All in a sniff: olfaction as a model for active sensing

Global coordination of brain activity by the breathing cycle

Neuronal activities that synchronize with the breathing rhythm have been found in humans and a host of mammalian species, not only in brain areas closely related to respiratory control or olfactory coding but also in areas linked to emotional and higher cognitive functions. In parallel, evidence is mounting for modulations of perception and action by the breathing cycle. In this Review, we discuss the extent to which brain activity locks to breathing across areas, levels of organization and brain states, and the physiological origins of this global synchrony. We describe how waves of sensory activity evoked by nasal airflow spread through brain circuits, synchronizing neuronal populations to the breathing cycle and modulating faster oscillations, cell assembly formation and cross-area communication, thereby providing a mechanistic link from breathing to neural coding, emotion and cognition. We argue that, through evolution, the breathing rhythm has come to shape network functions across species.

GABAergic neurons in central amygdala contribute to orchestrating anxiety-like behaviors and breathing patterns

Anxiety is characterized by dysregulated respiratory reactivity to emotional stimuli. The central amygdala (CeA) is a pivotal structure involved in processing emotional alterations, but its involvement in orchestrating anxiety-like behaviors and specific breathing patterns remains largely unexplored. Our findings demonstrate that the acute restraint stress (ARS) induces anxiety-like behaviors in mice, marked by prolonged grooming time and faster respiratory frequency (RF). Conversely, silencing GABAergic CeA neurons reduces post-ARS anxiety-like behaviors, as well as the associated increases in grooming time and RF. In actively behaving mice, stimulation of GABAergic CeA neurons elicits anxiety-like behaviors, concurrently prolongs grooming time, accelerates RF through a CeA-thalamic paraventricular nucleus (PVT) circuit. In either behaviorally quiescent or anesthetized mice, stimulation of these neurons significantly increases RF but does not induce anxiety-like behaviors through the CeA-lateral parabrachial nucleus (LPBN) circuit. Collectively, GABAergic CeA neurons are instrumental in orchestrating anxiety-like behaviors and breathing patterns primarily through the CeA-PVT and CeA-LPBN circuits, respectively.

Design Principles From Natural Olfaction for Electronic Noses

Natural olfactory systems possess remarkable sensitivity and precision beyond what is currently achievable by engineered gas sensors. Unlike their artificial counterparts, noses are capable of distinguishing scents associated with mixtures of volatile molecules in complex, typically fluctuating environments and can adapt to changes. This perspective examines the multifaceted biological principles that provide olfactory systems their discriminatory prowess, and how these ideas can be ported to the design of electronic noses for substantial improvements in performance across metrics such as sensitivity and ability to speciate chemical mixtures. The topics examined herein include the fluid dynamics of odorants in natural channels; specificity and kinetics of odorant interactions with olfactory receptors and mucus linings; complex signal processing that spatiotemporally encodes physicochemical properties of odorants; active sampling techniques, like biological sniffing and nose repositioning; biological priming; and molecular chaperoning. Each of these components of natural olfactory systems are systmatically investigated, as to how they have been or can be applied to electronic noses. While not all artificial sensors can employ these strategies simultaneously, integrating a subset of bioinspired principles can address issues like sensitivity, drift, and poor selectivity, offering advancements in many sectors such as environmental monitoring, industrial safety, and disease diagnostics.

Hierarchical behavioral analysis framework as a platform for standardized quantitative identification of behaviors

Behavior is composed of modules that operate based on inherent logic. Understanding behavior and its neural mechanisms is facilitated by clear structural behavioral analysis. Here, we developed a hierarchical behavioral analysis framework (HBAF) that efficiently reveals the organizational logic of these modules by analyzing high-dimensional behavioral data. By creating a spontaneous behavior atlas for male and female mice, we discovered that spontaneous behavior patterns are hardwired, with sniffing serving as the hub node for movement transitions. The sniffing-to-grooming ratio accurately distinguished the spontaneous behavioral states in a high-throughput manner. These states are influenced by emotional status, circadian rhythms, and lighting conditions, leading to unique behavioral characteristics, spatiotemporal features, and dynamic patterns. By implementing the straightforward and achievable spontaneous behavior paradigm, HBAF enables swift and accurate assessment of animal behavioral states and bridges the gap between a theoretical understanding of the behavioral structure and practical analysis using comprehensive multidimensional behavioral information.

Activation of glutamatergic neurons in the organum vasculosum of the lamina terminalis induces thirst-driven sniffing

Sniffing is a specialized respiratory behavior that enables rodents to localize and track objects in their environment. The organum vasculosum of the lamina terminalis (OVLT) is critically involved in the regulation of thirst and water intake, yet its role in controlling thirst-driven exploratory sniffing behaviors remains unclear. This study demonstrates that hypertonic stimulation significantly increases sniffing and activates OVLT glutamatergic (OVLTGlut) neurons. Photostimulation of both OVLTGlut neurons and their axon terminals within the paraventricular nucleus of the hypothalamus (PVN) induces robust sniffing. Furthermore, ablation of PVN neurons projecting to the preBötzinger complex not only reduces the sniffing time induced by photostimulation of OVLTGlut neurons projecting to the PVN but also prolongs the drinking latency. These findings identify the OVLTGlut-PVN-preBötzinger complex circuit as a pivotal regulator of thirst-driven sniffing, providing insights into the neural mechanisms underlying thirst and exploratory behavior.

Rhythm Facilitates Auditory Working Memory via Beta-Band Encoding and Theta-Band Maintenance

Rhythm, as a prominent characteristic of auditory experiences such as speech and music, is known to facilitate attention, yet its contribution to working memory (WM) remains unclear. Here, human participants temporarily retained a 12-tone sequence presented rhythmically or arrhythmically in WM and performed a pitch change-detection task. Behaviorally, while having comparable accuracy, rhythmic tone sequences showed a faster response time and lower response boundaries in decision-making. Electroencephalographic recordings revealed that rhythmic sequences elicited enhanced non-phase-locked beta-band (16 Hz-33 Hz) and theta-band (3 Hz-5 Hz) neural oscillations during sensory encoding and WM retention periods, respectively. Importantly, the two-stage neural signatures were correlated with each other and contributed to behavior. As beta-band and theta-band oscillations denote the engagement of motor systems and WM maintenance, respectively, our findings imply that rhythm facilitates auditory WM through intricate oscillation-based interactions between the motor and auditory systems that facilitate predictive attention to auditory sequences.

