Arc visualization of odor objects reveals experience-dependent ensemble sharpening, separation, and merging in anterior piriform cortex in adult rat

Olfactory Dysfunction Associated with Cognitive Decline in an Elderly Population

BACKGROUND

In many neurological disorders, including Alzheimer disease, early olfactory dysfunction is observed.

OBJECTIVE

In order to determine if deficits in olfactory memory are present in the elderly and if olfactory dysfunction correlates with cognitive impairment in the aging population, olfactory testing has been done on seniors from the NuAge cohort accepting to participate in the Olfactory Response Cognition and Aging (ORCA) secondary sub-study. The t-Mini Mental Statement Examination and the Telephone Interview for Cognitive Status tests were done to assess cognition levels.

RESULTS

Overall, 94% of the ORCA cohort displayed olfactory dysfunction. Deficits in olfactory memory were also present. A correlation was observed between olfactory function and cognitive test scores. Moreover, in women who smoked, there was an association between olfactory memory and cognitive scores.

CONCLUSION

Our results suggest that olfactory dysfunction may predict impending cognitive decline and highlights the need for olfactory training in seniors to improve olfaction and overall well-being.

The Role of the Stimulus in Olfactory Plasticity

Plasticity, the term we use to describe the ability of a nervous system to change with experience, is the evolutionary adaptation that freed animal behavior from the confines of genetic determinism. This capacity, which increases with brain complexity, is nowhere more evident than in vertebrates, especially mammals. Though the scientific study of brain plasticity dates back at least to the mid-19th century, the last several decades have seen unprecedented advances in the field afforded by new technologies. Olfaction is one system that has garnered particular attention in this realm because it is the only sensory modality with a lifelong supply of new neurons, from two niches no less! Here, we review some of the classical and contemporary literature dealing with the role of the stimulus or lack thereof in olfactory plasticity. We have restricted our comments to studies in mammals that have used dual tools of the field: stimulus deprivation and stimulus enrichment. The former manipulation has been implemented most frequently by unilateral naris occlusion and, thus, we have limited our comments to research using this technique. The work reviewed on deprivation provides substantial evidence of activity-dependent processes in both developing and adult mammals at multiple levels of the system from olfactory sensory neurons through to olfactory cortical areas. However, more recent evidence on the effects of deprivation also establishes several compensatory processes with mechanisms at every level of the system, whose function seems to be the restoration of information flow in the face of an impoverished signal. The results of sensory enrichment are more tentative, not least because of the actual manipulation: What odor or odors? At what concentrations? On what schedule? All of these have frequently not been sufficiently rationalized or characterized. Perhaps it is not surprising, then, that discrepant results are common in sensory enrichment studies. Despite this problem, evidence has accumulated that even passively encountered odors can "teach" olfactory cortical areas to better detect, discriminate, and more efficiently encode them for future encounters. We discuss these and other less-established roles for the stimulus in olfactory plasticity, culminating in our recommended "aspirations" for the field going forward.

Effects of stochastic coding on olfactory discrimination in flies and mice

Sparse coding can improve discrimination of sensory stimuli by reducing overlap between their representations. Two factors, however, can offset sparse coding's benefits: similar sensory stimuli have significant overlap and responses vary across trials. To elucidate the effects of these 2 factors, we analyzed odor responses in the fly and mouse olfactory regions implicated in learning and discrimination-the mushroom body (MB) and the piriform cortex (PCx). We found that neuronal responses fall along a continuum from extremely reliable across trials to extremely variable or stochastic. Computationally, we show that the observed variability arises from noise within central circuits rather than sensory noise. We propose this coding scheme to be advantageous for coarse- and fine-odor discrimination. More reliable cells enable quick discrimination between dissimilar odors. For similar odors, however, these cells overlap and do not provide distinguishing information. By contrast, more unreliable cells are decorrelated for similar odors, providing distinguishing information, though these benefits only accrue with extended training with more trials. Overall, we have uncovered a conserved, stochastic coding scheme in vertebrates and invertebrates, and we identify a candidate mechanism, based on variability in a winner-take-all (WTA) inhibitory circuit, that improves discrimination with training.

