated in five replications by a trained panel of 13 subjects. The application of the Olsson predictive model for odour intensity and quality perception gave quite a good estimation of the evolution of single component identification in the mixture when the intensity proportion of unmixed components varied. However, this model was unable to account for the odour quality dominance in mixtures of iso-intense components. An alternative linear logistic model was proposed to study the qualitative dominance of the woody note in the three mixtures when the perceived intensities of each unmixed compound were equal.     

Chemosensory responses in isolated olfactory receptor neurons from Necturus maculosus

Olfactory receptor neurons were isolated without enzymes from the mudpuppy, Necturus maculosus, and tested for chemosensitivity. The cells responded to odorants with changes in firing frequency and alterations in excitability that were detected with tight-seal patch electrodes using on-cell and whole-cell recording conditions. Chemosensitive cells exhibited two primary response characteristics: excitation and inhibition. Both types of primary response were observed in different cells stimulated by mixtures of amino acids as well as by the single compound L-alanine, suggesting that there may be more than one transduction pathway for some odorants. Using the normal whole-cell recording method, the chemosensitivity of competent cells washed out rapidly; a resistive whole-cell method was used to record odorant responses under current-clamp conditions. In response to chemical stimulation, excitability appeared to be modulated in several different ways in different cells: odorants induced hyperpolarizing or depolarizing receptor potentials, elicited or inhibited transient, rhythmic generator potentials, and altered excitability without changing the membrane potential or input resistance. These effects suggest that olfactory transduction is mediated through at least three different pathways with effects on four or more components of the membrane conductance. Polychotomous pathways such as these may be important for odor discrimination and for sharpening the "odor image" generated in the olfactory epithelium.     

The addition of CO2 to traditional taste solutions alters taste quality

Previous studies of the effect of carbonation on taste perception have suggested that it may be negligible, manifesting primarily in increases in the perceived intensity of weak salt and sour stimuli. Assuming CO2 solutions in the mouth stimulate only trigeminal nerve endings, this result is not altogether surprising; however, there are neurophysiological data indicating that CO2 stimulates gustatory as well as trigeminal fibers. In that case, carbonation might alter the quality profile of a stimulus without producing substantial changes in overall taste intensity--much as occurs when qualitatively different taste stimuli are mixed. To address this possibility, subjects were asked to rate the total taste intensity of moderate concentrations of stimuli representing each of the basic tastes and their binary combinations, with an without added carbonation. They then subdivided total taste intensity into the proportions of sweetness, saltiness, sourness, bitterness and 'other taste qualities' they perceived. The addition of carbonation produced only small increases in ratings of total taste intensity. However, rather dramatic alterations in the quality profiles of stimuli were observed, particularly for sweet and salty tastes. The nature of the interaction is consistent with a direct effect of carbonation/CO2 on the gustatory system, although the possibility that at least some of the observed effects reflect trigeminal-gustatory interactions cannot be ruled out.      

Contextual effects in the perception of quinine HCl/NaCl mixtures

Stimulus distribution, stimulus spacing and stimulus range affectmean intensity ratings of solutions of unmixed tastants. Thepresent study compares contextual effects for mixture ratingswith those for unmixed stimuli: if ratings for mixed and unmixedstimuli are differentially affected by context, the degree ofmixture suppression inferred from the responses is context-dependent.Bitterness intensity ratings for unmixed quinine and quinineHCl/NaCl mixtures were not differentially affected by shiftsin stimulus frequency distribution. The subjects' tendency tobe consistent in their responses to identical stimuli, resultsin a transfer of stimulus context of previous sessions to thenext.