Multiple learning parameters differentially regulate olfactory generalization

Mice in a labyrinth show rapid learning, sudden insight, and efficient exploration

Animals learn certain complex tasks remarkably fast, sometimes after a single experience. What behavioral algorithms support this efficiency? Many contemporary studies based on two-alternative-forced-choice (2AFC) tasks observe only slow or incomplete learning. As an alternative, we study the unconstrained behavior of mice in a complex labyrinth and measure the dynamics of learning and the behaviors that enable it. A mouse in the labyrinth makes ~2000 navigation decisions per hour. The animal explores the maze, quickly discovers the location of a reward, and executes correct 10-bit choices after only 10 reward experiences — a learning rate 1000-fold higher than in 2AFC experiments. Many mice improve discontinuously from one minute to the next, suggesting moments of sudden insight about the structure of the labyrinth. The underlying search algorithm does not require a global memory of places visited and is largely explained by purely local turning rules.

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.

Training with varying odor concentrations: implications for odor detection thresholds in canines

Detection dogs are required to detect trace quantities of substances, many times in the parts per billion or parts per trillion concentration range. Frequently, detection of trace quantities is not explicitly trained but rather assumed when dogs show proficiency at higher concentrations to which they are trained. The aim of this study was to evaluate the effect of the odor concentration of the training sample on the minimum concentration dogs will subsequently detect. We expected that dogs may not spontaneously generalize to trace odor concentration when trained with higher concentrations, but when trained to a range of lower concentrations, dogs will show superior detection to lower untrained concentrations. A total of 11 dogs were randomly assigned to 2 groups and were trained to alert to isoamyl acetate at 0.01% odor dilution (v/v with mineral oil) using a 3-alternative forced choice test. Once reaching proficiency, odor detection threshold was assessed using a 2-down 1-up descending staircase procedure. Next, experimental dogs received training with systematically lower concentrations of isoamyl acetate and threshold re-assessed. Control dogs were yoked to experimental dogs in terms of training time, but only received training to the 0.01% dilution between threshold assessments. Experimental dogs showed significantly improved detection thresholds, outperforming control dogs by detecting an average dilution about 100-fold lower. Results suggest that explicitly training for lower concentrations is critical for generalization for trace odor detection.

Stimulus Control of Odorant Concentration: Pilot Study of Generalization and Discrimination of Odor Concentration in Canines

Simple Summary

Dogs are deployed worldwide for detection tasks, but little is known about how they spontaneously generalize between concentration variations of their trained odor. This study found that dogs spontaneously generalized within a 10-fold concentration range lower than the training stimulus. Further, dogs could be trained to discriminate between concentrations within that 10-fold range. However, discrimination training did not affect dogs’ spontaneous generalization to the odor concentration unless discrimination training occurred in compound with generalization testing, suggesting that relative stimulus control of the target and non-target concentrations might be important in determining whether dogs will respond.

Abstract

Despite dogs’ widespread use as detection systems, little is known about how dogs generalize to variations of an odorant’s concentration. Further, it is unclear whether dogs can be trained to discriminate between similar concentration variations of an odorant. Four dogs were trained to an odorant (0.01 air dilution of isoamyl acetate) in an air-dilution olfactometer, and we assessed spontaneous generalization to a range of concentrations lower than the training stimulus (Generalization Test 1). Dogs generalized to odors within a 10-fold range of the training odorant. Next, we conducted discrimination training to suppress responses to concentrations lower than a concentration dogs showed initial responding towards in Generalization Test 1 (0.0025 air dilution). Dogs successfully discriminated between 0.0025 and 0.01, exceeding 90% accuracy. However, when a second generalization test was conducted (Generalization Test 2), responding at the 0.0025 concentration immediately recovered and was no different than in Generalization Test 1. Dogs were then tested in another generalization test (Compound Discrimination and Generalization) in which generalization probes were embedded within discrimination trials, and dogs showed suppression of responding to the 0.0025 concentration and lower concentrations in this preparation. These data suggest dogs show limited spontaneous generalization across odor concentration and that dogs can be trained to discriminate between similar concentrations of the same odorant. Stimulus control, however, may depend on the negative stimulus, suggesting olfactory concentration generalization may depend on relative stimulus control. These results highlight the importance of considering odor concentration as a dimension for generalization in canine olfactory research.