Breathing Modulates Network Activity in Frontal Brain Regions during Anxiety

Anxiety elicits various physiological responses, including changes in the respiratory rate and neuronal activity within specific brain regions such as the medial prefrontal cortex (mPFC). Previous research suggests that the olfactory bulb (OB) modulates the mPFC through respiration-coupled oscillations (RCOs), which have been linked to fear-related freezing behavior. Nevertheless, the impact of breathing on frontal brain networks during other negative emotional responses, such as anxiety-related states characterized by higher breathing rates, remains unclear. To address this, we subjected rats to the elevated plus maze (EPM) paradigm while simultaneously recording respiration and local field potentials in the OB and mPFC. Our findings demonstrate distinct respiratory patterns during EPM exploration: slower breathing frequencies prevailed in the closed arms, whereas faster frequencies were observed in the open arms, independent of locomotor activity, indicating that anxiety-like states are associated with increased respiratory rates. Additionally, we identified RCOs at different frequencies, mirroring the bimodal distribution of respiratory frequencies. RCOs exhibited higher power during open-arm exploration, when they showed greater coherence with breathing at faster frequencies. Furthermore, we confirmed that nasal respiration drives RCOs in frontal brain regions and found a stronger effect during faster breathing. Interestingly, we observed that the frequency of prefrontal gamma oscillations modulated by respiration increased with breathing frequency. Overall, our study provides evidence for a significant influence of breathing on prefrontal cortex networks during anxious states, shedding light on the complex interplay between respiratory physiology and emotional processing.

Dopaminergic signaling to ventral striatum neurons initiates sniffing behavior

Sniffing is a motivated behavior displayed by nearly all terrestrial vertebrates. While sniffing is associated with acquiring and processing odors, sniffing is also intertwined with affective and motivated states. The systems which influence the display of sniffing are unclear. Here, we report that dopamine release into the ventral striatum in mice is coupled with bouts of sniffing and that stimulation of dopaminergic terminals in these regions drives increases in respiratory rate to initiate sniffing whereas inhibition of these terminals reduces respiratory rate. Both the firing of individual neurons and the activity of post-synaptic D1 and D2 dopamine receptor-expressing neurons are coupled with sniffing and local antagonism of D1 and D2 receptors squelches sniffing. Together, these results support a model whereby sniffing can be initiated by dopamine's actions upon ventral striatum neurons. The nature of sniffing being integral to both olfaction and motivated behaviors implicates this circuit in a wide array of functions.

Communication subspace dynamics of the canonical olfactory pathway

Understanding how different brain areas communicate is crucial for elucidating the mechanisms underlying cognition. A possible way for neural populations to interact is through a communication subspace, a specific region in the state-space enabling the transmission of behaviorally relevant spiking patterns. In the olfactory system, it remains unclear if different populations employ such a mechanism. Our study reveals that neuronal ensembles in the main olfactory pathway (olfactory bulb to olfactory cortex) interact through a communication subspace, which is driven by nasal respiration and allows feedforward and feedback transmission to occur segregated along the sniffing cycle. Moreover, our results demonstrate that subspace communication depends causally on the activity of both areas, is hindered during anesthesia, and transmits a low-dimensional representation of odor.

Simple olfactory navigation in air and water

Two simple algorithms based on combining odor concentration differences across time and space along with information on the flow direction are tested for their ability to locate an odor source in four different odor landscapes. Image data taken from air plumes in three different regimes and a water plume are used as test environments for a bilateral ("stereo sampling") algorithm using concentration differences across two sensors and a "casting" algorithm that uses successive samples to decide orientation. Agents are started at random locations and orientations in the landscape and allowed to move until they reach the source of the odor (success) or leave the imaged area (failure). Parameters for the algorithm are chosen to optimize success and to minimize path length to the source. Success rates over 90% are consistently obtained with path lengths that can be as low as twice the starting distance from the source in air and four times the distance in the highly turbulent water plumes. We find that parameters that optimize success often lead to more exploratory pathways to the source. Information about the direction from which the odor is coming is necessary for successful navigation in the water plume and reduces the path length in the three tested air plumes.

Direct Piriform-to-Auditory Cortical Projections Shape Auditory-Olfactory Integration

In a real-world environment, the brain must integrate information from multiple sensory modalities, including the auditory and olfactory systems. However, little is known about the neuronal circuits governing how odors influence and modulate sound processing. Here, we investigated the mechanisms underlying auditory-olfactory integration using anatomical, electrophysiological, and optogenetic approaches, focusing on the auditory cortex as a key locus for cross-modal integration. First, retrograde and anterograde viral tracing strategies revealed a direct projection from the piriform cortex to the auditory cortex. Next, using in vivo electrophysiological recordings of neuronal activity in the auditory cortex of awake male or female mice, we found that odors modulate auditory cortical responses to sound. Finally, we used in vivo optogenetic manipulations during electrophysiology to demonstrate that olfactory modulation in the auditory cortex, specifically, odor-driven enhancement of sound responses, depends on direct input from the piriform cortex. Together, our results identify a novel role of piriform-to-auditory cortical circuitry in shaping olfactory modulation in the auditory cortex, shedding new light on the neuronal mechanisms underlying auditory-olfactory integration.

Do rodents smell with sound?

Chemosensation via olfaction is a critical process underlying social interactions in many different species. Past studies of olfaction in mammals often have focused on its mechanisms in isolation from other systems, limiting the generalizability of findings from olfactory research to perceptual processes in other modalities. Studies of chemical communication, in particular, have progressed independently of research on vocal behavior and acoustic communication. Those bioacousticians who have considered how sound production and reception might interact with olfaction often portray odors as cues to the kinds of vocalizations that might be functionally useful. In the olfaction literature, vocalizations are rarely mentioned. Here, we propose that ultrasonic vocalizations may affect what rodents smell by altering the deposition of inhaled particles and that rodents coordinate active sniffing with sound production specifically to enhance reception of pheromones. In this scenario, rodent vocalizations may contribute to a unique mode of active olfactory sensing, in addition to whatever roles they serve as social signals. Consideration of this hypothesis highlights the perceptual advantages that parallel coordination of multiple sensorimotor processes may provide to individuals exploring novel situations and environments, especially those involving dynamic social interactions.

