Odor Pairs Research Articles

Adaptive Adjustment of the Generalization-Discrimination Balance in Larval Drosophila

Learnt predictive behavior faces a dilemma: predictive stimuli will never ‘replay’ exactly as during the learning event, requiring generalization. In turn, minute differences can become meaningful, prompting discrimination. To provide a study case for an adaptive adjustment of this generalization-discrimination balance, the authors ask whether Drosophila melanogaster larvae are able to either generalize or discriminate between two odors (1-octen-3-ol and 3-octanol), depending on the task. The authors find that after discriminatively rewarding one but not the other odor, larvae show conditioned preference for the rewarded odor. On the other hand, no odor specificity is observed after nondiscriminative training, even if the test involves a choice between both odors. Thus, for this odor pair at least, discrimination training is required to confer an odor-specific memory trace. This requires that there is at least some difference in processing between the two odors already at the beginning of the training. Therefore, as a default, there is a small yet salient difference in processing between 1-octen-3-ol and 3-octanol; this difference is ignored after nondiscriminative training (generalization), whereas it is accentuated by odor-specific reinforcement (discrimination). Given that, as the authors show, both faculties are lost in anosmic Or83b1 mutants, this indicates an adaptive adjustment of the generalization-discrimination balance in larval Drosophila, taking place downstream of Or83b-expressing sensory neurons.

The Cross-Adaptation of Green and Citrus Odorants

In 1998, a set of straight chain aldehydes ranging in carbon chain length from seven to ten carbons were identified for their ability to excite olfactory receptor I-7 (OR-I7; Zhao et al. 1998). Several studies have examined the differences in physiological response to these odorants in mammals as well as the different psychophysical properties of these odorants. Octanal (C8) is the primary agonist of OR-I7 while hexanal, C6, does not excite OR-I7. The primary psychophysical difference between these two odorants is their perceptual quality, C8 is characterized as citrus and C6 is described as green (Kittel et al. 2008). Because ligands with similar odor qualities have been shown to cross-adapt, while those with differing odor qualities do not, one would predict C6 would not cross-adapt with C8, decanal (C10), and undecanal (C11) but C8, C10, and C11 would cross-adapt with each other. This study uses squeeze bottle sensory tests to examine the cross-adaptation and self-adaptation of hexanal, octanal, C10, and C11. Individuals evaluated the intensity of each odorant before and after smelling an adapting stimulus. Self-adaptation was measured for all four odorants. The strongest observed cross-adaptation occurred between odorant pairs C10–C11 while C10–C8, and C10–C11 showed slightly less cross-adaptation. Most importantly, cross-adaptation occurred between C6 and the three other stimuli to a significantly lesser degree. As predicted, C6 did not cross-adapt with any of the three other odorants; however, the three citrus odorants did readily cross-adapt suggesting adaptation is a peripheral process.

Olfactory discrimination varies in mice with different levels of α7-nicotinic acetylcholine receptor expression

Previous studies have shown that schizophrenics have decreased expression of α7-nicotinic acetylcholine (α7) receptors in the hippocampus and other brain regions, paranoid delusions, disorganized speech, deficits in auditory gating (i.e., inability to inhibit neuronal responses to repetitive auditory stimuli), and difficulties in odor discrimination and detection. Here we use mice with decreased α7 expression that also show a deficit in auditory gating to determine if these mice have similar deficits in olfaction. In the adult mouse olfactory bulb (OB), α7 expression localizes in the glomerular layer; however, the functional role of α7 is unknown. We show that inbred mouse strains (i.e., C3H and C57) with varying α7 expressions (e.g., α7 wild-type [α7+/+], α7 heterozygous knock-out [α7+/−] and α7 homozygous knock-out mice [α7−/−]) significantly differ in odor discrimination and detection of chemically-related odorant pairs. Using [125I] α-bungarotoxin (α-BGT) autoradiography, α7 expression was measured in the OB. As previously demonstrated, α-BGT binding was localized to the glomerular layer. Significantly more expression of α7 was observed in C57 α7+/+ mice compared to C3H α7+/+ mice. Furthermore, C57 α7+/+ mice were able to detect a significantly lower concentration of an odor in a mixture compared to C3H α7+/+ mice. Both C57 and C3H α7+/+ mice discriminated between chemically-related odorants sooner than α7+/− or α7−/− mice. These data suggest that α7-nicotinic-receptors contribute strongly to olfactory discrimination and detection in mice and may be one of the mechanisms producing olfactory dysfunction in schizophrenics.