Noradrenergic Activity in the Olfactory Bulb Is a Key Element for the Stability of Olfactory Memory

Memory stability is essential for animal survival when environment and behavioral state change over short or long time spans. The stability of a memory can be expressed by its duration, its perseverance when conditions change as well as its specificity to the learned stimulus. Using optogenetic and pharmacological manipulations in male mice, we show that the presence of noradrenaline in the olfactory bulb during acquisition renders olfactory memories more stable. We show that while inhibition of noradrenaline transmission during an odor-reward acquisition has no acute effects, it alters perseverance, duration, and specificity of the memory. We use a computational approach to propose a proof of concept model showing that a single, simple network effect of noradrenaline on olfactory bulb dynamics can underlie these seemingly different behavioral effects. Our results show that acute changes in network dynamics can have long-term effects that extend beyond the network that was manipulated.SIGNIFICANCE STATEMENT Olfaction guides the behavior of animals. For successful survival, animals have to remember previously learned information and at the same time be able to acquire new memories. We show here that noradrenaline in the olfactory bulb, the first cortical relay of the olfactory information, is important for creating stable and specific olfactory memories. Memory stability, as expressed in perseverance, duration and specificity of the memory, is enhanced when noradrenergic inputs to the olfactory bulb are unaltered. We show that, computationally, our diverse behavioral results can be ascribed to noradrenaline-driven changes in neural dynamics. These results shed light on how very temporary changes in neuromodulation can have a variety of long-lasting effects on neural processing and behavior.

A Systematic Framework for Olfactory Bulb Signal Transformations

We describe an integrated theory of olfactory systems operation that incorporates experimental findings across scales, stages, and methods of analysis into a common framework. In particular, we consider the multiple stages of olfactory signal processing as a collective system, in which each stage samples selectively from its antecedents. We propose that, following the signal conditioning operations of the nasal epithelium and glomerular-layer circuitry, the plastic external plexiform layer of the olfactory bulb effects a process of category learning-the basis for extracting meaningful, quasi-discrete odor representations from the metric space of undifferentiated olfactory quality. Moreover, this early categorization process also resolves the foundational problem of how odors of interest can be recognized in the presence of strong competitive interference from simultaneously encountered background odorants. This problem is fundamentally constraining on early-stage olfactory encoding strategies and must be resolved if these strategies and their underlying mechanisms are to be understood. Multiscale general theories of olfactory systems operation are essential in order to leverage the analytical advantages of engineered approaches together with our expanding capacity to interrogate biological systems.

Experience enhances certainty about olfactory stimuli under bulbar cholinergic control

We present evidence that experience and cholinergic modulation in an early sensory network interact to improve certainty about olfactory stimuli. The data we present are in agreement with existing theoretical ideas about the functional role of acetylcholine but highlight the importance of early sensory networks in addition to cortical networks. We use a simple behavioral paradigm in mice which allows us to measure certainty about a stimulus via the response amplitude to a condition and novel stimuli. We conclude that additional learning increases certainty and that the slope of this relationship can be modulated by activation of muscarinic cholinergic receptors in the olfactory bulb.

Methodological Considerations in Canine Olfactory Detection Research

Dogs are increasingly used in a wide range of detection tasks including explosives, narcotics, medical, and wildlife detection. Research on detection dog performance is important to understand olfactory capabilities, behavioral characteristics, improve training, expand deployment practices, and advance applied canine technologies. As such, it is important to understand the influence of specific variables on the quantification of detection dog performance such as test design, experimental controls, odor characteristics, and statistical analysis. Methods for testing canine scent detection vary influencing the outcome metrics of performance and the validity of results. Operators, management teams, policy makers, and law enforcement rely on scientific data to make decisions, design policies, and advance canine technologies. A lack of scientific information and standardized protocols in the detector dog industry adds difficulty and inaccuracies when making informed decisions about capability, vulnerability, and risk analysis. Therefore, the aim of this review is to highlight important methodological issues and expand on considerations for conducting scientifically valid detection dog research.