How do mammals convert dynamic odor information into neural maps for landscape navigation?

Odors are transported by seemingly chaotic plumes, whose spatiotemporal structure contains rich information about space, with olfaction serving as a gateway for obtaining and processing this spatial information. Beyond tracking odors, olfaction provides localization and chemical communication cues for detecting conspecifics and predators, and linking external environments to internal cognitive maps. In this Essay, we discuss recent physiological, behavioral, and methodological advancements in mammalian olfactory research to present our current understanding of how olfaction can be used to navigate the environment. We also examine potential neural mechanisms that might convert dynamic olfactory inputs into environmental maps along this axis. Finally, we consider technological applications of odor dynamics for developing bio-inspired sensor technologies, robotics, and computational models. By shedding light on the principles underlying the processing of odor dynamics, olfactory research will pave the way for innovative solutions that bridge the gap between biology and technology, enriching our understanding of the natural world.

Visual Processing by Hierarchical and Dynamic Multiplexing

The complexity of natural environments requires highly flexible mechanisms for adaptive processing of single and multiple stimuli. Neuronal oscillations could be an ideal candidate for implementing such flexibility in neural systems. Here, we present a framework for structuring attention-guided processing of complex visual scenes in humans, based on multiplexing and phase coding schemes. Importantly, we suggest that the dynamic fluctuations of excitability vary rapidly in terms of magnitude, frequency and wave-form over time, i.e., they are not necessarily sinusoidal or sustained oscillations. Different elements of single objects would be processed within a single cycle (burst) of alpha activity (7-14 Hz), allowing for the formation of coherent object representations while separating multiple objects across multiple cycles. Each element of an object would be processed separately in time-expressed as different gamma band bursts (>30 Hz)-along the alpha phase. Since the processing capacity per alpha cycle is limited, an inverse relationship between object resolution and size of attentional spotlight ensures independence of the proposed mechanism from absolute object complexity. Frequency and wave-shape of those fluctuations would depend on the nature of the object that is processed and on cognitive demands. Multiple objects would further be organized along the phase of slower fluctuations (e.g., theta), potentially driven by saccades. Complex scene processing, involving covert attention and eye movements, would therefore be associated with multiple frequency changes in the alpha and lower frequency range. This framework embraces the idea of a hierarchical organization of visual processing, independent of environmental temporal dynamics.

Human olfactory perception embeds fine temporal resolution within a single sniff

A sniff in humans typically lasts one to three seconds and is commonly considered to produce a long-exposure shot of the chemical environment that sets the temporal limit of olfactory perception. To break this limit, we devised a sniff-triggered apparatus that controls odorant deliveries within a sniff with a precision of 18 milliseconds. Using this apparatus, we show through rigorous psychophysical testing of 229 participants (649 sessions) that two odorants presented in one order and its reverse become perceptually discriminable when the stimulus onset asynchrony is merely 60 milliseconds (Cohen's d = 0.48; 95% confidence interval, (55, 59); 120-millisecond difference). Discrimination performance improves with the length of stimulus onset asynchrony and is independent of explicit knowledge of the temporal order of odorants or the relative amount of odorant molecules accumulated in a sniff. Our findings demonstrate that human olfactory perception is sensitive to chemical dynamics within a single sniff and provide behavioural evidence for a temporal code of odour identity.

Hunger signalling in the olfactory bulb primes exploration, food-seeking and peripheral metabolism

OBJECTIVE

Although the metabolic state of an organism affects olfactory function, the precise mechanisms and their impact on behavior and metabolism remain unknown. Here, we assess whether ghrelin receptors (GHSRs) in the olfactory bulb (OB) increase olfactory function and influence foraging behaviors and metabolism.

METHODS

We performed a detailed behavioural and metabolic analysis in mice lacking GHSRs in the OB (OBGHSR deletion). We also analsyed OB scRNA-seq and spatial transcriptomic datasets to assess GHSR+ cells in the main and accessory olfactory bulbs, as well as the anterior olfactory nucleus.

RESULTS

OBGHSR deletion affected olfactory discrimination and habituation to both food and non-food odors. Anxiety-like and depression-like behaviors were significantly greater after OBGHSR deletion, whereas exploratory behavior was reduced, with the greatest effect under fasted conditions. OBGHSR deletion impacted feeding behavior as evidenced by altered bout number and duration, as well as buried food-seeking. OBGHSR deletion increased body weight and fat mass, spared fat utilisation on a chow diet and impaired glucose metabolism indicating metabolic dysfunction. Cross referenced analysis of OB scRNA-seq and spatial transcriptomic datasets revealed GHSR+ glutamate neurons in the main and accessory olfactory bulbs, as well as the anterior olfactory nucleus. Ablation of glutamate neurons in the OB reduced ghrelin-induced food finding and phenocopied results seen after OBGHSR deletion.

CONCLUSIONS

OBGHSRs help to maintain olfactory function, particularly during hunger, and facilitate behavioral adaptations that optimise food-seeking in anxiogenic environments, priming metabolic pathways in preparation for food consumption.

Odor representation and coding by the mitral/tufted cells in the olfactory bulb

The olfactory bulb (OB) is the first relay station in the olfactory system and functions as a crucial hub. It can represent odor information precisely and accurately in an ever-changing environment. As the only output neurons in the OB, mitral/tufted cells encode information such as odor identity and concentration. Recently, the neural strategies and mechanisms underlying odor representation and encoding in the OB have been investigated extensively. Here we review the main progress on this topic. We first review the neurons and circuits involved in odor representation, including the different cell types in the OB and the neural circuits within and beyond the OB. We will then discuss how two different coding strategies-spatial coding and temporal coding-work in the rodent OB. Finally, we discuss potential future directions for this research topic. Overall, this review provides a comprehensive description of our current understanding of how odor information is represented and encoded by mitral/tufted cells in the OB.