Experience-dependent evolution of odor mixture representations in piriform cortex

Rodents can learn from exposure to rewarding odors to make better and quicker decisions. The piriform cortex is thought to be important for learning complex odor associations; however, it is not understood exactly how it learns to remember discriminations between many, sometimes overlapping, odor mixtures. We investigated how odor mixtures are represented in the posterior piriform cortex (pPC) of mice while they learn to discriminate a unique target odor mixture against hundreds of nontarget mixtures. We find that a significant proportion of pPC neurons discriminate between the target and all other nontarget odor mixtures. Neurons that prefer the target odor mixture tend to respond with brief increases in firing rate at odor onset compared to other neurons, which exhibit sustained and/or decreased firing. We allowed mice to continue training after they had reached high levels of performance and find that pPC neurons become more selective for target odor mixtures as well as for randomly chosen repeated nontarget odor mixtures that mice did not have to discriminate from other nontargets. These single unit changes during overtraining are accompanied by better categorization decoding at the population level, even though behavioral metrics of mice such as reward rate and latency to respond do not change. However, when difficult ambiguous trial types are introduced, the robustness of the target selectivity is correlated with better performance on the difficult trials. Taken together, these data reveal pPC as a dynamic and robust system that can optimize for both current and possible future task demands at once.

Altered Odor-Evoked Electrophysiological Responses in the Anterior Piriform Cortex of Conscious APP/PS1 Mice

Background: Olfactory decline is an indicator of early-stage Alzheimer’s disease (AD). Although the anterior piriform cortex (aPC) is an important brain area involved in processing olfactory input, little is known about how its neuronal activity is affected in early-stage AD. Objective: To elucidate whether odor-induced electrophysiological responses are altered in the aPC of 3-5-month-old APP/PS1 mice. Methods: Using head-fixed multi-channel recording techniques in APP/PS1 AD mouse model to uncover potential aberrance of the aPC neuronal firing and local field potential (LFP) in response to vanillin. Results: We show that the firing rate of aPC neurons evoked by vanillin is significantly reduced in conscious APP/PS1 mice. LFP analysis demonstrates reduced low- and high-gamma (γ low, γ high) oscillations during both the baseline and odor stimulation periods in APP/PS1 mice. Moreover, according to spike-field coherence (SFC) analysis, APP/PS1 mice show decreased coherence between odor-evoked spikes and γ low rhythms, while the coherence with γ high rhythms and the ΔSFC of the oscillations is unaffected. Furthermore, APP/PS1 mice show reduced phase-locking strength in the baseline period, such that there is no difference between baseline and odor-stimulation conditions. This contrasts markedly with wild type mice, where phase-locking strength decreases on stimulation. Conclusion: The abnormalities in both the neuronal and oscillatory activities of the aPC may serve as electrophysiological indicators of underlying olfactory decline in early AD.

Understanding Categorical Learning in Neural Circuits Through the Primary Olfactory Cortex

Knowing which elements in the environment are associated with various opportunities and dangers is advantageous. A major role of mammalian sensory systems is to provide information about the identity of such elements which can then be used for adaptive action planning by the animal. Identity-tuned sensory representations are categorical, invariant to nuances in the sensory stream and depend on associative learning. Although categorical representations are well documented across several sensory modalities, these tend to situate synaptically far from the sensory organs which reduces experimenter control over input-output transformations. The formation of such representations is a fundamental neural computation that remains poorly understood. Odor representations in the primary olfactory cortex have several characteristics that qualify them as categorical and identity-tuned, situated only two synapses away from the sensory epithelium. The formation of categorical representations is likely critically dependent on—and dynamically controlled by—recurrent circuitry within the primary olfactory cortex itself. Experiments suggest that the concerted activity of several neuromodulatory systems plays a decisive role in shaping categorical learning through complex interactions with recurrent activity and plasticity in primary olfactory cortex circuits. In this perspective we discuss missing pieces of the categorical learning puzzle, and why several features of olfaction make it an attractive model system for this challenge.