Generalization and Discrimination of Molecularly Similar Odorants in Detection Canines and the Influence of Training

Operationally-deployed canine detectors are often trained on one or a limited number of materials representing a single target odor, and training frequently occurs using materials of a high purity grade in controlled scenarios with minimal other background odors. Conversely, in the field, canine detectors are expected to generalize and identify variations of the target odor, while discriminating from similar extraneous or background odors. This exemplifies the balance between generalization and discrimination required for effective canine detectors. This research explored the tendency for detection canines to generalize or discriminate between similar odorants. Two groups of related odorants were used in two separate studies; (1) odorants of similar functional groups with differing carbon chains, and (2) odorants of similar carbon chain length but differing functional groups. Within each odorant set, the effect of training was addressed by incrementally increasing the number of odorants each canine was trained to detect. Initially, discrimination increased with increasing molecular dissimilarity in both odorant groups. After further training on additional related odorants, generalization increased across the set of odorants of the same carbon chain length, but there were no significant changes in either generalization or discrimination across the set of odorants of the same functional group. The results suggest that the canines in this study were more likely to generalize across compounds of the same chain length with differing functional group than across compounds of the same functional group, but differing chain lengths. Furthermore, some variation in performance between individual canines indicated that the tendency to generalize differed with experience, breed, and other factors affecting olfaction.

Olfactory Generalization in Detector Dogs

Generalizing to target odor variations while retaining specificity against non-targets is crucial to the success of detector dogs under working conditions. As such, the importance of generalization should be considered in the formulation of effective training strategies. Research investigating olfactory generalization from pure singular compounds to more complex odor mixtures helps to elucidate animals' olfactory generalization tendencies and inform ways to alter the generalization gradient by broadening or narrowing the range of stimuli to which dogs will respond. Olfactory generalization depends upon both intrinsic factors of the odors, such as concentration, as well as behavioral and cognitive factors related to training and previous experience. Based on the current research, some training factors may influence generalization. For example, using multiple target exemplars appears to be the most effective way to promote elemental processing and broaden the generalization gradient, whereas increasing the number of training instances with fewer exemplars can narrow the gradient, thereby increasing discrimination. Overall, this research area requires further attention and study to increase our understanding of olfactory generalization in dogs, particularly detector dogs, to improve training and detection outcomes.

Modeling habituation of introduced predators to unrewarding bird odors for conservation of ground-nesting shorebirds

Foraging mammalian predators face a myriad of odors from potential prey. To be efficient, they must focus on rewarding odors while ignoring consistently unrewarding ones. This may be exploited as a nonlethal conservation tool if predators can be deceived into ignoring odors of vulnerable secondary prey. To explore critical design components and assess the potential gains to prey survival of this technique, we created an individual-based model that simulated the hunting behavior of three introduced mammalian predators on one of their secondary prey (a migratory shorebird) in the South Island of New Zealand. Within this model, we heuristically assessed the outcome of habituating the predators to human-deployed unrewarding bird odors before the bird's arrival at their breeding grounds, i.e., the predators were "primed." Using known home range sizes and probabilities of predators interacting with food lures, our model suggests that wide-ranging predators should encounter a relatively large number of odor points (between 10 and 115) during 27 d of priming when odor is deployed within high-resolution grids (100-150 m). Using this information, we then modeled the effect of different habituation curves (exponential and sigmoidal) on the probability of predators depredating shorebird nests. Our results show that important gains in nest survival can be achieved regardless of the shape of the habituation curve, but particularly if predators are fast olfactory learners (exponential curve), and even if some level of dishabituation occurs after prey become available. Predictions from our model can inform the amount and pattern in which olfactory stimuli need to be deployed in the field to optimize encounters by predators, and the relative gains that can be expected from reduced predation pressure on secondary prey under different scenarios of predator learning. Habituating predators to odors of threatened secondary prey may have particular efficacy as a conservation tool in areas where lethal predator control is not possible or ethical, or where even low predator densities can be detrimental to prey survival. Our approach is also relevant for determining interaction probabilities for devices other than odor points, such as bait stations and camera traps.

