Predicting odorant quality perceptions from multidimensional scaling of olfactory bulb glomerular activity patterns.

Odorants and their perceptions differ along multiple dimensions, requiring that a critical examination of any putative neural code directly assess the multidimensional nature of the encoding process. Previous work has examined simple, systematic odorant differences that, regardless of coding strategy, would be expected to produce simple, systematic predictions in neural and behavioral responses. In the present study, an odorant identification confusion matrix task that extracts precise quality relationships across odorants was used to determine whether spatially specific glomerular activity patterns predict perceptual quality relationships for odorants that cannot easily be classified a priori along a single chemical dimension. Multidimensional scaling (MDS) analysis of odorant pattern similarity measures derived from the comparison of [14C]-2-deoxyglucose glomerular activity pattern data yielded a two-dimensional odorant activity space that was highly significantly predictive of similarly obtained odorant perceptual spaces, uniformly across animals. These results strongly support the relevance of global spatial patterns in the olfactory bulb to the encoding of odor quality. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Intensity Modulation of Olfactory Acuity.

Acuity is fundamental to sensory systems, establishing the foundation for detectable differences in stimulus quality and consequently shaping animals' sensory capacities. In the olfactory system, which samples intrinsically high-dimensional chemical information, acuity for odor quality is measurable by means of ad hoc dimensions based on behaviorally confirmed sets of sequentially similar odorants. The authors measure olfactory acuity in mice using a rewarded forced-choice odor generalization task and show that mice exhibit greater olfactory acuity in response to higher concentration (1,0 Pa) odorants than to lower concentration (0.01 Pa) odorants. Results suggest that the dynamic modulation of sensory acuity-not necessarily its maximization-is an important component of olfactory processing and reflects the salience of odorant stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Odorant identification and quality perception following methyl bromide-induced lesions of the olfactory epithelium.

Using a 5-odorant identification confusion matrix task, the authors assessed the consequence of olfactory epithelial damage on odorant quality perception in the rat. After establishing prelesion identification performance, each rat's epithelium was subjected to 330 ppm methyl bromide gas for 6 hr. Comparison of prelesion and 3-day postlesion performance demonstrated a significant decrease in identification as a consequence of 95%-98% epithelial destruction. Further, there was a differential effect of lesion on the ability of different animals to identify the different individual odorants. Evaluation of the anatomical state of the epithelium relative to performance on the identification task demonstrated a significant relationship between the extent and location of anatomical sparing and changes in individual odorant identifications. Assessment of pre- and postlesion quality perception for the individual rats demonstrated a highly significant shift in quality perception that was independent of any decrease in performance. These results provide strong support for the proposition that the regional variations in mucosal sensitivities within and across olfactory receptor gene expression zones are fundamentally important for the encoding of odorant quality. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The Effect of Verbal Context on Olfactory Perception.

Three experiments were conducted to examine how verbal context and sensory stimulation interact to influence odor hedonic perception. Eight common odors were presented in their natural and synthetic forms, and verbal labels designating name and source (natural, synthetic) information were either explicitly given, self-generated, falsely provided, or not provided. Results revealed that verbal information about source influenced hedonic ratings whether or not the odorant itself was also present. When odorants were presented without verbal labels, olfactory evaluations were based in sensation. Name and source information contributed different levels of meaning and influence to perceptual evaluations. The findings are discussed with reference to an experiential-collocation model for odor-label interactions and a dual-coding hypothesis for olfactory perception. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of perceptual and structural similarity in cross-adaptation

Cross-adaptation, the decrease in sensitivity to one odorant following exposure to a different odorant, is affected by odorant similarity, both perceptual and structural, but the precise relationship is obscure. The present series of studies was designed to explore various aspects of perceptual and structural similarity as they relate to cross-adaptation. In Experiment 1, cross-adaptation was assessed between androstenone and five odorants that share a common urinous note with androstenone, but retain unique perceptual characteristics; only the compound judged most perceptually similar to androstenone cross-adapted it. In Experiment 2, odorants both perceptually and structurally similar (androstenone and androstanone) displayed significant, mutual cross-adaptation. Furthermore, magnitude estimates for androstanone were significantly reduced following exposure to 3-methylidene-5 alpha-androstane (3M5A), a structurally similar, perceptually odorless compound. This finding appears to be the first demonstration that an odorless compound can affect, via cross-adaptation, the perception of an odorous compound. Finally, in Experiment 3, significant, asymmetric cross-adaptation was observed between compounds that are perceptually and structurally dissimilar (4-cyclohexylcyclohexanone [4-CHCH] and androstenone). These findings indicate that the role of similarity in cross-adaptation is difficult to quantify and emphasize the numerous odorant characteristics that can affect cross-adaptation.     