Novelty-like activation of locus coeruleus protects against deleterious human pretangle tau effects while stress-inducing activation worsens its effects

The earliest abnormality associated with Alzheimer's disease (AD) is the presence of persistently phosphorylated pretangle tau in locus coeruleus (LC) neurons. LC neuron numbers and fiber density are positive predictors of cognition prior to death. Using an animal model of LC pretangle tau, we ask if LC activity patterns influence the sequelae of pretangle tau. We seeded LC neurons with a pretangle human tau gene. We provided daily novelty- or stress-associated optogenetic activation patterns to LC neurons for 6 weeks in mid-adulthood and, subsequently, probed cognitive and anatomical changes. Prior LC phasic stimulation prevented spatial and olfactory discrimination deficits and preserved LC axonal density. A stress-associated activation pattern increased indices of anxiety and depression, did not improve cognition, and worsened LC neuronal health. These results argue that variations in environmental experiences associated with differing LC activity patterns may account for individual susceptibility to development of AD in humans.

Cortical feedback and gating in odor discrimination and generalization

A central question in neuroscience is how context changes perception. In the olfactory system, for example, experiments show that task demands can drive divergence and convergence of cortical odor responses, likely underpinning olfactory discrimination and generalization. Here, we propose a simple statistical mechanism for this effect based on unstructured feedback from the central brain to the olfactory bulb, which represents the context associated with an odor, and sufficiently selective cortical gating of sensory inputs. Strikingly, the model predicts that both convergence and divergence of cortical odor patterns should increase when odors are initially more similar, an effect reported in recent experiments. The theory in turn predicts reversals of these trends following experimental manipulations and in neurological conditions that increase cortical excitability.

Locus Coeruleus Activation Patterns Differentially Modulate Odor Discrimination Learning and Odor Valence in Rats

The locus coeruleus (LC) produces phasic and tonic firing patterns that are theorized to have distinct functional consequences. However, how different firing modes affect learning and valence encoding of sensory information are unknown. Here, we show bilateral optogenetic activation of rat LC neurons using 10-Hz phasic trains of either 300 ms or 10 s accelerated acquisition of a similar odor discrimination. Similar odor discrimination learning was impaired by noradrenergic blockade in the piriform cortex (PC). However, 10-Hz phasic light-mediated learning facilitation was prevented by a dopaminergic antagonist in the PC, or by ventral tegmental area (VTA) silencing with lidocaine, suggesting a LC–VTA–PC dopamine circuitry involvement. Ten-hertz tonic stimulation did not alter odor discrimination acquisition, and was ineffective in activating VTA DA neurons. For valence encoding, tonic stimulation at 25 Hz induced conditioned odor aversion, whereas 10-Hz phasic stimulations produced an odor preference. Both conditionings were prevented by noradrenergic blockade in the basolateral amygdala (BLA). Cholera Toxin B retro-labeling showed larger engagement of nucleus accumbens-projecting neurons in the BLA with 10-Hz phasic activation, and larger engagement of central amygdala projecting cells with 25-Hz tonic light. These outcomes argue that the LC activation patterns differentially influence both target networks and behavior.

Structure and flexibility in cortical representations of odour space

The cortex organizes sensory information to enable discrimination and generalization^1–4. Systematic representations of chemical odor space have not been described in olfactory cortex, and so it remains unclear how odor relationships are encoded to place chemically distinct but similar odors, like lemon and orange, into perceptual categories, like citrus^5–7. Here we demonstrate that both the piriform cortex (PCx) and its sensory inputs from the olfactory bulb represent chemical odor relationships through correlated patterns of activity. However, cortical odor codes differ from those in the bulb: cortex more strongly clusters together representations for related odors, selectively rewrites pairwise odor relationships, and better matches odor perception. The bulb-to-cortex transformation depends upon the associative network originating within PCx, and can be reshaped by passive odor experience. Thus, cortex actively builds a structured representation of chemical odor space that highlights odor relationships; this representation is similar across individuals but remains plastic, suggesting a means through which the olfactory system can assign related odor cues to common and yet personalized percepts.