Kinase activity in the olfactory bulb is required for odor memory consolidation

Long-term fear memory formation in the hippocampus and neocortex depends upon brain-derived neurotrophic factor (BDNF) signaling after acquisition. Incremental, appetitive odor discrimination learning is thought to depend substantially on the differentiation of adult-born neurons within the olfactory bulb (OB)—a process that is closely associated with BDNF signaling. We sought to elucidate the role of neurotrophin signaling within the OB on odor memory consolidation. Male mice were trained on odor–reward associative discriminations after bilateral infusion of the kinase inhibitor K252a, or vehicle control, into the OB. K252a is a partially selective inhibitor of tyrosine kinase (Trk) receptors, including the TrkB receptor for BDNF, though it also inhibits other plasticity-related kinases such as PKC and CaMKII/IV. K252a infusion into the OB did not impair odor acquisition or short-term (2 h) memory for the learned discriminations, but significantly impaired long-term (48 h) odor memory (LTM). This LTM deficit also was associated with reduced selectivity for the conditioned odorant in a reward-seeking digging task. Infusions of K252a immediately prior to testing did not impair LTM recall. These results indicate that kinase activation in the OB is required for the consolidation of odor memory of incrementally acquired information.

Noradrenergic effects on olfactory perception and learning

We here review modulation of olfactory guided behavioral tasks by noradrenaline. In this review we focus on modulation of the main olfactory system in adult rodents. We detail behavioral paradigms commonly used and discuss how sensory perception and learning can be measured using these paradigms. We then describe neuromodulatory effects on several aspects of olfactory processing, including detection and encoding. We describe how memory duration, specificity and duration are affected by noradrenergic modulation.

Associative Learning Through Acquired Salience

Most associative learning studies describe the salience of stimuli as a fixed learning-rate parameter. Presumptive saliency signals, however, have also been linked to motivational and attentional processes. An interesting possibility, therefore, is that discriminative stimuli could also acquire salience as they become powerful predictors of outcomes. To explore this idea, we first characterized and extracted the learning curves from mice trained with discriminative images offering varying degrees of structural similarity. Next, we fitted a linear model of associative learning coupled to a series of mathematical representations for stimulus salience. We found that the best prediction, from the set of tested models, was one in which the visual salience depended on stimulus similarity and a non-linear function of the associative strength. Therefore, these analytic results support the idea that the net salience of a stimulus depends both on the items' effective salience and the motivational state of the subject that learns about it. Moreover, this dual salience model can explain why learning about a stimulus not only depends on the effective salience during acquisition but also on the specific learning trajectory that was used to reach this state. Our mathematical description could be instrumental for understanding aberrant salience acquisition under stressful situations and in neuropsychiatric disorders like schizophrenia, obsessive-compulsive disorder, and addiction.

The hippocampus, medial prefrontal cortex, and selective memory retrieval: evidence from a rodent model of the retrieval-induced forgetting effect

Inhibition is an important component of many cognitive functions, including memory. For example, the retrieval-induced forgetting (RIF) effect occurs when extra practice with some items from a study list inhibits the retrieval of the nonpracticed items relative to a baseline condition that does not involve extra practice. Although counterintuitive, the RIF phenomenon may be important for resolving interference by inhibiting potentially competing retrieval targets. Neuroimaging studies suggest that the hippocampus and prefrontal cortex are involved in the RIF effect, but controlled lesion studies have not yet been performed. We developed a rodent model of the RIF training procedure and trained control rats and rats with temporary inactivation of the hippocampus or medial prefrontal cortex (mPFC). Rats were trained on a list of odor cues, presented in cups of digging medium with a buried reward, followed by additional practice trials with a subset of the cues. We then tested the rats' memories for the cues and their association with reward by presenting them with unbaited cups containing the test odorants and measuring how long they persisted in digging. Control rats exhibited a robust RIF effect in which memory for the nonpracticed odors was significantly inhibited. Thus, extra practice with some odor cues inhibited memory for the others, relative to a baseline condition that involved an identical amount of training. Inactivation of either the hippocampus or the mPFC blocked the RIF effect. We also constructed a computational model of a representational learning circuit to simulate the RIF effect. We show in this model that "sideband suppression" of similar memory representations can reproduce the RIF effect and that alteration of the suppression parameters and learning rate can reproduce the lesion effects seen in our rats. Our results suggest that the RIF effect is widespread and that inhibitory processes are an important feature of memory function.