How synchronization properties among second-order sensory neurons can mediate stimulus salience.

Spatial patterns of glomerular activity in the vertebrate olfactory bulb and arthropod antennal lobe reflect an important component of first-order olfactory representation and contribute to odorant identification. Higher concentration odor stimuli evoke broader glomerular activation patterns, resulting in greater spatial overlap among different odor representations. However, behavioral studies demonstrate results contrary to what these data might suggest: Honeybees are more, not less, able to discriminate among odorants applied at higher concentrations. Using a computational model of the honeybee antennal lobe, the authors show that changes in synchronization patterns among antennal lobe projection neurons, as observed electrophysiologically, could parsimoniously underlie these observations. The results suggest that stimulus salience, as defined behaviorally, is directly correlated with the degree of synchronization among second-order olfactory neurons. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Olfactory bulb habituation to odor stimuli.

Habituation is a simple form of memory, yet its neurobiological mechanisms are only beginning to be understood in mammals. In the olfactory system, the neural correlates of habituation at a fast experimental timescale involving very short intertrial intervals (tens of seconds) have been shown to depend on synaptic adaptation in olfactory cortex. In contrast, behavioral habituation to odorants on a longer timescale with intertrial intervals of several minutes depends on processes in the olfactory bulb, as demonstrated by pharmacological studies. We here show that behavioral habituation to odorants on this longer timescale has a neuronal activity correlate in the olfactory bulb. Spiking responses of mitral cells in the rat olfactory bulb adapt to, and recover from, repeated odorant stimulation with 5-min intertrial intervals with a time course similar to that of behavioral habituation. Moreover, both the behavioral and neuronal effects of odor habituation require functioning N-methyl-d-aspartic acid receptors in the olfactory bulb. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A critical test of the overlap hypothesis for odor mixture perception.

The overlap hypothesis of mixture perception is based on the observation that mixtures of perceptually similar odorants tend to smell different from their components (configural), whereas mixtures of dissimilar odorants smell like their components (elemental). Because input patterns of perceptually similar odorants tend to overlap more than dissimilar ones, it has been hypothesized that component pattern overlap can predict a mixture's perceptual quality, with high overlap predicting a configural response and low overlap an elemental response. The authors used 7 pairs of odorants chosen for different degrees of overlap in their monomolecular 2-deoxyglucose activation patterns to test the theory in a go/no-go behavioral assay that measured generalization from binary mixtures to components. The authors show that individual component odorant input patterns are not sufficient to predict mixture quality, falsifying the overlap hypothesis. An important finding is that different odorant pairs with similar glomerular overlap showed opposite behavioral–perceptual responses, suggesting nonlinear effects at the receptor or glomerular level or the critical involvement of higher order areas. Thus, the authors posit that imaging the mixtures themselves may provide additional information needed to reliably predict mixture quality. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Olfactory discrimination of aliphatic odorants in South African fur seals (arctocephalus pusillus).

Using a food-rewarded, two-choice, instrumental conditioning paradigm we assessed the ability of South African fur seals, Arctocephalus pusillus, to discriminate between members of five chemical classes of aliphatic odorants presumed to differ in their abundance in the marine chemical environment. We found that the fur seals were able to distinguish between 24 of the 25 odor pairs presented and thus have a well-developed ability to discriminate between structurally related odorants, that aliphatic n-acetic esters were significantly more poorly discriminated by the fur seals than aliphatic n-aldehydes and n-carboxylic acids, and a lack of correlations between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length. These results suggest that the sense of smell may play an important and hitherto underestimated role in regulating the behavior of fur seals. Further, they support the notion that regular connections between the perceived quality of odorants and their molecular structural properties are not a general phenomenon but appear to be odorant class- and species-specific. Our data support the hypothesis that a species' chemical environment may affect its olfactory capabilities. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Selective loss of cholinergic neurons projecting to the olfactory system increases perceptual generalization between similar, but not dissimilar, odorants.

The neuromodulator acetylcholine is thought to modulate information processing in the olfactory system. The authors used 192 IgG-saporin, a lesioning agent selective for basal forebrain cholinergic neurons, to determine whether selective lesions of cholinergic neurons projecting to the olfactory bulb and cortex affect odor perception in rats. Lesioned and sham-operated rats were tested in an olfactory generalization paradigm with sets of chemically related odorants (n-aliphatic aldehydes, acids, and alcohols). Lesioned rats generalized more between chemically similar odorants but did not differ from controls in their response to chemically unrelated odorants or in acquisition of the conditioned odor. Results show that cholinergic inputs to the olfactory system influence perceptual qualities of odorants and confirm predictions made by computational models of this system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Noradrenergic neuromodulation in the olfactory bulb modulates odor habituation and spontaneous discrimination.