Odor coding in piriform cortex: mechanistic insights into distributed coding

Olfaction facilitates a large variety of animal behaviors such as feeding, mating, and communication. Recent work has begun to reveal the logic of odor transformations that occur throughout the olfactory system to form the odor percept. In this review, we describe the coding principles and mechanisms by which the piriform cortex and other olfactory areas encode three key odor features: odor identity, intensity, and valence. We argue that the piriform cortex produces a multiplexed odor code that allows non-interfering representations of distinct features of the odor stimulus to facilitate odor recognition and learning, which ultimately drives behavior.

An experimental model of Braak's pretangle proposal for the origin of Alzheimer's disease: the role of locus coeruleus in early symptom development

Background

The earliest brain pathology related to Alzheimer’s disease (AD) is hyperphosphorylated soluble tau in the noradrenergic locus coeruleus (LC) neurons. Braak characterizes five pretangle tau stages preceding AD tangles. Pretangles begin in young humans and persist in the LC while spreading from there to other neuromodulatory neurons and, later, to the cortex. While LC pretangles appear in all by age 40, they do not necessarily result in AD prior to death. However, with age and pretangle spread, more individuals progress to AD stages. LC neurons are lost late, at Braak stages III–IV, when memory deficits appear. It is not clear if LC hyperphosphorylated tau generates the pathology and cognitive changes associated with preclinical AD. We use a rat model expressing pseudohyperphosphorylated human tau in LC to investigate the hypothesis that LC pretangles generate preclinical Alzheimer pathology.

Methods

We infused an adeno-associated viral vector carrying a human tau gene pseudophosphorylated at 14 sites common in LC pretangles into 2–3- or 14–16-month TH-Cre rats. We used odor discrimination to probe LC dysfunction, and we evaluated LC cell and fiber loss.

Results

Abnormal human tau was expressed in LC and exhibited somatodendritic mislocalization. In rats infused at 2–3 months old, 4 months post-infusion abnormal LC tau had transferred to the serotonergic raphe neurons. After 7 months, difficult similar odor discrimination learning was impaired. Impairment was associated with reduced LC axonal density in the olfactory cortex and upregulated β1-adrenoceptors. LC infusions in 14–16-month-old rats resulted in more severe outcomes. By 5–6 months post-infusion, rats were impaired even in simple odor discrimination learning. LC neuron number was reduced. Human tau appeared in the microglia and cortical neurons.

Conclusions

Our animal model suggests, for the first time, that Braak’s hypothesis that human AD originates with pretangle stages is plausible. LC pretangle progression here generates both preclinical AD pathological changes and cognitive decline. The odor discrimination deficits are similar to human odor identification deficits seen with aging and preclinical AD. When initiated in aged rats, pretangle stages progress rapidly and cause LC cell loss. These age-related outcomes are associated with a severe learning impairment consistent with memory decline in Braak stages III–IV.

Electronic supplementary material

The online version of this article (10.1186/s13195-019-0511-2) contains supplementary material, which is available to authorized users.

In the Piriform Cortex, the Primary Impetus for Information Encoding through Synaptic Plasticity Is Provided by Descending Rather than Ascending Olfactory Inputs

Information encoding by means of persistent changes in synaptic strength supports long-term information storage and memory in structures such as the hippocampus. In the piriform cortex (PC), that engages in the processing of associative memory, only short-term synaptic plasticity has been described to date, both in vitro and in anesthetized rodents in vivo. Whether the PC maintains changes in synaptic strength for longer periods of time is unknown: Such a property would indicate that it can serve as a repository for long-term memories. Here, we report that in freely behaving animals, frequency-dependent synaptic plasticity does not occur in the anterior PC (aPC) following patterned stimulation of the olfactory bulb (OB). Naris closure changed action potential properties of aPC neurons and enabled expression of long-term potentiation (LTP) by OB stimulation, indicating that an intrinsic ability to express synaptic plasticity is present. Odor discrimination and categorization in the aPC is supported by descending inputs from the orbitofrontal cortex (OFC). Here, OFC stimulation resulted in LTP (>4 h), suggesting that this structure plays an important role in promoting information encoding through synaptic plasticity in the aPC. These persistent changes in synaptic strength are likely to comprise a means through which long-term memories are encoded and/or retained in the PC.