Properties and mechanisms of olfactory learning and memory

Memories are dynamic physical phenomena with psychometric forms as well as characteristic timescales. Most of our understanding of the cellular mechanisms underlying the neurophysiology of memory, however, derives from one-trial learning paradigms that, while powerful, do not fully embody the gradual, representational, and statistical aspects of cumulative learning. The early olfactory system—particularly olfactory bulb—comprises a reasonably well-understood and experimentally accessible neuronal network with intrinsic plasticity that underlies both one-trial (adult aversive, neonatal) and cumulative (adult appetitive) odor learning. These olfactory circuits employ many of the same molecular and structural mechanisms of memory as, for example, hippocampal circuits following inhibitory avoidance conditioning, but the temporal sequences of post-conditioning molecular events are likely to differ owing to the need to incorporate new information from ongoing learning events into the evolving memory trace. Moreover, the shapes of acquired odor representations, and their gradual transformation over the course of cumulative learning, also can be directly measured, adding an additional representational dimension to the traditional metrics of memory strength and persistence. In this review, we describe some established molecular and structural mechanisms of memory with a focus on the timecourses of post-conditioning molecular processes. We describe the properties of odor learning intrinsic to the olfactory bulb and review the utility of the olfactory system of adult rodents as a memory system in which to study the cellular mechanisms of cumulative learning.

Characterizing olfactory perceptual similarity using carbon chain discrimination in Fischer 344 rats

Performance on olfactory tests can be influenced by a number of stimulus characteristics including chemical structure, concentration, perceptual similarity, and previous experience with the test odorants. Few of these parameters have been extensively characterized in the Fischer 344 rat strain. To investigate how odor quality affects perception in this rat strain, we measured how graded perceptual similarity, created by varying carbon chain length across a series of homologous alcohol pairs, influenced odor discrimination using a liquid-motivated go/no-go task. We employed an automated, liquid-dilution olfactometer to train Fischer 344 rats (N = 8) on a 2-odor discrimination task. Six odorants (1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, and 1-octanol) were arranged to produce 15 novel odorant pairs differing between 1 and 5 carbon atoms; testing sessions included presentation of only 1 pseudorandomly assigned pair daily (200 trials). Results show that although rats can learn to discriminate between any 2 odorant pairs, performance declines systematically as the pairs become more structurally similar and, therefore, more perceptually confusing. As such, the easier discrimination pairs produced reliable ceiling effects across all rats, whereas performance for the difficult discrimination pairs was consistently worse, even after repeated testing. These data emphasize the importance of considering odorant stimulus dimensions in experimental designs employing olfactory stimuli. Moreover, establishing baseline olfactory performance in Fischer 344 rats may be particularly useful for predicting age-related cognitive decline in this model.

Perceptual judgements and chronic imaging of altered odour maps indicate comprehensive stimulus template matching in olfaction

Lesion experiments suggest that odour input to the olfactory bulb contains significant redundant signal such that rodents can discern odours using minimal stimulus-related information. Here we investigate the dependence of odour-quality perception on the integrity of glomerular activity by comparing odour-evoked activity maps before and after epithelial lesions. Lesions prevent mice from recognizing previously experienced odours and differentially delay discrimination learning of unrecognized and novel odour pairs. Poor recognition results not from mice experiencing an altered concentration of an odour but from perception of apparent novel qualities. Consistent with this, relative intensity of glomerular activity following lesions is altered compared with maps recorded in shams and by varying odour concentration. Together, these data show that odour recognition relies on comprehensively matching input patterns to a previously generated stimulus template. When encountering novel odours, access to all glomerular activity ensures rapid generation of new templates to perform accurate perceptual judgements.