A Beta Oscillation Network in the Rat Olfactory System During a 2-Alternative Choice Odor Discrimination Task

We previously showed that in a two-alternative choice (2AC) task, olfactory bulb (OB) gamma oscillations (approximately 70 Hz in rats) were enhanced during discrimination of structurally similar odorants (fine discrimination) versus discrimination of dissimilar odorants (coarse discrimination). In other studies (mostly employing go/no-go tasks) in multiple labs, beta oscillations (15-35 Hz) dominate the local field potential (LFP) signal in olfactory areas during odor sampling. Here we analyzed the beta frequency band power and pairwise coherence in the 2AC task. We show that in a task dominated by gamma in the OB, beta oscillations are also present in three interconnected olfactory areas (OB and anterior and posterior pyriform cortex). Only the beta band showed consistently elevated coherence during odor sniffing across all odor pairs, classes (alcohols and ketones), and discrimination types (fine and coarse), with stronger effects in first than in final criterion sessions (>70% correct). In the first sessions for fine discrimination odor pairs, beta power for incorrect trials was the same as that for correct trials for the other odor in the pair. This pattern was not repeated in coarse discrimination, in which beta power was elevated for correct relative to incorrect trials. This difference between fine and coarse odor discriminations may relate to different behavioral strategies for learning to differentiate similar versus dissimilar odors. Phase analysis showed that the OB led both pyriform areas in the beta frequency band during odor sniffing. We conclude that the beta band may be the means by which information is transmitted from the OB to higher order areas, even though task specifics modify dominance of one frequency band over another within the OB.

How big is the gap between olfactory detection and recognition of aliphatic aldehydes?

The aim of the present study was to determine the magnitude of the difference in concentration between olfactory detection and recognition thresholds of aliphatic aldehydes. To this end, we first determined olfactory detection thresholds for n-butanal, n-pentanal, n-hexanal, n-heptanal, and n-octanal in a group of 16 subjects and then assessed their ability to discriminate between all possible binary pairs of the same odorants presented at different concentrations above their individual detection thresholds. We found that the gap between detection and recognition of aliphatic aldehydes is odorant pair dependent and, at the group level, spans at least a factor of 100. However, single subjects successfully discriminated between certain aldehyde pairs presented at a factor as low as 3 above detection threshold. Our approach to determining olfactory recognition thresholds, using a performance-based measure rather than verbal labeling, not only avoids the problem of semantic ambiguity and arguable criteria, but also is applicable to nonhuman species, allowing for interspecific comparisons of recognition thresholds and of the gap between detection and recognition of odorants. The raw discrimination data from this study are available as a supplement from http://app.psychonomic-journals.org/content/supplemental.

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.

Dynamics of Input Patterns Modulate the Behavior of a Model of Olfactory Bulb Function

Input patterns to the olfactory bulb are dynamic and change in an odor-specific manner as measured by selective calcium imaging of olfactory bulb input. To our knowledge, none of the published models of olfactory bulb function uses dynamic input patterns. Therefore we tested how dynamic input alters the behavior of a simple model consisting of two layers. The membrane potential of the first-layer neurons, integrate-and-fire neurons corresponding to mitral cells, was modulated with a subthreshold oscillation at respiration frequency. The membrane potential of the second-layer neurons was used to discriminate input patterns. We implemented oscillating input with amplitudes and latencies different for each mitral cell. Not only varying the input amplitudes but also de-synchronizing the input, and varying the relation between latency and input amplitude, individually changed the model's performance significantly. The discrimination time was affected more easily than the number of second-layer neurons that can differentiate an odor pair. Increasing the de-synchronization, i.e., the spread of latency values, reduced the differences in response time between strong and weak stimulus pairs without reducing the number of reacting cells. Input phase relative to the subthreshold oscillation altered the effect of de-synchronization. Thus dynamic input changes performance parameters of models of olfactory information processing that can be verified experimentally.

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.