Pheromone-Induced Odor Associative Fear Learning in Rats

Alarm pheromones alert conspecifics to the presence of danger. Can pheromone communication aid in learning specific cues? Such facilitation has an evident evolutionary advantage. We use two associative learning paradigms to test this hypothesis. The first is stressed cage mate-induced conditioning. One pair-housed adult rat received 4 pairings of terpinene + shock over 30 min. Ten minutes after return to the home cage, its companion rat was removed and exposed to terpinene. Single-housed controls were exposed to either terpinene or shock only. Companion rats showed terpinene-specific freezing, which was prevented by β-adrenoceptor blockade. Using Arc to index neuronal activation in response to terpinene re-exposure, stressed cage-mate induced associative learning was measured. Companion rats showed increased neuronal activity in the accessory olfactory bulb, while terpinene + shock-conditioned rats showed increased activity in the main olfactory bulb. Both groups had enhanced activity in the anterior basolateral amygdala and central amygdala. To test involvement of pheromone mediation, in the 2nd paradigm, we paired terpinene with soiled bedding from odor + shock rats or a rat alarm pheromone. Both conditioning increased rats’ freezing to terpinene. Blocking NMDA receptors in the basolateral amygdala prevented odor-specific learning suggesting shock and pheromone-paired pathways converge in the amygdala. An alarm pheromone thus enables cue-specific learning as well as signalling danger.

Balancing the Robustness and Efficiency of Odor Representations during Learning

For reliable stimulus identification, sensory codes have to be robust by including redundancy to combat noise, but redundancy sacrifices coding efficiency. To address how experience affects the balance between the robustness and efficiency of sensory codes, we probed odor representations in the mouse olfactory bulb during learning over a week, using longitudinal two-photon calcium imaging. When mice learned to discriminate between two dissimilar odorants, responses of mitral cell ensembles to the two odorants gradually became less discrete, increasing the efficiency. In contrast, when mice learned to discriminate between two very similar odorants, the initially overlapping representations of the two odorants became progressively decorrelated, enhancing the robustness. Qualitatively similar changes were observed when the same odorants were experienced passively, a condition that would induce implicit perceptual learning. These results suggest that experience adjusts odor representations to balance the robustness and efficiency depending on the similarity of the experienced odorants.

The role of piriform associative connections in odor categorization

Distributed neural activity patterns are widely proposed to underlie object identification and categorization in the brain. In the olfactory domain, pattern-based representations of odor objects are encoded in piriform cortex. This region receives both afferent and associative inputs, though their relative contributions to odor perception are poorly understood. Here, we combined a placebo-controlled pharmacological fMRI paradigm with multivariate pattern analyses to test the role of associative connections in sustaining olfactory categorical representations. Administration of baclofen, a GABA(B) agonist known to attenuate piriform associative inputs, interfered with within-category pattern separation in piriform cortex, and the magnitude of this drug-induced change predicted perceptual alterations in fine-odor discrimination performance. Comparatively, baclofen reduced pattern separation between odor categories in orbitofrontal cortex, and impeded within-category generalization in hippocampus. Our findings suggest that odor categorization is a dynamic process concurrently engaging stimulus discrimination and generalization at different stages of olfactory information processing, and highlight the importance of associative networks in maintaining categorical boundaries.

DOI:http://dx.doi.org/10.7554/eLife.13732.001

Imagine bringing your groceries home and tucking them into the refrigerator. You’ll probably organize the items by categories: lemons and oranges into the fruit drawer, carrots and cauliflower into the vegetable drawer. Categorization is essential, allowing us to interact with the world in the most efficient way possible. If the differences between objects are not relevant to the task at hand, the brain will group objects together based on their shared properties and develop mental representations of the “categories”. Importantly, we are still aware of the distinctions between objects within the same category.

Categories of odor (for example, minty or fruity) are represented in a part of the brain called the olfactory (or piriform) cortex, which receives information from odor cues as well as “top-down” information from other areas of the brain. But how do these top-down pathways influence odor categorization?