Sniffing can be initiated by dopamine's actions on ventral striatum neurons

Sniffing is a motivated behavior displayed by nearly all terrestrial vertebrates. While sniffing is associated with acquiring and processing odors, sniffing is also intertwined with affective and motivated states. The neuromodulatory systems which influence the display of sniffing are unclear. Here, we report that dopamine release into the ventral striatum is coupled with bouts of sniffing and that stimulation of dopaminergic terminals in these regions drives increases in respiratory rate to initiate sniffing whereas inhibition of these terminals reduces respiratory rate. Both the firing of individual neurons and the activity of post-synaptic D1 and D2 receptor-expressing neurons in the ventral striatum are also coupled with sniffing and local antagonism of D1 and D2 receptors squelches sniffing. Together, these results support a model whereby sniffing can be initiated by dopamine's actions upon ventral striatum neurons. The nature of sniffing being integral to both olfaction and motivated behaviors implicates this circuit in a wide array of functions.

Development of a noninvasive olfactory stimulation fMRI system in marmosets

Olfactory dysfunction is associated with aging and the earliest stages of neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases; it is thought to be an early biomarker of cognitive decline. In marmosets, a small non-human primate model used in brain research, olfactory pathway activity during olfactory stimulation has not been well studied because of the difficulty in clearly switching olfactory stimuli inside a narrow MRI. Here, we developed an olfactory-stimulated fMRI system using a small-aperture MRI machine. The olfactory presentation system consisted of two tubes, one for supply and one for suction of olfactory stimulants and a balloon valve. A balloon valve installed in the air supply tube controlled the presentation of the olfactory stimulant, which enabled sharp olfactory stimulation within MRI, such as 30 s of stimulation repeated five times at five-minute intervals. The olfactory stimulation system was validated in vivo and in a simulated system. fMRI analysis showed a rapid increase in signal values within 30 s of olfactory stimulation in eight regions related to the sense of smell. As these regions include those associated with Alzheimer's and Parkinson's diseases, olfactory stimulation fMRI may be useful in clarifying the relationship between olfactory dysfunction and dementia in non-human primates.

Fast-spiking interneuron detonation drives high-fidelity inhibition in the olfactory bulb

Inhibitory circuits in the mammalian olfactory bulb (OB) dynamically reformat olfactory information as it propagates from peripheral receptors to downstream cortex. To gain mechanistic insight into how specific OB interneuron types support this sensory processing, we examine unitary synaptic interactions between excitatory mitral and tufted cells (MTCs), the OB projection neurons, and a conserved population of anaxonic external plexiform layer interneurons (EPL-INs) using pair and quartet whole-cell recordings in acute mouse brain slices. Physiological, morphological, neurochemical, and synaptic analyses divide EPL-INs into distinct subtypes and reveal that parvalbumin-expressing fast-spiking EPL-INs (FSIs) perisomatically innervate MTCs with release-competent dendrites and synaptically detonate to mediate fast, short-latency recurrent and lateral inhibition. Sparse MTC synchronization supralinearly increases this high-fidelity inhibition, while sensory afferent activation combined with single-cell silencing reveals that individual FSIs account for a substantial fraction of total network-driven MTC lateral inhibition. OB output is thus powerfully shaped by detonation-driven high-fidelity perisomatic inhibition.

Common principles for odour coding across vertebrates and invertebrates

The olfactory system is an ideal and tractable system for exploring how the brain transforms sensory inputs into behaviour. The basic tasks of any olfactory system include odour detection, discrimination and categorization. The challenge for the olfactory system is to transform the high-dimensional space of olfactory stimuli into the much smaller space of perceived objects and valence that endows odours with meaning. Our current understanding of how neural circuits address this challenge has come primarily from observations of the mechanisms of the brain for processing other sensory modalities, such as vision and hearing, in which optimized deep hierarchical circuits are used to extract sensory features that vary along continuous physical dimensions. The olfactory system, by contrast, contends with an ill-defined, high-dimensional stimulus space and discrete stimuli using a circuit architecture that is shallow and parallelized. Here, we present recent observations in vertebrate and invertebrate systems that relate the statistical structure and state-dependent modulation of olfactory codes to mechanisms of perception and odour-guided behaviour.

Wireless monitoring of respiration with EEG reveals relationships between respiration, behavior, and brain activity in freely moving mice

Active sampling in the olfactory domain is a fundamental aspect of mouse behavior, and there is increasing evidence that respiration-entrained neural activity outside of the olfactory system sets an important global brain rhythm. It is therefore crucial to accurately measure breathing during natural behaviors. We develop a new approach to do this in freely moving animals, by implanting a telemetry-based pressure sensor into the right jugular vein, which allows for wireless monitoring of thoracic pressure. After verifying this technique against standard head-fixed respiration measurements, we combined it with EEG and EMG recording and used evolving partial coherence analysis to investigate the relationship between respiration and brain activity across a range of experiments in which the mice could move freely. During voluntary exploration of odors and objects, we found that the association between respiration and cortical activity in the delta and theta frequency range decreased, whereas the association between respiration and cortical activity in the alpha range increased. During sleep, however, the presentation of an odor was able to cause a transient increase in sniffing without changing dominant sleep rhythms (delta and theta) in the cortex. Our data align with the emerging idea that the respiration rhythm could act as a synchronizing scaffold for specific brain rhythms during wakefulness and exploration, but suggest that respiratory changes are less able to impact brain activity during sleep. Combining wireless respiration monitoring with different types of brain recording across a variety of behaviors will further increase our understanding of the important links between active sampling, passive respiration, and neural activity.NEW & NOTEWORTHY Animals can alter their respiration rate to actively sample their environment, and increasing evidence suggests that neurons across the brain align their firing to this changing rhythm. We developed a new approach to measure sniffing in freely moving mice while simultaneously recording brain activity, and uncovered how specific cortical rhythms changed their coherence with respiration rhythm during natural behaviors and across arousal states.

