Odor Memory Research Articles

A review on the neural bases of episodic odor memory: from laboratory-based to autobiographical approaches.

Odors are powerful cues that trigger episodic memories. However, in light of the amount of behavioral data describing the characteristics of episodic odor memory, the paucity of information available on the neural substrates of this function is startling. Furthermore, the diversity of experimental paradigms complicates the identification of a generic episodic odor memory network. We conduct a systematic review of the literature depicting the current state of the neural correlates of episodic odor memory in healthy humans by placing a focus on the experimental approaches. Functional neuroimaging data are introduced by a brief characterization of the memory processes investigated. We present and discuss laboratory-based approaches, such as odor recognition and odor associative memory, and autobiographical approaches, such as the evaluation of odor familiarity and odor-evoked autobiographical memory. We then suggest the development of new laboratory-ecological approaches allowing for the controlled encoding and retrieval of specific multidimensional events that could open up new prospects for the comprehension of episodic odor memory and its neural underpinnings. While large conceptual differences distinguish experimental approaches, the overview of the functional neuroimaging findings suggests relatively stable neural correlates of episodic odor memory.

Beta and gamma oscillatory activities associated with olfactory memory tasks: different rhythms for different functional networks?

Olfactory processing in behaving animals, even at early stages, is inextricable from top down influences associated with odor perception. The anatomy of the olfactory network (olfactory bulb, piriform, and entorhinal cortices) and its unique direct access to the limbic system makes it particularly attractive to study how sensory processing could be modulated by learning and memory. Moreover, olfactory structures have been early reported to exhibit oscillatory population activities easy to capture through local field potential recordings. An attractive hypothesis is that neuronal oscillations would serve to “bind” distant structures to reach a unified and coherent perception. In relation to this hypothesis, we will assess the functional relevance of different types of oscillatory activity observed in the olfactory system of behaving animals. This review will focus primarily on two types of oscillatory activities: beta (15–40 Hz) and gamma (60–100 Hz). While gamma oscillations are dominant in the olfactory system in the absence of odorant, both beta and gamma rhythms have been reported to be modulated depending on the nature of the olfactory task. Studies from the authors of the present review and other groups brought evidence for a link between these oscillations and behavioral changes induced by olfactory learning. However, differences in studies led to divergent interpretations concerning the respective role of these oscillations in olfactory processing. Based on a critical reexamination of those data, we propose hypotheses on the functional involvement of beta and gamma oscillations for odor perception and memory.

Generalization and discrimination tasks yield concordant measures of perceived distance between odours and their binary mixtures in larval Drosophila.

Similarity between odours is notoriously difficult to measure. Widely used behavioural approaches in insect olfaction research are cross-adaptation, masking, as well as associative tasks based on olfactory learning and the subsequent testing for how specific the established memory is. A concern with such memory-based approaches is that the learning process required to establish an odour memory may alter the way the odour is processed, such that measures of perception taken at the test are distorted. The present study was therefore designed to see whether behavioural judgements of perceptual distance are different for two different memory-based tasks, namely generalization and discrimination. We used odour–reward learning in larval Drosophila as a study case. In order to challenge the larvae's olfactory system, we chose to work with binary mixtures and their elements (1-octanol, n-amyl acetate, 3-octanol, benzaldehyde and hexyl acetate). We determined the perceptual distance between each mixture and its elements, first in a generalization task, and then in a discrimination task. It turns out that scores of perceptual distance are correlated between both tasks. A re-analysis of published studies looking at element-to-element perceptual distances in larval reward learning and in adult punishment learning confirms this result. We therefore suggest that across a given set of olfactory stimuli, associative training does not grossly alter the pattern of perceptual distances.

Induction of aversive learning through thermogenetic activation of Kenyon cell ensembles in Drosophila.