Temporal processing of olfactory stimuli during retronasal perception

Odorants can be perceived via the nose during an inhalation or sniff (orthonasal perception) and via the mouth, nasopharynx and nasal cavity during mastication or drinking (retronasal perception). Previous data suggest that orthonasal perception provides a more efficient route with greater difficulty being reported when detecting [Halpern BP. Retronasal and orthonasal smelling. Chemosense 2004;6:1–7; Voirol E, Daget N. Comparative study of nasal and retronasal olfactory perception. Lebensmittel-Wissenschaft Technol 1986;19:316–9] and identifying [Heilmann S, Hummel T. A new method for comparing orthonasal and retronasal olfaction. Behav Neurosci 2004;118:412–9; Sun BC, Halpern BP. Identification of air-phase retronasal and orthonasal odorant pairs. Chem Senses 2005;30:1–14] single odorants retronasally. Whether the poorer sensitivity obtained via the retronasal route is largely due to the greater adsorption of odorants by the nasopharyngeal mucus compared to the nasal mucus thereby reducing their peak concentration and/or slowing their passage, has not been resolved. Importantly, the question of whether solubility of odorants in mucus or water predicts the outcomes for perception of stimuli presented via the retronasal route has not been resolved. Accordingly, the present study investigates this question by determining whether the solubility of an odorant in mucus predicts which component of a binary odour mixture is perceived first during retronasal perception. The results indicate that solubility in mucus rather than solubility in water is a better predictor of which odour will be perceived first and identified more readily during the retronasal perception of a binary mixture. In addition, lower intensity levels of single odorants occurred via the retronasal route suggesting that adsorption was greater via this route. Whether this was due to nasopharyngeal mucus having a greater adsorptive area or different composition compared to the orthonasal pathway is not known.

Eight different types of dopaminergic neurons innervate the Drosophila mushroom body neuropil: anatomical and physiological heterogeneity

We examined tyrosine hydroxylase (TH-GAL4) expression and anti-TH immunoreactivity in the Drosophila protocerebrum and characterized single cell clones of the TH-GAL4 neurons. Eight clusters of putative dopaminergic neurons were characterized. Neurons in three of the clusters project to the mushroom body neuropil: PAM neurons project to the medial portion of the horizontal lobes; PPL1 neurons project to the vertical lobes, the junction area, the heel and distal peduncle; and PPL2ab neurons project to the calyx. Five types of PPL1 neurons were discovered that innervate different zones of the mushroom body lobes. Functional imaging experiments showed that the dopaminergic processes in four of the zones differ in response properties to odor, electric shock, or following the pairing of odor and electric shock. These results indicate that distinct dopaminergic neurons define separate zones of the mushroom body lobes and are probably involved in different functions. Differences in functional response properties of these neurons suggest that they are involved in different behavioral processes.

Sequential unfolding of novelty and pleasantness appraisals of odors: Evidence from facial electromyography and autonomic reactions.

We investigated the effects of odors on appraisal processes and consequent emotional responses. The main goal was to test whether an odor is detected as novel or familiar before it is evaluated as pleasant or unpleasant. Participants performed a recognition task in which they were presented with pairs of unpleasant or pleasant odors (sample and target odors). Within a pair, the sample and target were either identical or different to assess participants' novelty detection; unpleasant and pleasant target odors were contrasted to examine participants' appraisal of intrinsic pleasantness. We measured facial expressions using electromyography and physiological reactions using electrocardiogram and electrodermal activity in response to odors. The earliest effects on facial muscles and heart rate occurred in response to novelty detection. Later effects on facial muscles and heart rate were related to pleasantness evaluation. This study is the first to demonstrate the existence of a sequence of appraisal checks for odors eliciting emotional reaction.

Towards an image of a memory trace

Learning and memory lead to functional and structural changes in the brain, ultimately providing a basis for adaptive behavior. The honeybee is an elegant model for the study of learning and memory formation as it permits both the visualization of neural activity related to the events occurring in olfactory learning and the behavioral assessment of olfactory learning (Galizia and Menzel, 2000). The formation of odor memories in the honeybee is thought to involve the two primary processing centers of the olfactory system, the antennal lobe (AL) and the mushroom body (MB). The intrinsic neurons of the MB – the Kenyon cells (KCs), located within the lip region of the MB calyx – are the site of convergence of the neural pathways that transmit odor information from the projection neurons (PNs) of the AL and reward information from the VUMmx1 neuron (Hammer, 1997). In recent years, imaging studies performed in the honeybee AL and MB lip have indicated that pairing odor and reward induces changes in neural activity (Faber and Menzel, 2001; Faber et al., 1999), reinforcing the anatomical suggestion that KCs are likely to undergo associative plasticity during learning. This study, Szyszka et al. (2008) provides new evidence that odor-evoked activity of KCs can be modified by sensory experience, and the changes can be associative or non-associative. Szyszka et al. (2008) visualized odor representations in the MB of the honeybee by imaging the calcium responses of a subpopulation of KCs – the clawed KCs, by retrogradely labeling the cell population using dye injections into the ventral alpha/gamma lobe. This allowed the authors to image, in a relatively selective way, the bulk dendritic activity of KCs over neuropil. Szyszka et al. (2008) first examined KC activity in response to repeated stimulation with the same odor and found that it led to a decrease in KC responses, a non-associative form of plasticity resembling a phenomenon previously described in locust PNs (Stopfer and Laurent, 1999). Separate experiments found that honeybee PNs did not show repetition-induced depression, indicating that the effect is not a generalized run-down. The authors then compared KC activity before and after pairing one odor with sucrose reward and tested how the associative pairing procedure affects KC responses to the rewarded odor (CS+) and to the unrewarded odor (CS−). During pre-training, KC responses to both CS+ and CS− decreased with repeated stimulation. After pairing CS+ with sucrose CS+ responses recovered and CS− responses decreased further (Figure ​(Figure1).1). Intriguingly, examination of spatiotemporal patterns within the imaged region before and after conditioning suggested additional changes in both CS+ and CS− evoked KC activity following conditioning. Figure 1 KC responses are subject to associative and non-associative plasticity. (A) Scheme of honeybee antennal lobes and mushroom bodies (MB). Calcium responses in the somata and dendrites of clawed Kenyon cells (clawed KCs, red) were recorded in the MB calyx ... The results of this study support the suggestion that KC odor responses can be modified by sensory experience and that these changes can be associative, extending the earlier work of the Menzel group (Faber and Menzel, 2001; Faber et al., 1999). Although a causal link between this activity and learning and memory remains to be established, the strengthening of support for a leading candidate cellular substrate is an encouraging observation that can be followed up in several ways. Firstly, it should be possible to examine the same cell population at the molecular and cellular levels in order to elucidate the processes underlying these modifications. Secondly, the changes in spatiotemporal patterns hint at restructuring of KC odor representations that might reflect a correlate of information storage. It may now be feasible, using similar imaging approaches but at single cell resolution, to resolve the details of the changes. Finally, it should be possible, albeit technically challenging, to take advantage of the possibility of behavioral measurements in combination with imaging to try to directly link KC changes to memory storage itself.