A two-layer biophysical model of cholinergic neuromodulation in olfactory bulb

Cholinergic inputs from the basal forebrain regulate multiple olfactory bulb (OB) functions, including odor discrimination, perceptual learning, and short-term memory. Previous studies have shown that nicotinic cholinergic receptor activation sharpens mitral cell chemoreceptive fields, likely via intraglomerular circuitry. Muscarinic cholinergic activation is less well understood, though muscarinic receptors are implicated in olfactory learning and in the regulation of synchronized oscillatory dynamics in hippocampus and cortex. To understand the mechanisms underlying cholinergic neuromodulation in OB, we developed a biophysical model of the OB neuronal network including both glomerular layer and external plexiform layer (EPL) computations and incorporating both nicotinic and muscarinic neuromodulatory effects. Our simulations show how nicotinic activation within glomerular circuits sharpens mitral cell chemoreceptive fields, even in the absence of EPL circuitry, but does not facilitate intrinsic oscillations or spike synchronization. In contrast, muscarinic receptor activation increases mitral cell spike synchronization and field oscillatory power by potentiating granule cell excitability and lateral inhibitory interactions within the EPL, but it has little effect on mitral cell firing rates and hence does not sharpen olfactory representations under a rate metric. These results are consistent with the theory that EPL interactions regulate the timing, rather than the existence, of mitral cell action potentials and perform their computations with respect to a spike timing-based metric. This general model suggests that the roles of nicotinic and muscarinic receptors in olfactory bulb are both distinct and complementary to one another, together regulating the effects of ascending cholinergic inputs on olfactory bulb transformations.

Rabbit neonates and human adults perceive a blending 6-component odor mixture in a comparable manner

Young and adult mammals are constantly exposed to chemically complex stimuli. The olfactory system allows for a dual processing of relevant information from the environment either as single odorants in mixtures (elemental perception) or as mixtures of odorants as a whole (configural perception). However, it seems that human adults have certain limits in elemental perception of odor mixtures, as suggested by their inability to identify each odorant in mixtures of more than 4 components. Here, we explored some of these limits by evaluating the perception of three 6-odorant mixtures in human adults and newborn rabbits. Using free-sorting tasks in humans, we investigated the configural or elemental perception of these mixtures, or of 5-component sub-mixtures, or of the 6-odorant mixtures with modified odorants' proportion. In rabbit pups, the perception of the same mixtures was evaluated by measuring the orocephalic sucking response to the mixtures or their components after conditioning to one of these stimuli. The results revealed that one mixture, previously shown to carry the specific odor of red cordial in humans, was indeed configurally processed in humans and in rabbits while the two other 6-component mixtures were not. Moreover, in both species, such configural perception was specific not only to the 6 odorants included in the mixture but also to their respective proportion. Interestingly, rabbit neonates also responded to each odorant after conditioning to the red cordial mixture, which demonstrates their ability to perceive elements in addition to configuration in this complex mixture. Taken together, the results provide new insights related to the processing of relatively complex odor mixtures in mammals and the inter-species conservation of certain perceptual mechanisms; the results also revealed some differences in the expression of these capacities between species putatively linked to developmental and ecological constraints.

Cholinergic modulation during acquisition of olfactory fear conditioning alters learning and stimulus generalization in mice

We investigated the role of cholinergic neurotransmission in olfactory fear learning. Mice receiving pairings of odor and foot shock displayed fear to the trained odor the following day. Pretraining injections of the nicotinic antagonist mecamylamine had no effect on subsequent freezing, while the muscarinic antagonist scopolamine significantly reduced freezing. To test whether cholinergic manipulation affected fear generalization, mice were presented with odors similar to the trained odor. Generalization was increased following pretraining scopolamine, while the muscarinic agonist oxotremorine decreased generalization. These results suggest that muscarinic neurotransmission during the acquisition of olfactory association modulates both the strength and specificity of learning.

Exploiting olfactory learning in alien rats to protect birds' eggs

Predators must ignore unhelpful background "noise" within information-rich environments and focus on useful cues of prey activity to forage efficiently. Learning to disregard unrewarding cues should happen quickly, weakening future interest in the cue. Prey odor, which is rapidly investigated by predators, may be particularly appropriate for testing whether consistently unrewarded cues are ignored, and whether such behavior can be exploited to benefit prey. Using wild free-ranging populations of black rats, Rattus rattus, an alien predator of global concern, we tested whether the application of bird-nesting odors before the introduction of artificial nests (odor preexposure), enhanced the survival of birds eggs (prey) compared with areas where prey and nesting odors were introduced concurrently. In areas where predators had encountered prey odor before prey being available, the subsequently introduced eggs showed 62% greater survival than in areas where prey and odor were introduced together. We suggest that black rats preexposed to prey odor learned to ignore the unrewarding cue, leading to a significant improvement in prey survival that held for the 7-d monitoring period. Exploiting rapid learning that underpins foraging decisions by manipulating sensory contexts offers a nonlethal, but effective approach to reducing undesirable predatory impacts. Techniques based on olfactory preexposure may provide prey with protection during critical periods of vulnerability, such as immediately following a prey reintroduction. These results also highlight the potential benefits to species conservation to be gained from a greater understanding of the cognitive mechanisms driving alien predator behavior within ecological contexts.