Drosophila represents a model organism to analyze neuronal mechanisms underlying learning and memory. Kenyon cells of the Drosophila mushroom body are required for associative odor learning and memory retrieval. But is the mushroom body sufficient to acquire and retrieve an associative memory? To answer this question we have conceived an experimental approach to bypass olfactory sensory input and to thermogenetically activate sparse and random ensembles of Kenyon cells directly. We found that if the artifical activation of Kenyon cell ensembles coincides with a salient, aversive stimulus learning was induced. The animals adjusted their behavior in a subsequent test situation and actively avoided reactivation of these Kenyon cells. Our results show that Kenyon cell activity in coincidence with a salient aversive stimulus can suffice to form an associative memory. Memory retrieval is characterized by a closed feedback loop between a behavioral action and the reactivation of sparse ensembles of Kenyon cells.

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.

Sleep and olfactory cortical plasticity.

In many systems, sleep plays a vital role in memory consolidation and synaptic homeostasis. These processes together help store information of biological significance and reset synaptic circuits to facilitate acquisition of information in the future. In this review, we describe recent evidence of sleep-dependent changes in olfactory system structure and function which contribute to odor memory and perception. During slow-wave sleep, the piriform cortex becomes hypo-responsive to odor stimulation and instead displays sharp-wave activity similar to that observed within the hippocampal formation. Furthermore, the functional connectivity between the piriform cortex and other cortical and limbic regions is enhanced during slow-wave sleep compared to waking. This combination of conditions may allow odor memory consolidation to occur during a state of reduced external interference and facilitate association of odor memories with stored hedonic and contextual cues. Evidence consistent with sleep-dependent odor replay within olfactory cortical circuits is presented. These data suggest that both the strength and precision of odor memories is sleep-dependent. The work further emphasizes the critical role of synaptic plasticity and memory in not only odor memory but also basic odor perception. The work also suggests a possible link between sleep disturbances that are frequently co-morbid with a wide range of pathologies including Alzheimer’s disease, schizophrenia and depression and the known olfactory impairments associated with those disorders.

Olfactory LOVER: behavioral and neural correlates of autobiographical odor memory.

Autobiographical memories (AMs) are personally experienced events that may be localized in time and space. In the present work we present an overview targeting memories evoked by the sense of smell. Overall, research indicates that autobiographical odor memory is different than memories evoked by our primary sensory systems; sight, and hearing. Here, observed differences from a behavioral and neuroanatomical perspective are presented. The key features of an olfactory evoked AM may be referred to the LOVER acronym-Limbic, Old, Vivid, Emotional, and Rare.

On the age-related loss of smell function

As already shown more than 100 years ago aging is accompanied by a decrease in intranasal chemosensory sensitivity. Numerous studies confirmed this finding for various odorants and different olfactory tasks, e.g., odor identification, odor detection, odor discrimination, or odor memory. In addition, aging is accompanied by a greater tendency for olfactory adaptation and slower recovery of threshold sensitivity. These age-related changes are detected by means of psychophysical, electrophysiological, and imaging techniques. The anatomical correlate is found both at the level of the olfactory epithelium and at higher centers involved in the processing of olfactory information. Although considerably less data are available regarding age-related changes of intranasal trigeminal chemoreception, its function also appears to decrease in an age-related manner similar to what is seen in the olfactory system. Thus, aging of chemosensory perception seems to be a multi-factorial process. However, age-related changes of the olfactory system do not appear to be an inevitable fate. Studies are needed to explain in what could be done to prevent or delay the age-related change.

Oxidative stress and age-related olfactory memory impairment in the honeybee Apis mellifera.