Cell type-specific and frequency-dependent centrifugal modulation in olfactory bulb output neurons in vivo

Mitral/tufted cells (M/TCs) form complex local circuits with interneurons in the olfactory bulb and are powerfully inhibited by these interneurons. The horizontal limb of the diagonal band of Broca (HDB), the only GABAergic/inhibitory source of centrifugal circuit with the olfactory bulb, is known to target olfactory bulb interneurons, and we have shown targeting also to olfactory bulb glutamatergic neurons in vitro. However, the net efficacy of these circuits under different patterns of activation in vivo and the relative balance between the various targeted intact local and centrifugal circuits was the focus of this study. Here channelrhodopsin-2 (ChR2) was expressed in HDB GABAergic neurons to investigate the short-term plasticity of HDB-activated disinhibitory rebound excitation of M/TCs. Optical activation of HDB interneurons increased spontaneous M/TC firing without odor presentation and increased odor-evoked M/TC firing. HDB activation induced disinhibitory rebound excitation (burst or cluster of spiking) in all classes of M/TCs. This excitation was frequency dependent, with short-term facilitation only at higher HDB stimulation frequency (5 Hz and above). However, frequency-dependent HDB regulation was more potent in the deeper layer M/TCs compared with more superficial layer M/TCs. In all neural circuits the balance between inhibition and excitation in local and centrifugal circuits plays a critical functional role, and this patterned input-dependent regulation of inhibitory centrifugal inputs to the olfactory bulb may help maintain the precise balance across the populations of output neurons in different environmental odors, putatively to sharpen the enhancement of tuning specificity of individual or classes of M/TCs to odors.NEW & NOTEWORTHY Neuronal local circuits in the olfactory bulb are modulated by centrifugal long circuits. In vivo study here shows that inhibitory horizontal limb of the diagonal band of Broca (HDB) modulates all five types of mitral/tufted cells (M/TCs), by direct inhibitory circuits HDB → M/TCs and indirect disinhibitory long circuits HDB → interneurons → M/TCs. The HDB net effect exerts excitation in all types of M/TCs but more powerful in deeper layer output neurons as HDB activation frequency increases, which may sharpen the tuning specificity of classes of M/TCs to odors during sensory processing.

Anesthetized animal experiments for neuroscience research

Brain research has progressed with anesthetized animal experiments for a long time. Recent progress in research techniques allows us to measure neuronal activity in awake animals combined with behavioral tasks. The trends became more prominent in the last decade. This new research style triggers the paradigm shift in the research of brain science, and new insights into brain function have been revealed. It is reasonable to consider that awake animal experiments are more ideal for understanding naturalistic brain function than anesthetized ones. However, the anesthetized animal experiment still has advantages in some experiments. To take advantage of the anesthetized animal experiments, it is important to understand the mechanism of anesthesia and carefully handle the obtained data. In this minireview, we will shortly summarize the molecular mechanism of anesthesia in animal experiments, a recent understanding of the neuronal activities in a sensory system in the anesthetized animal brain, and consider the advantages and disadvantages of the anesthetized and awake animal experiments. This discussion will help us to use both research conditions in the proper manner.

Fast-spiking interneuron detonation drives high-fidelity inhibition in the olfactory bulb

Inhibitory circuits in the mammalian olfactory bulb (OB) dynamically reformat olfactory information as it propagates from peripheral receptors to downstream cortex. To gain mechanistic insight into how specific OB interneuron types support this sensory processing, we examine unitary synaptic interactions between excitatory mitral and tufted cells (MTCs), the OB projection cells, and a conserved population of anaxonic external plexiform layer interneurons (EPL-INs) using pair and quartet whole-cell recordings in acute mouse brain slices. Physiological, morphological, neurochemical, and synaptic analyses divide EPL-INs into distinct subtypes and reveal that parvalbumin-expressing fast-spiking EPL-INs (FSIs) perisomatically innervate MTCs with release-competent dendrites and synaptically detonate to mediate fast, short-latency recurrent and lateral inhibition. Sparse MTC synchronization supralinearly increases this high-fidelity inhibition, while sensory afferent activation combined with single-cell silencing reveals that individual FSIs account for a substantial fraction of total network-driven MTC lateral inhibition. OB output is thus powerfully shaped by detonation-driven high-fidelity perisomatic inhibition.

A mechanosensory feedback that uncouples external and self-generated sensory responses in the olfactory cortex

Sampling behaviors have sensory consequences that can hinder perceptual stability. In olfaction, sniffing affects early odor encoding, mimicking a sudden change in odor concentration. We examined how the inhalation speed affects the representation of odor concentration in the main olfactory cortex. Neurons combine the odor input with a global top-down signal preceding the sniff and a mechanosensory feedback generated by the air passage through the nose during inhalation. Still, the population representation of concentration is remarkably sniff invariant. This is because the mechanosensory and olfactory responses are uncorrelated within and across neurons. Thus, faster odor inhalation and an increase in concentration change the cortical activity pattern in distinct ways. This encoding strategy affords tolerance to potential concentration fluctuations caused by varying inhalation speeds. Since mechanosensory reafferences are widespread across sensory systems, the coding scheme described here may be a canonical strategy to mitigate the sensory ambiguities caused by movements.

Sensory neurobiology: Muscles power pheromone sensation

While we understand how the five main sensory organs enable and facilitate stimulus detection, little is known about how the vomeronasal organ enables pheromone sensation. A new study finds specialized muscles poised to coordinate stimulus delivery, dynamics, and arousal.