Implementation of olfactory bulb glomerular-layer computations in a digital neurosynaptic core

We present a biomimetic system that captures essential functional properties of the glomerular layer of the mammalian olfactory bulb, specifically including its capacity to decorrelate similar odor representations without foreknowledge of the statistical distributions of analyte features. Our system is based on a digital neuromorphic chip consisting of 256 leaky-integrate-and-fire neurons, 1024 × 256 crossbar synapses, and address-event representation communication circuits. The neural circuits configured in the chip reflect established connections among mitral cells, periglomerular cells, external tufted cells, and superficial short-axon cells within the olfactory bulb, and accept input from convergent sets of sensors configured as olfactory sensory neurons. This configuration generates functional transformations comparable to those observed in the glomerular layer of the mammalian olfactory bulb. Our circuits, consuming only 45 pJ of active power per spike with a power supply of 0.85 V, can be used as the first stage of processing in low-power artificial chemical sensing devices inspired by natural olfactory systems.

On-Center/Inhibitory-Surround Decorrelation via Intraglomerular Inhibition in the Olfactory Bulb Glomerular Layer

Classical lateral inhibition, which relies on spatially ordered neural representations of physical stimuli, cannot decorrelate sensory representations in which stimulus properties are represented non-topographically. Recent theoretical and experimental studies indicate that such a non-topographical representation of olfactory stimuli predominates in olfactory bulb, thereby refuting the classical view that olfactory decorrelation is mediated by lateral inhibition comparable to that in the retina. Questions persist, however, regarding how well non-topographical decorrelation models can replicate the inhibitory “surround” that has been observed experimentally (with respect to odor feature-similarity) in olfactory bulb principal neurons, analogous to the spatial inhibitory surround generated by lateral inhibition in retina. Using two contrasting scenarios of stimulus representation – one “retinotopically” organized and one in which receptive fields are unpredictably distributed as they are in olfactory bulb – we here show that intracolumnar inhibitory interactions between local interneurons and principal neurons successfully decorrelate similar sensory representations irrespective of the scenario of representation. In contrast, lateral inhibitory interactions between these same neurons in neighboring columns are only able to effectively decorrelate topographically organized representations. While anatomical substrates superficially consistent with both types of inhibition exist in olfactory bulb, of the two only local intraglomerular inhibition suffices to mediate olfactory decorrelation.

Sequential mechanisms underlying concentration invariance in biological olfaction

Concentration invariance-the capacity to recognize a given odorant (analyte) across a range of concentrations-is an unusually difficult problem in the olfactory modality. Nevertheless, humans and other animals are able to recognize known odors across substantial concentration ranges, and this concentration invariance is a highly desirable property for artificial systems as well. Several properties of olfactory systems have been proposed to contribute to concentration invariance, but none of these alone can plausibly achieve full concentration invariance. We here propose that the mammalian olfactory system uses at least six computational mechanisms in series to reduce the concentration-dependent variance in odor representations to a level at which different concentrations of odors evoke reasonably similar representations, while preserving variance arising from differences in odor quality. We suggest that the residual variance then is treated like any other source of stimulus variance, and categorized appropriately into "odors" via perceptual learning. We further show that naïve mice respond to different concentrations of an odorant just as if they were differences in quality, suggesting that, prior to odor categorization, the learning-independent compensatory mechanisms are limited in their capacity to achieve concentration invariance.

Imprinted Rasgrf1 expression in neonatal mice affects olfactory learning and memory

Rasgrf1 is genomically imprinted; only the paternally inherited allele is expressed in the neonatal mouse brain until weaning, at which time expression becomes biallelic. Whereas Rasgrf1 has been implicated in learning and memory via knockout studies in adult mice, the effect of its normal imprinted expression on these phenotypes has not yet been examined. Neonatal mice with experimentally manipulated patterns of imprinted Rasgrf1 expression were assessed on an associative olfactory task. Neonates lacking the normally expressed wild-type paternal allele exhibited significant impairment in olfactory associative memory. Adult animals in which neonatal imprinting had been manipulated were also behaviorally assessed; while neonatal imprinting significantly affects body weight even into adulthood, no learning and memory phenotype attributable to imprinting was observed in adults. Additional analyses of neonates showed imprinted Rasgrf1 transcript selective to olfactory bulb even in mice that were null for Rasgrf1 in the rest of the brain and showed that Rasgrf1 affects Ras and Rac activation in the brain. Taken together, these results indicate that Rasgrf1 expression from the wild-type paternal allele contributes to learning and memory in neonatal mice.