is one of the most impor-tant insect species to study olfactorylearning and memory because it canbe conditioned to respond with feed-ing movements of the mouthparts (pro-boscis)to avariety offloral odors by usingProboscis Extension Reflex (PER) condi-tioning. Honeybee workers switch fromin-hive (nursing) to outside-hive (forag-ing) tasks depending on their age, colonydemand, and outside conditions. Workersuse their sophisticated cognitive abilitiesin both foraging, and task performancewithin the colony (Menzel and Giurfa,2001; Frasnelli et al., 2014). The honey-bee olfactory system possesses olfactorysensory neurons inside the cuticle-coveredsensillae along the antennae, which areequivalenttoolfactoryepitheliawithinthenasal cavity in vertebrates. From antennathe information is carried via axons ofolfactory sensory neurons directly to theantennal lobe (AL) that is equivalent tothe olfactory bulb (OB) in vertebrates.This information is processed in the ALthen relayed by projection neurons tothe mushroom bodies (MB), which con-tribute to memory consolidation asso-ciated with long-term potentiation andsynaptic organization (Hourcade et al.,2010).Theexhibitingsynapticplasticityinthe MB is similar to that of mammalianhippocampal synaptic plasticity (Bliss andCollingridge, 1993).The neuronal synaptic plasticity canbe regulated by important signalingmolecules called as “biogenic amines”that modulate synaptic morphology,number of synapses, and receptors,influencing animal behaviors includ-ing complex behaviors such as learningand memory formation in both ver-tebrates and invertebrates. Prominentexamples of biogenic amines includeepinephrine, norepinephrine, dopamine,serotonin, octopamine, and tyramine.Norepinephrine and epinephrine arepreferentially synthesized by vertebrates;whereas octopamine and tyramine arepreferentially synthesized by invertebrates.Biogenic amines exert their activity byinteracting with specific G-protein cou-pled receptors, causing changes in thelevels of intracellular second messengers(Scheiner et al., 2006). In vertebrates,both the OB and cortex receive heavyinputs from cholinergic, noradrenergic,and serotonergic modulatory systems,exerting profound effects on both odorprocessing and odor memory by actingon both inhibitory local interneurons γ-amino butyric acid (GABA) and outputneurons in both regions (Fletcher andChen, 2010). The primary sensory affer-ents from the olfactory neuroepitheliumto OB can be modulated by a presynap-tic inhibition-mediated by GABA anddopamine released from bulbar interneu-rons. Increased levels of octopamine inthe AL mediate an important role in areinforcementpathwayinvolvingolfactorylearning and memory in

Odor memories regulate olfactory receptor expression in the sensory periphery

Odor learning induces structural and functional modifications throughout the olfactory system, but it is currently unknown whether this plasticity extends to the olfactory receptors (Or) in the sensory periphery. Here, we demonstrate that odor learning induces plasticity in olfactory receptor expression in the honeybee, Apis mellifera. Using quantitative RT-PCR analysis, we show that six putative floral scent receptors were differentially expressed in the bee antennae depending on the scent environment that the bees experienced. Or151, which we characterized using an in vitro cell expression system as a broadly tuned receptor binding floral odorants such as linalool, and Or11, the specific receptor for the queen pheromone 9-oxo-decenoic acid, were significantly down-regulated after honeybees were conditioned with the respective odorants in an olfactory learning paradigm. Electroantennogram recordings showed that the neural response of the antenna was similarly reduced after odor learning. Long-term odor memory was essential for inducing these changes, suggesting that the molecular mechanisms involved in olfactory memory also regulate olfactory receptor expression. Our study demonstrates for the first time that olfactory receptor expression is experience-dependent and modulated by scent conditioning, providing novel insight into how molecular regulation at the periphery contributes to plasticity in the olfactory system.

A "Misfit" Theory of Spontaneous Conscious Odor Perception (MITSCOP): reflections on the role and function of odor memory in everyday life.

Our senses have developed as an answer to the world we live in (Gibson, 1966) and so have the forms of memory that accompany them. All senses serve different purposes and do so in different ways. In vision, where orientation and object recognition are important, memory is strongly linked to identification. In olfaction, the guardian of vital functions such as breathing and food ingestion, perhaps the most important (and least noticed and researched) role of odor memory is to help us not to notice the well-known odors or flavors in our everyday surroundings, but to react immediately to the unexpected ones. At the same time it provides us with a feeling of safety when our expectancies are met. All this happens without any smelling intention or conscious knowledge of our expectations. Identification by odor naming is not involved in this and people are notoriously bad at it. Odors are usually best identified via the episodic memory of the situation in which they once occurred. Spontaneous conscious odor perception normally only occurs in situations where attention is demanded, either because the inhaled air or the food smell is particularly good or particularly bad and people search for its source or because people want to actively enjoy the healthiness and pleasantness of their surroundings or food. Odor memory is concerned with novelty detection rather than with recollection of odors. In this paper, these points are illustrated with experimental results and their consequences for doing ecologically valid odor memory research are drawn. Furthermore, suggestions for ecologically valid research on everyday odor memory and some illustrative examples are given.