Bao et al. asked how the brain solves the problem of categorizing odors. For the experiments, human volunteers smelled six familiar odors belonging to three different categories while their brain activity was monitored using a magnetic resonance imaging (fMRI) scanner. Then, half of the participants were given a drug called baclofen that prevents top-down inputs, but not odor cues, from reaching the piriform cortex, while the rest received a placebo. After five days of taking the medication, all of the volunteers had another session of fMRI where they had to categorize the same odors as before.

The experiments show that when comparing the fMRI scans before and after the drug treatment, the representations of odors belonging to the same category became more distinct in the piriform cortex in the placebo group. Put differently, as the volunteers were repeatedly exposed to odors of well-known categories, they became better at discriminating individual odors within the same category. However, these changes were disrupted in the group of volunteers that took baclofen.

Bao et al.’s findings indicate that this “practice makes perfect” approach to recognizing odors relies on top-down inputs into the piriform cortex. In future work it will be important to study the roles of these inputs in learning new categories of odors, and to investigate whether the mechanisms identified here apply to other sensory information and to more abstract knowledge.

DOI:http://dx.doi.org/10.7554/eLife.13732.002

Arc-Expressing Neuronal Ensembles Supporting Pattern Separation Require Adrenergic Activity in Anterior Piriform Cortex: An Exploration of Neural Constraints on Learning

Arc ensembles in adult rat olfactory bulb (OB) and anterior piriform cortex (PC) were assessed after discrimination training on highly similar odor pairs. Nonselective α- and β-adrenergic antagonists or saline were infused in the OB or anterior PC during training. OB adrenergic blockade slowed, but did not prevent, odor discrimination learning. After criterion performance, Arc ensembles in anterior piriform showed enhanced stability for the rewarded odor and pattern separation for the discriminated odors as described previously. Anterior piriform adrenergic blockade prevented acquisition of similar odor discrimination and of OB ensemble changes, even with extended overtraining. Mitral and granule cell Arc ensembles in OB showed enhanced stability for rewarded odor only in the saline group. Pattern separation was not seen in the OB. Similar odor discrimination co-occurs with increased stability in rewarded odor representations and pattern separation to reduce encoding overlap. The difficulty of similar discriminations may relate to the necessity to both strengthen rewarded representations and weaken overlap across similar representations.

SIGNIFICANCE STATEMENT

We show for the first time that adrenoceptors in anterior piriform cortex (aPC) must be engaged for adult rats to learn to discriminate highly similar odors. Loss of adrenergic activation in olfactory bulb (OB) slows, but does not prevent, discrimination learning. Both increased stability of the rewarded odor representation and increased pattern separation of the rewarded and unrewarded odors in aPC accompany successful discrimination. In the OB, rewarded odors increase in ensemble stability, but there is no evidence of pattern separation. We suggest that the slow acquisition of similar odor discriminations is related to the differing plasticity requirements for increased stability and pattern separation.

Lack of spatial segregation in the representation of pheromones and kairomones in the mouse medial amygdala

The nervous system is organized to detect, internally represent and process sensory information to generate appropriate behaviors. Despite the crucial importance of odors that elicit instinctive behaviors, such as pheromones and kairomones, their neural representation remains little characterized in the mammalian brain. Here we used expression of the immediate early gene product c-Fos as a marker of neuronal activity to find that a wide range of pheromones and kairomones produces activation in the medial nucleus of the amygdala, a brain area anatomically connected with the olfactory sensory organs. We see that activity in this nucleus depends on vomeronasal organ input, and that distinct vomeronasal stimuli activate a dispersed ensemble of cells, without any apparent spatial segregation. This activity pattern does not reflect the chemical category of the stimuli, their valence or the induced behaviors. These findings will help build a complete understanding of how odor information is processed in the brain to generate instinctive behaviors.