Value-related learning in the olfactory bulb occurs through pathway-dependent perisomatic inhibition of mitral cells

Associating values to environmental cues is a critical aspect of learning from experiences, allowing animals to predict and maximise future rewards. Value-related signals in the brain were once considered a property of higher sensory regions, but their wide distribution across many brain regions is increasingly recognised. Here, we investigate how reward-related signals begin to be incorporated, mechanistically, at the earliest stage of olfactory processing, namely, in the olfactory bulb. In head-fixed mice performing Go/No-Go discrimination of closely related olfactory mixtures, rewarded odours evoke widespread inhibition in one class of output neurons, that is, in mitral cells but not tufted cells. The temporal characteristics of this reward-related inhibition suggest it is odour-driven, but it is also context-dependent since it is absent during pseudo-conditioning and pharmacological silencing of the piriform cortex. Further, the reward-related modulation is present in the somata but not in the apical dendritic tuft of mitral cells, suggesting an involvement of circuit components located deep in the olfactory bulb. Depth-resolved imaging from granule cell dendritic gemmules suggests that granule cells that target mitral cells receive a reward-related extrinsic drive. Thus, our study supports the notion that value-related modulation of olfactory signals is a characteristic of olfactory processing in the primary olfactory area and narrows down the possible underlying mechanisms to deeper circuit components that contact mitral cells perisomatically.

What Does the Human Olfactory System Do, and How Does It Do It?

Historically, the human sense of smell has been regarded as the odd stepchild of the senses, especially compared to the sensory bravado of seeing, touching, and hearing. The idea that the human olfaction has little to contribute to our experience of the world is commonplace, though with the emergence of COVID-19 there has rather been a sea change in this understanding. An ever increasing body of work has convincingly highlighted the keen capabilities of the human nose and the sophistication of the human olfactory system. Here, we provide a concise overview of the neuroscience of human olfaction spanning the last 10-15 years, with focus on the peripheral and central mechanisms that underlie how odor information is processed, packaged, parceled, predicted, and perturbed to serve odor-guided behaviors. We conclude by offering some guideposts for harnessing the next decade of olfactory research in all its shapes and forms.

The Motor of Time: Coupling Action to Temporally Predictable Events Heightens Perception

Timing and motor function share neural circuits and dynamics, which underpin their close and synergistic relationship. For instance, the temporal predictability of a sensory event optimizes motor responses to that event. Knowing when an event is likely to occur lowers response thresholds, leading to faster and more efficient motor behavior though in situations of response conflict can induce impulsive and inappropriate responding. In turn, through a process of active sensing, coupling action to temporally predictable sensory input enhances perceptual processing. Action not only hones perception of the event's onset or duration, but also boosts sensory processing of its non-temporal features such as pitch or shape. The effects of temporal predictability on motor behavior and sensory processing involve motor and left parietal cortices and are mediated by changes in delta and beta oscillations in motor areas of the brain.

Stimulus-Induced Theta-Band LFP Oscillations Format Neuronal Representations of Social Chemosignals in the Mouse Accessory Olfactory Bulb

Social communication is crucial for the survival of many species. In most vertebrates, a dedicated chemosensory system, the vomeronasal system (VNS), evolved to process ethologically relevant chemosensory cues. The first central processing stage of the VNS is the accessory olfactory bulb (AOB), which sends information to downstream brain regions via AOB mitral cells (AMCs). Recent studies provided important insights about the functional properties of AMCs, but little is known about the principles that govern their coordinated activity. Here, we recorded local field potentials (LFPs) and single-unit activity in the AOB of adult male and female mice during presentation of natural stimuli. Our recordings reveal prominent LFP theta-band oscillatory episodes with a characteristic spatial pattern across the AOB. Throughout an experiment, the AOB network shows varying degrees of similarity to this pattern, in a manner that depends on the sensory stimulus. Analysis of LFP signal polarity and single-unit activity indicates that oscillatory episodes are generated locally within the AOB, likely representing a reciprocal interaction between AMCs and granule cells. Notably, spike times of many AMCs are constrained to the negative LFP oscillation phase in a manner that can drastically affect integration by downstream processing stages. Based on these observations, we propose that LFP oscillations may gate, bind, and organize outgoing signals from individual AOB neurons to downstream processing stages. Our findings suggest that, as in other neuronal systems and brain regions, population-level oscillations play a key role in organizing and enhancing transmission of socially relevant chemosensory information.SIGNIFICANCE STATEMENT The accessory olfactory bulb (AOB) is the first central stage of the vomeronasal system, a chemosensory system dedicated to processing cues from other organisms. Information from the AOB is conveyed to other brain regions via activity of its principal neurons, AOB mitral cells (AMCs). Here, we show that socially relevant sensory stimulation of the mouse vomeronasal system leads not only to changes in AMC activity, but also to distinct theta-band (∼5 Hz) oscillatory episodes in the local field potential. Notably AMCs favor the negative phase of these oscillatory events. Our findings suggest a novel mechanism for the temporal coordination of distributed patterns of neuronal activity, which can serve to efficiently activate downstream processing stages.

Active smelling in the American cockroach

Motion plays an essential role in sensory acquisition. From changing the position in which information can be acquired to fine-scale probing and active sensing, animals actively control the way they interact with the environment. In olfaction, movement impacts the time and location of odour sampling as well as the flow of odour molecules around the olfactory organs. Employing a detailed spatiotemporal analysis, we investigated how insect antennae interact with the olfactory environment in a species with a well-studied olfactory system - the American cockroach. Cockroaches were tested in a wind-tunnel setup during the presentation of odours with different attractivity levels: colony extract, butanol and linalool. Our analysis revealed significant changes in antennal kinematics when odours were presented, including a shift towards the stream position, an increase in vertical movement and high-frequency local oscillations. Nevertheless, the antennal shifting occurred predominantly in a single antenna while the overall range covered by both antennae was maintained throughout. These findings hold true for both static and moving stimuli and were more pronounced for attractive odours. Furthermore, we found that upon odour encounter, there was an increase in the occurrence of high-frequency antennal sweeps and vertical strokes, which were shown to impact the olfactory environment's statistics directly. Our study lays out a tractable system for exploring the tight coupling between sensing and movement, in which antennal sweeps, in parallel to mammalian sniffing, are actively involved in facilitating odour capture and transport, generating odour intermittency in environments with low air movement where cockroaches dwell.