Investigation of Neurophysiological Changes during Olfactory Habituation in Drosophila melanogaster

Event Abstract Back to Event Investigation of Neurophysiological Changes during Olfactory Habituation in Drosophila melanogaster Isabell Twick1*, John A. Lee1 and Mani Ramaswami1, 2 1 Trinity College Dublin, Ireland 2 National Centre for Biological Science, India Brain systems filter irrelevant sensory information. Habituation to inconsequential stimuli allows salient stimuli to be selectively broadcasted to attentional, emotional, and learning systems. Individuals diagnosed with autism spectrum disorder are impaired in habituation processes. This defect could contribute causally to stimulus hypersensitivity and altered salience mapping. Despite its importance, habituation mechanisms are poorly understood. Both, humans and fruit flies show habituation to odour stimuli. As their olfactory systems are remarkably conserved, Drosophila melanogaster with its genetic tools and well-defined olfactory circuitry is ideal to reveal conserved mechanisms of olfactory habituation (Twick et al., 2014) . Genetic and behavioral experiments in the Ramaswami lab suggest that continuous exposure to an odorant results in enhanced inhibitory transmission onto projection neurons activated by the odorant (Das, et al., 2011; Ramaswami, 2014). However, this simple model for habituation still needs to be tested on the basis of neurophysiological observations and further experiments will be required to elucidate potential mechanisms. My aim is to test and elaborate this enhanced inhibition model through experiments, which directly assess neurophysiological changes that occur during olfactory habituation. I have established a calcium imaging protocol that allows me to study changes in neuronal activity in a living preparation. Using this protocol I have shown alterations in the spatio-temporal activation patterns of a range of olfactory neuronal types after continuous odour exposure. I will continue to address whether and how the neuromodulator dopamine suppresses neurophysiological changes associated with olfactory habituation as indicated by behavioral experiments. Acknowledgements We thank Ronald Calabrese for insightful discussions on neuronal dynamics and circuitries and his advice and help with our experimental set up. References Twick I, Lee JA, Ramaswami M (2014) "Chapter 1 - Olfactory Habituation in Drosophila-Odor Encoding and its Plasticity in the Antennal Lobe." in Progress in Brain Research: Odor Memory and Perception, eds E. Barkai and D. Wilson (New York, NY: Elsevier), 177–203. Das S, Sadanandappa MK, Dervan A, Larkin A, Lee JA, Sudhakaran IP, Priya R, Heidari R, Holohan EE, Pimentel A, Gandhi A, Ito K, Sanyal S, Wang JW, Rodrigues V, Ramaswami M (2011) Plasticity of local GABAergic interneurons drives olfactory habituation. Proc Natl Acad Sci USA 108:646–654. Ramaswami M (2014) Network Plasticity in Adaptive Filtering and Behavioral Habituation. Neuron 82:1216-1229. Keywords: habituation, Drosophila, antennal lobe, spatiotemporal code, Olfaction, calcium imaging, Olfactory Habituation, local interneurons Conference: Neuroscience Ireland Young Neuroscientists Symposium 2014 , Dublin, Ireland, 20 Sep - 20 Sep, 2014. Presentation Type: Poster Presentation Topic: Early Career Neuroscience Citation: Twick I, Lee JA and Ramaswami M (2014). Investigation of Neurophysiological Changes during Olfactory Habituation in Drosophila melanogaster. Front. Neurosci. Conference Abstract: Neuroscience Ireland Young Neuroscientists Symposium 2014 . doi: 10.3389/conf.fnins.2014.87.00042 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 19 Sep 2014; Published Online: 19 Sep 2014. * Correspondence: Miss. Isabell Twick, Trinity College Dublin, Dublin, Ireland, twicki@tcd.ie Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Isabell Twick John A Lee Mani Ramaswami Google Isabell Twick John A Lee Mani Ramaswami Google Scholar Isabell Twick John A Lee Mani Ramaswami PubMed Isabell Twick John A Lee Mani Ramaswami Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Chapter 10 - Neural Mechanisms of Odor Rule Learning