Compensatory responses to age-related decline in odor quality acuity: cholinergic neuromodulation and olfactory enrichment

The perceptual differentiation of odors can be measured behaviorally using generalization gradients. The steepness of these gradients defines a form of olfactory acuity for odor quality that depends on neural circuitry within the olfactory bulb and is regulated by cholinergic activity therein as well as by associative learning. Using this system as a reduced model for age-related cognitive decline, we show that aged mice, while maintaining almost the same baseline behavioral performance as younger mice, are insensitive to the effects of acutely elevated acetylcholine, which sharpens generalization gradients in young adult mice. Moreover, older mice exhibit evidence of chronically elevated acetylcholine levels in the olfactory bulb, suggesting that their insensitivity to further elevated levels of acetylcholine may arise because the maximum capacity of the system to respond to acetylcholine has already been reached. We propose a model in which an underlying, age-related, progressive deficit is mitigated by a compensatory cholinergic feedback loop that acts to retard the behavioral effects of what would otherwise be a substantial age-related decline in olfactory plasticity. We also treated mice with 10-day regimens of olfactory environmental enrichment and/or repeated systemic injections of the acetylcholinesterase inhibitor physostigmine. Each treatment alone sharpened odor quality acuity, but administering both treatments together had no greater effect than either alone. Age was not a significant main effect in this study, suggesting that some capacity for acetylcholine-dependent plasticity is still present in aged mice despite their sharply reduced ability to respond to acute increases in acetylcholine levels. These results suggest a dynamical framework for understanding age-related decline in neural circuit processing in which the direct effects of aging can be mitigated, at least temporarily, by systemic compensatory responses. In particular, a decline in cholinergic efficacy can precede any breakdown in cholinergic production, which may help explain the limited effectiveness of cholinergic replacement therapies in combating cognitive decline.

Elemental and configural processing of odour mixtures in the newborn rabbit

The processing of odour mixtures by young organisms is poorly understood. Recently, the perception of an AB mixture, known to engage configural perception in adult humans, was suggested also to be partially configural in newborn rabbits. In particular, pups did not respond to AB after they had learned A or B. However, two alternative hypotheses might be suggested to explain this result: the presence in the mixture of a novel odorant that inhibits the response to the learned stimulus, and the unevenness of the sensory and cognitive processes engaged during the conditioning and the behavioural testing. We conducted four experiments to explore these alternative hypotheses. In experiment 1, the learning of A or B ended in responses to mixtures including a novel odorant (AC or BC). Experiment 2 pointed to the absence of overshadowing. Therefore, a novelty effect cannot explain the non-response to AB after the learning of A or B. In experiment 3,pups having learned A or B in AC or BC did not respond to AB. However, they generalized odour information acquired in AB to AC or BC in experiment 4. Thus, the balancing of the perceptual tasks between the conditioning and retention test does not enhance the response to the AB mixture. To sum up, the present experiments give concrete support to the partially configural perception of specific odour mixtures by newborn rabbits.

Function follows form: ecological constraints on odor codes and olfactory percepts

Sensory system function has evolved to meet the biological needs of organisms, but it is less often regarded that sensory system form has by necessity evolved to contend with the stimulus. For an olfactory system extracting meaningful information from natural scents, the ecological milieu presents unique problems. Recent studies provide new insights into the perceptual and neural mechanisms underlying how odorant elements are assembled into odor wholes, how odor percepts are reconstructed from degraded inputs, and how learning and experience sculpt olfactory categorical perception. These data show that spatial ensemble activity patterns in piriform cortex are closely linked to the perceptual meaning and identity of odor objects, substantiating theoretical models that emphasize the importance of distributed templates for the perception, discrimination, and recall of olfactory quality.