Sensorimotor transformation underlying odor-modulated locomotion in walking Drosophila

Most real-world behaviors - such as odor-guided locomotion - are performed with incomplete information. Activity in olfactory receptor neuron (ORN) classes provides information about odor identity but not the location of its source. In this study, we investigate the sensorimotor transformation that relates ORN activation to locomotion changes in Drosophila by optogenetically activating different combinations of ORN classes and measuring the resulting changes in locomotion. Three features describe this sensorimotor transformation: First, locomotion depends on both the instantaneous firing frequency (f) and its change (df); the two together serve as a short-term memory that allows the fly to adapt its motor program to sensory context automatically. Second, the mapping between (f, df) and locomotor parameters such as speed or curvature is distinct for each pattern of activated ORNs. Finally, the sensorimotor mapping changes with time after odor exposure, allowing information integration over a longer timescale.

Autogenous cerebral processes: an invitation to look at the brain from inside out

While external stimulation can reliably trigger neuronal activity, cerebral processes can operate independently from the environment. In this study, we conceptualize autogenous cerebral processes (ACPs) as intrinsic operations of the brain that exist on multiple scales and can influence or shape stimulus responses, behavior, homeostasis, and the physiological state of an organism. We further propose that the field should consider exploring to what extent perception, arousal, behavior, or movement, as well as other cognitive functions previously investigated mainly regarding their stimulus-response dynamics, are ACP-driven.

Interoceptive rhythms in the brain

Sensing internal bodily signals, or interoception, is fundamental to maintain life. However, interoception should not be viewed as an isolated domain, as it interacts with exteroception, cognition and action to ensure the integrity of the organism. Focusing on cardiac, respiratory and gastric rhythms, we review evidence that interoception is anatomically and functionally intertwined with the processing of signals from the external environment. Interactions arise at all stages, from the peripheral transduction of interoceptive signals to sensory processing and cortical integration, in a network that extends beyond core interoceptive regions. Interoceptive rhythms contribute to functions ranging from perceptual detection up to sense of self, or conversely compete with external inputs. Renewed interest in interoception revives long-standing issues on how the brain integrates and coordinates information in distributed regions, by means of oscillatory synchrony, predictive coding or multisensory integration. Considering interoception and exteroception in the same framework paves the way for biological modes of information processing specific to living organisms.

The Wire Is Not the Territory: Understanding Representational Drift in Olfaction With Dynamical Systems Theory

Representational drift is a phenomenon of increasing interest in the cognitive and neural sciences. While investigations are ongoing for other sensory cortices, recent research has demonstrated the pervasiveness in which it occurs in the piriform cortex for olfaction. This gradual weakening and shifting of stimulus-responsive cells has critical implications for sensory stimulus-response models and perceptual decision-making. While representational drift may complicate traditional sensory processing models, it could be seen as an advantage in olfaction, as animals live in environments with constantly changing and unpredictable chemical information. Non-topographical encoding in the olfactory system may aid in contextualizing reactions to promiscuous odor stimuli, facilitating adaptive animal behavior and survival. This article suggests that traditional models of stimulus-(neural) response mapping in olfaction may need to be reevaluated and instead motivates the use of dynamical systems theory as a methodology and conceptual framework.

The evolution of social timing

Sociality and timing are tightly interrelated in human interaction as seen in turn-taking or synchronised dance movements. Sociality and timing also show in communicative acts of other species that might be pleasurable, but also necessary for survival. Sociality and timing often co-occur, but their shared phylogenetic trajectory is unknown: How, when, and why did they become so tightly linked? Answering these questions is complicated by several constraints; these include the use of divergent operational definitions across fields and species, the focus on diverse mechanistic explanations (e.g., physiological, neural, or cognitive), and the frequent adoption of anthropocentric theories and methodologies in comparative research. These limitations hinder the development of an integrative framework on the evolutionary trajectory of social timing and make comparative studies not as fruitful as they could be. Here, we outline a theoretical and empirical framework to test contrasting hypotheses on the evolution of social timing with species-appropriate paradigms and consistent definitions. To facilitate future research, we introduce an initial set of representative species and empirical hypotheses. The proposed framework aims at building and contrasting evolutionary trees of social timing toward and beyond the crucial branch represented by our own lineage. Given the integration of cross-species and quantitative approaches, this research line might lead to an integrated empirical-theoretical paradigm and, as a long-term goal, explain why humans are such socially coordinated animals.

Active Sensing in Bees Through Antennal Movements Is Independent of Odor Molecule

When sampling odors, many insects are moving their antennae in a complex but repeatable fashion. Previous studies with bees have tracked antennal movements in only two dimensions, with a low sampling rate and with relatively few odorants. A detailed characterization of the multimodal antennal movement patterns as function of olfactory stimuli is thus wanted. The aim of this study is to test for a relationship between the scanning movements and the properties of the odor molecule. We tracked several key locations on the antennae of bumblebees at high frequency and in three dimensions while stimulating the insect with puffs of 11 common odorants released in a low-speed continuous flow. Water and paraffin were used as negative controls. Movement analysis was done with the neural network Deeplabcut. Bees use a stereotypical oscillating motion of their antennae when smelling odors, similar across all bees, independently of the identity of the odors and hence their diffusivity and vapor pressure. The variability in the movement amplitude among odors is as large as between individuals. The main type of oscillation at low frequencies and large amplitude is triggered by the presence of an odor and is in line with previous work, as is the speed of movement. The second oscillation mode at higher frequencies and smaller amplitudes is constantly present. Antennae are quickly deployed when a stimulus is perceived, decorrelate their movement trajectories rapidly, and oscillate vertically with a large amplitude and laterally with a smaller one. The cone of airspace thus sampled was identified through the 3D understanding of the motion patterns. The amplitude and speed of antennal scanning movements seem to be function of the internal state of the animal, rather than determined by the odorant. Still, bees display an active olfactory sampling strategy. First, they deploy their antennae when perceiving an odor. Second, fast vertical scanning movements further increase the odorant capture rate. Finally, lateral movements might enhance the likelihood to locate the source of odor, similarly to the lateral scanning movement of insects at odor plume boundaries.