Computational modeling suggests distinct, location-specific function of norepinephrine in olfactory bulb and piriform cortex

Noradrenergic modulation from the locus coerulus is often associated with the regulation of sensory signal-to-noise ratio. In the olfactory system, noradrenergic modulation affects both bulbar and cortical processing, and has been shown to modulate the detection of low concentration stimuli. We here implemented a computational model of the olfactory bulb and piriform cortex, based on known experimental results, to explore how noradrenergic modulation in the olfactory bulb and piriform cortex interact to regulate odor processing. We show that as predicted by behavioral experiments in our lab, norepinephrine can play a critical role in modulating the detection and associative learning of very low odor concentrations. Our simulations show that bulbar norepinephrine serves to pre-process odor representations to facilitate cortical learning, but not recall. We observe the typical non-uniform dose—response functions described for norepinephrine modulation and show that these are imposed mainly by bulbar, but not cortical processing.

Coding of odor stimulus features among secondary olfactory structures

Sensory systems must represent stimuli in manners dependent upon a wealth of factors, including stimulus intensity and duration. One way the brain might handle these complex functions is to assign the tasks throughout distributed nodes, each contributing to information processing. We sought to explore this important aspect of sensory network function in the mammalian olfactory system, wherein the intensity and duration of odor exposure are critical contributors to odor perception. This is a quintessential model for exploring processing schemes given the distribution of odor information by olfactory bulb mitral and tufted cells into several anatomically distinct secondary processing stages, including the piriform cortex (PCX) and olfactory tubercle (OT), whose unique contributions to odor coding are unresolved. We explored the coding of PCX and OT neuron responses to odor intensity and duration. We found that both structures similarly partake in representing descending intensities of odors by reduced recruitment and modulation of neurons. Additionally, while neurons in the OT adapt to odor exposure, they display reduced capacity to adapt to either repeated presentations of odor or a single prolonged odor presentation compared with neurons in the PCX. These results provide insights into manners whereby secondary olfactory structures may, at least in some cases, uniquely represent stimulus features.

Visualizing the engram: learning stabilizes odor representations in the olfactory network

The nature of memory is a central issue in neuroscience. How does our representation of the world change with learning and experience? Here we use the transcription of Arc mRNA, which permits probing the neural representations of temporally separated events, to address this in a well characterized odor learning model. Rat pups readily associate odor with maternal care. In pups, the lateralized olfactory networks are independent, permitting separate training and within-subject control. We use multiday training to create an enduring memory of peppermint odor. Training stabilized rewarded, but not nonrewarded, odor representations in both mitral cells and associated granule cells of the olfactory bulb and in the pyramidal cells of the anterior piriform cortex. An enlarged core of stable, likely highly active neurons represent rewarded odor at both stages of the olfactory network. Odor representations in anterior piriform cortex were sparser than typical in adult rat and did not enlarge with learning. This sparser representation of odor is congruent with the maturation of lateral olfactory tract input in rat pups. Cortical representations elsewhere have been shown to be highly variable in electrophysiological experiments, suggesting brains operate normally using dynamic and network-modulated representations. The olfactory cortical representations here are consistent with the generalized associative model of sparse variable cortical representation, as normal responses to repeated odors were highly variable (∼70% of the cells change as indexed by Arc). Learning and memory modified rewarded odor ensembles to increase stability in a core representational component.

Learning modulation of odor representations: new findings from Arc-indexed networks

We first review our understanding of odor representations in rodent olfactory bulb (OB) and anterior piriform cortex (APC). We then consider learning-induced representation changes. Finally we describe the perspective on network representations gained from examining Arc-indexed odor networks of awake rats. Arc-indexed networks are sparse and distributed, consistent with current views. However Arc provides representations of repeated odors. Arc-indexed repeated odor representations are quite variable. Sparse representations are assumed to be compact and reliable memory codes. Arc suggests this is not necessarily the case. The variability seen is consistent with electrophysiology in awake animals and may reflect top-down cortical modulation of context. Arc-indexing shows that distinct odors share larger than predicted neuron pools. These may be low-threshold neuronal subsets. Learning’s effect on Arc-indexed representations is to increase the stable or overlapping component of rewarded odor representations. This component can decrease for similar odors when their discrimination is rewarded. The learning effects seen are supported by electrophysiology, but mechanisms remain to be elucidated.