Architecture of odor information processing in the olfactory system

Since the discovery of the superfamily of approximately 1000 odorant receptor genes in rodents, the structural simplicity as well as the complexity of the olfactory system have been revealed. The simple aspects include the one neuron-one receptor rule and the exclusive convergence of projections from receptor neurons expressing the same receptors to one or two glomeruli in the olfactory bulb. Odor decoding in the olfactory cortex or higher cortical areas is likely to be a complicated process that depends on the sequence of signal activation and the relative signal intensities of receptors overlapping for similar but different odors. The aim of the present study was to investigate odor information processing both in receptors and in the olfactory cortex. At the receptor level, the similarity and difference in receptor codes between a pair of chiral odorants were examined using the tissue-printing method for sampling all the epithelial zones. In order to dissect odor-driven signal processing in the olfactory cortex by reducing cross-talk with the non-olfactory activities, such as cyclic respiration or other sensory inputs, an in vitro preparation of isolated whole brain with an attached nose was developed, and the methodologies and resulting hypothesis of receptor-sensitivity-dependent hierarchical odor information coding were reviewed.

The GABAergic anterior paired lateral neuron suppresses and is suppressed by olfactory learning

GABAergic neurotransmitter systems are important for many cognitive processes, including learning and memory. We identified a single neuron in each hemisphere of the Drosophila brain - the anterior paired lateral (APL) neuron - as a GABAergic neuron that broadly innervated the mushroom bodies. Reducing GABA synthesis in the APL neuron enhanced olfactory learning, suggesting that APL suppressed learning by releasing the inhibitory neurotransmitter GABA. Functional optical imaging experiments revealed that APL responded to both odor and electric shock stimuli presented to the fly with increases of intracellular calcium and released neurotransmitter. More importantly, a memory trace formed in the APL neuron by pairing odor with electric shock. This trace was detected as a reduced calcium response in APL after conditioning specifically to the trained odor. These results demonstrated a mutual suppression between the GABAergic APL neuron and olfactory learning, and functional neuroplasticity of the GABAergic system due to learning.

Olfactory discrimination of aliphatic odorants at 1 ppm: too easy for CD-1 mice to show odor structure–activity relationships?

Using an operant conditioning paradigm we tested the ability of CD-1 mice to discriminate between 25 odorants comprising members of five homologous series of aliphatic odorants (C4-C8) presented at a gas phase concentration of 1 ppm. We found (a) that all mice significantly discriminated between all 50 stimulus pairs that involved odorants sharing the same functional group, but differing in carbon chain length, as well as between all 50 stimulus pairs that involved odorants sharing the same carbon chain length but differing in functional group, (b) a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length with the acetic esters and the 2-ketones, but not with the 1-alcohols, n-aldehydes, and n-carboxylic acids tested, and (c) that odorant pairs differing in functional group were significantly better discriminated than odorant pairs differing in carbon chain length. These findings demonstrate that CD-1 mice have excellent discrimination ability for structurally related aliphatic odorants, that correlations between discrimination performance and structural similarity of odorants are odorant class-specific rather than a general phenomenon, and that both carbon chain length and type of functional group play an important role for odor quality coding in mice.