Rats that are trained in a particularly difficult olfactory discrimination task demonstrate a dramatic increase in their capability to acquire memories of new odors once they have learned the first discrimination task. Such high-skill learning, termed "rule learning" or "learning set" (see Saar et al., 1998, 2001) is accompanied by a series postsynaptic cellular modifications which have three major traits: a.They are widespread throughout the piriform cortex network. Both physiological and morphological modifications are found in most of the studied neurons.b.The time course in which these modifications appear and disappear is strongly correlated with the time course in which the skill is acquired and decayed. However, memories for specific odors outlast these modifications by far. Thus, the identified modifications are related to rule learning (learning how to learn), rather than to long-term memory for the specific odors for which the rats are trained.c.While the above-mentioned changes act to enhance single-cell excitability, others reduced it; synaptic inhibition is enhanced after learning and the subunit composition of the NMDA receptor is modified in a manner that favors activity-induced synaptic weakening over synaptic enhancement.

Chapter 9 - Circuit Oscillations in Odor Perception and Memory

Olfactory system neural oscillations as seen in the local field potential have been studied for many decades. Recent research has shown that there is a functional role for the most studied gamma oscillations (40-100Hz in rats and mice, and 20Hz in insects), without which fine odor discrimination is poor. When these oscillations are increased artificially, fine discrimination is increased, and when rats learn difficult and highly overlapping odor discriminations, gamma is increased in power. Because of the depth of study on this oscillation, it is possible to point to specific changes in neural firing patterns as represented by the increase in gamma oscillation amplitude. However, we know far less about the mechanisms governing beta oscillations (15-30Hz in rats and mice), which are best associated with associative learning of responses to odor stimuli. These oscillations engage every part of the olfactory system that has so far been tested, plus the hippocampus, and the beta oscillation frequency band is the one that is most reliably coherent with other regions during odor processing. Respiratory oscillations overlapping with the theta frequency band (2-12Hz) are associated with odor sniffing and normal breathing in rats. They also show coupling in some circumstances between olfactory areas and rare coupling between the hippocampus and olfactory bulb. The latter occur in specific learning conditions in which coherence strength is negatively or positively correlated with performance, depending on the task. There is still much to learn about the role of neural oscillations in learning and memory, but techniques that have been brought to bear on gamma oscillations (current source density, computational modeling, slice physiology, behavioral studies) should deliver much needed knowledge of these events.

Chapter 11 - Cortical Odor Processing in Health and Disease

The olfactory system has a rich cortical representation, including a large archicortical component present in most vertebrates, and in mammals neocortical components including the entorhinal and orbitofrontal cortices. Together, these cortical components contribute to normal odor perception and memory. They help transform the physicochemical features of volatile molecules inhaled or exhaled through the nose into the perception of odor objects with rich associative and hedonic aspects. This chapter focuses on how olfactory cortical areas contribute to odor perception and begins to explore why odor perception is so sensitive to disease and pathology. Odor perception is disrupted by a wide range of disorders including Alzheimer's disease, Parkinson's disease, schizophrenia, depression, autism, and early life exposure to toxins. This olfactory deficit often occurs despite maintained functioning in other sensory systems. Does the unusual network of olfactory cortical structures contribute to this sensitivity?

Chapter 6 - Adult Neurogenesis in the Olfactory System Shapes Odor Memory and Perception

The olfactory system is a dynamic place. In mammals, not only are sensory neurons located in the sensory organ renewed through adult life, but also its first central relay is reconstructed by continuous neuronal recruitment. Despite these numerous morphological and physiological changes, olfaction is a unique sensory modality endowed with a privileged link to memory. This raises a clear conundrum; how does the olfactory system balance its neuronal turnover with its participation in long-term memory? This review concentrates on the functional aspects of adult neurogenesis, addressing how the integration of late-born neurons participates in olfactory perception and memory. After outlining the properties of adult neurogenesis in the olfactory system, and after describing their regulation by internal and environmental factors, we ask how the process of odorant perception can be influenced by constant neuronal turnover. We then explore the possible functional roles that newborn neurons might have for olfactory memory. Throughout this review, and as we concentrate almost exclusively on mammalian models, we stress the idea that adult neurogenesis is yet another form of plasticity used by the brain to copes with a constantly changing olfactory world.