Noradrenergic projections from the locus coeruleus (LC) project to the olfactory bulb (OB), a cortical structure implicated in odor learning and perceptual differentiation among similar odorants. The authors tested the role of OB noradrenaline (NA) in short-term olfactory memory using an animal model of LC degeneration coupled with intrabulbar infusions of NA. Specifically, the authors lesioned cortical noradrenergic fibers in mice with the noradrenergic neurotoxin N-Ethyl-N-(2-chloroethyl)-2-bromobenzylamine hydrochloride (DSP4) and measured the effects on an olfactory habituation/spontaneous discrimination task. DSP4-treated mice failed to habituate to repeated odor presentations, indicating that they could not remember odors over the 5-min intertrial interval. The authors then infused NA bilaterally into the OBs of both DSP4-treated and nonlesioned control animals at two concentrations (10-3M and 10-5M, 2 μl/side). In DSP4-treated animals, NA administration at either concentration restored normal habituation and spontaneous discrimination performance, indicating that noradrenergic neuromodulation mediates these aspects of perceptual learning and that its efficacy does not require activity-dependent local regulation of NA release. Functional OB learning mechanisms may be necessary for normal odor recognition and differentiation among physically similar odorants. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Opposing Effects of D? and D? Receptor Activation on Odor Discrimination Learning.

Dopaminergic modulation of cortical activity has been implicated in the formation of reward associations. There is abundant evidence for dopaminergic effects on olfactory processing. Using an olfactory discrimination task, the authors show that D? and D? dopamine receptors can regulate rats' olfactory discrimination capacities and that the effects of receptor activation functionally oppose one another. Injection of either the D? agonist SKF 38393 (10 mg/kg) or the D? antagonist spiperone (0.62 mg/kg) facilitated the discrimination of similar odorants but had no effect on the discrimination of dissimilar odorants, whereas both the D, antagonist SCH 23390 (0.025 mg/kg) and the D? agonist quinpirole (0.2 mg/kg) significantly impaired rats' ability to discriminate similar and dissimilar odorants. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A modified coaxial compound micropipette for extracellular iontophoresis and intracellular recording: fabrication, performance and theory

Different olfactory cues elicit distinct behaviors such as attraction, avoidance, feeding, or mating. In the nematode C. elegans, these cues are sensed by a small number of olfactory neurons, each of which expresses several different odorant receptors. The type of behavioral response elicited by an odorant could be specified by the olfactory receptor or by the olfactory neuron in which the receptor is activated. The attractive odorant diacetyl is detected by the receptor protein ODR-10, which is normally expressed in the AWA olfactory neurons. The repulsive odorant 2-nonanone is detected by the AWB olfactory neurons. Transgenic animals that express ODR-10 in AWB rather than AWA avoid diacetyl, while maintaining qualitatively normal responses to other attractive and repulsive odorants. Animals that express ODR-10 simultaneously in AWA and AWB have a defective response to diacetyl, possibly because of conflicting olfactory inputs. Thus, an animal's preference for an odor is defined by the sensory neurons that express a given odorant receptor molecule.     

Binary mixture perception is affected by concentration of odor components.

Some controversy still exists as to how binary odorant mixtures are behaviorally perceived, despite many studies aimed at understanding this phenomenon. Binary mixture perception by rodents is a first step in elucidating how more complex odor blends may be perceived. Research thus far has examined how the degree of component similarity, olfactory receptor overlap, relative concentration of components, and even olfactory enrichment affect the behavioral perception of binary mixtures. These studies have aimed to categorize binary mixtures into 1 of 3 rigid categories, but often the results conflict as to which category a particular mixture belongs. In the present article, the authors used a habituation/discrimination paradigm to determine whether rats' perception of one component of a binary mixture of either perceptually similar or dissimilar components changed when the concentration of both components was varied together. The authors found that perception of a binary mixture changed with changing component concentration, such that one binary mixture could be categorized differently depending on component intensity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Trends in odor intensity for human and electronic noses: relative roles of odorant vapor pressure vs. molecularly specific odorant binding

Response data were collected for a carbon black-polymer composite electronic nose array during exposure to homologous series of alkanes and alcohols. The mean response intensity of the electronic nose detectors and the response intensity of the most strongly driven set of electronic nose detectors were essentially constant for members of a chemically homologous odorant series when the concentration of each odorant in the gas phase was maintained at a constant fraction of the odorant's vapor pressure. A similar trend is observed in human odor detection threshold values for these same homologous series of odorants. Because the thermodynamic activity of an odorant at equilibrium in a sorbent phase is equal to the partial pressure of the odorant in the gas phase divided by the vapor pressure of the odorant and because the activity coefficients are similar within these homologous series of odorants for sorption of the vapors into specific polymer films, the data imply that the trends in detector response can be understood based on the thermodynamic tendency to establish a relatively constant concentration of sorbed odorant into each of the polymeric films of the electronic nose at a constant fraction of the odorant's vapor pressure. Similarly, the data are consistent with the hypothesis that the odor detection thresholds observed in human psychophysical experiments for the odorants studied herein are driven predominantly by the similarity in odorant concentrations sorbed into the olfactory epithelium at a constant fraction of the odorant's vapor pressure.     