Constructing receiver operating characteristics (ROCs) with experimental animals: Cautionary notes: Figure 1.

Constructing receiver operating characteristics (ROCs) with experimental animals: Cautionary notes: Figure 1.

The effect of appropriate and inappropriate stimulus color on odor discrimination

Color can strongly affect participants' self-report of an odor's qualities. In Experiment 1, we examined whether color influences a more objective measure of odor quality, discrimination. Odor pairs, presented in their appropriate color (e.g., strawberry and cherry in red water), an inappropriate color (e.g., strawberry and cherry in green water), or uncolored water were presented for discrimination. Participants made significantly more errors when odors were discriminated in an inappropriate color. In Experiment 2, the same design was utilized, but with an articulatory suppression task (AST), to examine whether the effect of color was mediated by identification or by a more direct effect on the percept. Here, the AST significantly improved discrimination for the inappropriate color condition, relative to Experiment 1. Although color does affect a more objective measure of odor quality, this is mediated by conceptual, rather than perceptual, means.

Rapid Consolidation to aradishand Protein Synthesis-Dependent Long-Term Memory after Single-Session Appetitive Olfactory Conditioning inDrosophila

In Drosophila, formation of aversive olfactory long-term memory (LTM) requires multiple training sessions pairing odor and electric shock punishment with rest intervals. In contrast, here we show that a single 2 min training session pairing odor with a more ethologically relevant sugar reinforcement forms long-term appetitive memory that lasts for days. Appetitive LTM has some mechanistic similarity to aversive LTM in that it can be disrupted by cycloheximide, the dCreb2-b transcriptional repressor, and the crammer and tequila LTM-specific mutations. However, appetitive LTM is completely disrupted by the radish mutation that apparently represents a distinct mechanistic phase of consolidated aversive memory. Furthermore, appetitive LTM requires activity in the dorsal paired medial neuron and mushroom body alpha'beta' neuron circuit during the first hour after training and mushroom body alphabeta neuron output during retrieval, suggesting that appetitive middle-term memory and LTM are mechanistically linked. Last, experiments feeding and/or starving flies after training reveals a critical motivational drive that enables appetitive LTM retrieval.

Disruption of GABAA in the Insect Antennal Lobe Generally Increases Odor Detection and Discrimination Thresholds

Studies of olfactory function show that disruption of GABA A receptors within the insect antennal lobe (AL) disrupts discrimination of closely related odors, suggesting that local processing within the AL specifically enhances fine odor discrimination. It remains unclear, however, how extensively AL function has been disrupted in these circumstances. Here we psychophysically characterize the effect of GABA A blockade in the AL of the moth Manduca sexta. We used 2 GABA A antagonists and 3 Pavlovian-based behavioral assays of olfactory function. In all cases, we used matched saline-injected controls in a blind study. Using a stimulus generalization assay, we found that GABA A disruption abolished the differential response to related odors, suggesting that local processing mediates fine odor discrimination. We then assessed the effect of GABA A antagonist on discrimination thresholds. Moths were differentially conditioned to respond to one odor (reinforced conditioned stimulus [CS+]) but not a second (unreinforced conditioning stimulus [CS-]) then tested for a significant differential conditioned response between them across a series of increasing concentrations. Here, GABA A blockade disrupted discrimination of both similar and dissimilar odor pairs as indicated by generally increased discrimination thresholds. Finally, using a detection threshold assay, we established that GABA A blockade also increases detection thresholds. Because detection is a prerequisite of discrimination, this later finding suggests that disrupted discrimination may be due to impairment of the ability to detect. We conclude that the loss of ability to detect and subsequently discriminate is attributable to a loss of ability of the AL to provide a clear neural signal from background.

Differential regional expression of brain-derived neurotrophic factor following olfactory fear learning

We examined brain-derived neurotrophic factor (BDNF) mRNA expression across the olfactory system following fear conditioning. Mice received 10 pairings of odor with footshock or equivalent unpaired odors and shocks. We found increased BDNF mRNA in animals receiving paired footshocks in the multiple regions examined including the posterior piriform cortex (PPC) and basolateral amygdala (BLA). This was in contrast to the unpaired and odor-alone treatments, where BDNF mRNA was increased in the olfactory bulb (OB) and anterior piriform cortex (APC) only, but not the higher olfactory areas. We propose that odor exposure increases expression of BDNF in the OB and APC while the PPC and BLA increase BDNF mRNA only when associative learning occurs.