Chapter 4 - Visualizing Olfactory Learning Functional Imaging of Experience-Induced Olfactory Bulb Changes

The anatomical organization of sensory neuron input allows odor information to be transformed into odorant-specific spatial maps of mitral/tufted cell glomerular activity. In other sensory systems, neuronal representations of sensory stimuli can be reorganized or enhanced following learning or experience. Similarly, several studies have demonstrated both structural and physiological experience-induced changes throughout the olfactory system. As experience-induced changes within this circuit likely serve as an initial site for odor memory formation, the olfactory bulb is an ideal site for optical imaging studies of olfactory learning, as they allow for the visualization of experience-induced changes in the glomerular circuit following learning and how these changes impact of odor representations with the bulb. Presently, optical imaging techniques have been used to visualize experience-induced changes in glomerular odor representations in a variety of paradigms in short-term habituation, chronic odor exposure, and olfactory associative conditioning.

Distinct Hippocampal Time Cell Sequences Represent Odor Memories in Immobilized Rats

Previous studies have revealed the existence of hippocampal "time cells," principal neurons in CA1 that fire at specific moments in temporally organized experiences. However, in all these studies, animals were in motion; and so, temporal modulation might be due, at least in part, to concurrent or planned movement through space or self-generated movement (path integration). Here the activity of hippocampal CA1 neurons was recorded in head-fixed and immobile rats while they remembered odor stimuli across a delay period. Many neurons selectively and reliably activated at brief moments during the delay, as confirmed by several analyses of temporal modulation, during a strong ongoing θ rhythm. Furthermore, each odor memory was represented by a temporally organized ensemble of time cells composed mostly of neurons that were unique to each memory and some that fired at the same or different moments among multiple memories. These results indicate that ongoing or intended movement through space is not necessary for temporal representations in the hippocampus, and highlight the potential role of time cells as a mechanism for representing the flow of time in distinct memories.

Muscarinic acetylcholine receptors are required for nestmate-odor memory retrieval but not for olfaction in harvester ants (Pogonomyrmex barbatus)

The ability of social insects such as ants to distinguish between nestmates and non-nestmates is odor-based, learned and critical to the survival of the colony. However, little is known about how the brain processes the olfactory cues associated with nestmate recognition. Previous studies have shown that inhibition of muscarinic acetylcholine receptors with a competitive inhibitor, scopolamine affects nestmate recognition in honeybees, causing them to mistakenly attack nestmates. This suggests that muscarinic receptors may be required for either olfaction and/or retrieval of odor memory. The aim of this study was to further investigate the role of muscarinic receptors in nestmate recognition in another social insect, the harvester ant (Pogonomyrmex barbatus). Focal ants were fed either scopolamine dissolved in sucrose or sucrose alone (control) for approximately 72 hours, then placed in an arena with either a nestmate or a genetically unrelated non-nestmate. We videotaped their interactions and determined the frequency with which focal ants exhibited aggressive behaviors toward the introduced ants. Focal ants treated with scopolamine exhibited inappropriate aggressive behaviors toward nestmates with a significantly higher frequency than did the controls, but with the same frequency that they exhibited aggressive behaviors toward non-nestmates. These results suggest that scopolamine-treated ants can't distinguish nestmates from non-nestmates. However, scopolamine-treated focal ants did respond appropriately to the presence of an olfactory repellent, indicating that scopolamine does not disrupt olfaction. The data support a role for muscarinic receptors in odor memory retrieval rather than in odor perception, and indicate that this role is conserved in P. barbatus.

Sensory preconditioning in newborn rabbits: from common to distinct odor memories

This study evaluated whether olfactory preconditioning is functional in newborn rabbits and based on joined or independent memory of odorants. First, after exposure to odorants A+B, the conditioning of A led to high responsiveness to odorant B. Second, responsiveness to B persisted after amnesia of A. Third, preconditioning was also functional with two overlapping pairs of odorants (A+B and B+C) and amnesia of one odorant did not affect memory of the others. Thus, incidental pairing of odorants allows reinforcement of one odorant to implicitly reinforce the others, the bond then vanishes, and the memory of each element becomes independent.