Experience-induced fetal plasticity: The effect of gestational ethanol exposure on the behavioral and neurophysiologic olfactory response to ethanol odor in early postnatal and adult rats.

Human fetal ethanol exposure is strongly associated with ethanol avidity during adolescence. Evidence that intrauterine olfactory experience influences chemosensory-guided postnatal behaviors suggests that an altered response to ethanol odor resulting from fetal exposure may contribute to later abuse risk. Using behavioral and neurophysiological methods, the authors tested whether ethanol exposure via the dam's diet resulted in an altered responsiveness to ethanol odor in infant and adult rats. Compared with controls, (a) fetal exposure tuned the neurophysiologic response of the olfactory epithelium to ethanol odor at some expense to its responsiveness to other odorants in infantile rats--this effect was absent in adults; (b) the neural effect in infantile rats was paralleled by an altered behavioral response to ethanol odor that was specific to this odorant--this effect was also absent in adults; and (c) a significant component of the infantile behavioral effect was attributable to ethanol's effect on the olfactory neural modality. These data provide evidence for an important relationship between prenatal ethanol experience and postnatal behavioral responsiveness to the drug that is modulated or determined by olfactory function. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of training and experience on the perception of multicomponent odor mixtures.

This study examined whether a previously established (D. G. Laing and G. W. Francis; see record 1990-11967-001) limited capacity to discriminate and identify the components of olfactory mixtures resulted from the participants' lack of familiarity with the task, training designed to optimize cognitive and perceptual performance, or professional experience in odor discrimination. The participants were a trained panel of 10 women (23–43 years old), and an expert panel of 8 male professional perfumers and flavorists (25–55 years old). The individual chemical stimuli were 7 common dissimilar odorants of equal moderate intensity. An air dilution olfactometer delivered a single odorant or a mixture containing up to 5 odorants. The results indicated that for both panels only 3 or 4 components of a complex mixture could be discriminated and identified and that this capacity could not be increased by training. Therefore, the limit may be imposed physiologically or by processing constraints. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sniffing in Infant Rats During Sleep and Wakefulness.

Sniffing, a behavior that enhances detection and localization of odorants, is typically assumed to require behavioral arousal. In an effort to determine whether sniffing and arousal are dissociable, dimethyl disulfide (DMDS) was presented to 8-day-old rats while respiration and behavioral state were monitored. Pups sniffed in response to the highest concentrations of DMDS, exhibiting a lower olfactory threshold when awake. Surprisingly, sniffing occurred even while pups remained asleep. Sniffing was mediated by the olfactory system, as evidenced by the abolition of sniffing when the lateral olfactory tracts, were cut and the retention of rapid arousal in response to a trigeminal stimulant, acetic acid. Finally, sleeping pups presented with acetic acid awakened without sniffing. Thus, although olfactory threshold increases during sleep, sleeping does not preclude sniffing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Chemotopic, combinatorial, and noncombinatorial odorant representations in the olfactory bulb revealed using a voltage-sensitive axon tracer

Odor information is first represented in the brain by patterns of input activity across the glomeruli of the olfactory bulb (OB). To examine how odorants are represented at this stage of olfactory processing, we labeled anterogradely the axons of olfactory receptor neurons with the voltage-sensitive dye Di8-ANEPPQ in zebrafish. The activity induced by diverse natural odorants in afferent axons and across the array of glomeruli was then recorded optically. The results show that certain subregions of the OB are preferentially activated by defined chemical odorant classes. Within these subregions, "ordinary" odorants (amino acids, bile acids, and nucleotides) induce overlapping activity patterns involving multiple glomeruli, indicating that they are represented by combinatorial activity patterns. In contrast, two putative pheromone components (prostaglandin F2alpha and 17alpha, 20beta-dihydroxy-4-pregnene-3-one-20-sulfate) each induce a single focus of activity, at least one of which comes from a single, highly specific and sensitive glomerulus. These results indicate that the OB is organized into functional subregions processing classes of odorants. Furthermore, they suggest that individual odorants can be represented by "combinatorial" or "noncombinatorial" (focal) activity patterns and that the latter may serve to process odorants triggering distinct responses such as that of pheromones.