Modulatory dynamics of periodic and aperiodic activity in respiration-brain coupling

Bodily rhythms such as respiration are increasingly acknowledged to modulate neural oscillations underlying human action, perception, and cognition. Conversely, the link between respiration and aperiodic brain activity - a non-oscillatory reflection of excitation-inhibition (E:I) balance - has remained unstudied. Aiming to disentangle potential respiration-related dynamics of periodic and aperiodic activity, we applied recently developed algorithms of time-resolved parameter estimation to resting-state MEG and EEG data from two labs (total N = 78 participants). We provide evidence that fluctuations of aperiodic brain activity (1/f slope) are phase-locked to the respiratory cycle, which suggests that spontaneous state shifts of excitation-inhibition balance are at least partly influenced by peripheral bodily signals. Moreover, differential temporal dynamics in their coupling to non-oscillatory and oscillatory activity raise the possibility of a functional distinction in the way each component is related to respiration. Our findings highlight the role of respiration as a physiological influence on brain signalling.

Smell-related quality of life changes after total laryngectomy: a multi-centre study

PURPOSE

A total laryngectomy creates an alternate airway for gas exchange that bypasses the upper aerodigestive tract. The subsequent reduction in nasal airflow, and therefore, reduction in deposition of particles to the olfactory neuroepithelium leads to hyposmia or anosmia. The aim of this study was to assess the quality of life impairment conferred by anosmia following laryngectomy and identify any specific patient-related risk factors that are associated with poorer outcomes.

METHODS

Consecutive patients with a total laryngectomy presenting for review at three tertiary head and neck services (in Australia, the United Kingdom and India) over a 12-month period were recruited. Patient demographic and clinical data were collected, and each subject completed the validated assessment of self-reported olfactory functioning and olfaction-related quality of life questionnaire (ASOF). Dichotomous comparisons were performed using the student's unpaired t-test for continuous variables (SRP), a chi-squared test for categorical variables, and a Kendall's tau-b for ordinal variables (SOC) to assess for a correlation with poorer questionnaire scores.

RESULTS

A total of 66 laryngectomees (13.4% female; age 65.7 ± 8.6 years) were included in the study. The mean SRP score of the cohort was found to be 15.6 ± 7.4, while the mean ORQ score was noted to be 16.4 ± 8.1. No other specific risk factors associated with poorer quality of life were identified.

CONCLUSION

A significant quality of life detriment from hyposmia is conferred following laryngectomy. Further research to assess treatment options and the patient population that would best benefit from these interventions is required.

Nonequilibrium sensing of volatile compounds using active and passive analyte delivery

Although sensor technologies have allowed us to outperform the human senses of sight, hearing, and touch, the development of artificial noses is significantly behind their biological counterparts. This largely stems from the sophistication of natural olfaction, which relies on both fluid dynamics within the nasal anatomy and the response patterns of hundreds to thousands of unique molecular-scale receptors. We designed a sensing approach to identify volatiles inspired by the fluid dynamics of the nose, allowing us to extract information from a single sensor (here, the reflectance spectra from a mesoporous one-dimensional photonic crystal) rather than relying on a large sensor array. By accentuating differences in the nonequilibrium mass-transport dynamics of vapors and training a machine learning algorithm on the sensor output, we clearly identified polar and nonpolar volatile compounds, determined the mixing ratios of binary mixtures, and accurately predicted the boiling point, flash point, vapor pressure, and viscosity of a number of volatile liquids, including several that had not been used for training the model. We further implemented a bioinspired active sniffing approach, in which the analyte delivery was performed in well-controlled 'inhale-exhale' sequences, enabling an additional modality of differentiation and reducing the duration of data collection and analysis to seconds. Our results outline a strategy to build accurate and rapid artificial noses for volatile compounds that can provide useful information such as the composition and physical properties of chemicals, and can be applied in a variety of fields, including disease diagnosis, hazardous waste management, and healthy building monitoring.

Nasal anatomy and sniffing in respiration and olfaction of wild and domestic animals

Animals have been widely utilized as surrogate models for humans in exposure testing, infectious disease experiments, and immunology studies. However, respiratory diseases affect both humans and animals. These disorders can spontaneously affect wild and domestic animals, impacting their quality and quantity of life. The origin of such responses can primarily be traced back to the pathogens deposited in the respiratory tract. There is a lack of understanding of the transport and deposition of respirable particulate matter (bio-aerosols or viruses) in either wild or domestic animals. Moreover, local dosimetry is more relevant than the total or regionally averaged doses in assessing exposure risks or therapeutic outcomes. An accurate prediction of the total and local dosimetry is the crucial first step to quantifying the dose-response relationship, which in turn necessitates detailed knowledge of animals' respiratory tract and flow/aerosol dynamics within it. In this review, we examined the nasal anatomy and physiology (i.e., structure-function relationship) of different animals, including the dog, rat, rabbit, deer, rhombus monkey, cat, and other domestic and wild animals. Special attention was paid to the similarities and differences in the vestibular, respiratory, and olfactory regions among different species. The ventilation airflow and behaviors of inhaled aerosols were described as pertinent to the animals' mechanisms for ventilation modulation and olfaction enhancement. In particular, sniffing, a breathing maneuver that animals often practice enhancing olfaction, was examined in detail in different animals. Animal models used in COVID-19 research were discussed. The advances and challenges of using numerical modeling in place of animal studies were discussed. The application of this technique in animals is relevant for bidirectional improvements in animal and human health.

Behavioral State-Dependent Modulation of Prefrontal Cortex Activity by Respiration

Respiration-rhythmic oscillations in the local field potential emerge in the mPFC, a cortical region with a key role in the regulation of cognitive and emotional behavior. Respiration-driven rhythms coordinate local activity by entraining fast γ oscillations as well as single-unit discharges. To what extent respiration entrainment differently engages the mPFC network in a behavioral state-dependent manner, however, is not known. Here, we compared the respiration entrainment of mouse PFC local field potential and spiking activity (23 male and 2 female mice) across distinct behavioral states: during awake immobility in the home cage (HC), during passive coping in response to inescapable stress under tail suspension (TS), and during reward consumption (Rew). Respiration-driven rhythms emerged during all three states. However, prefrontal γ oscillations were more strongly entrained by respiration during HC than TS or Rew. Moreover, neuronal spikes of putative pyramidal cells and putative interneurons showed significant respiration phase-coupling throughout behaviors with characteristic phase preferences depending on the behavioral state. Finally, while phase-coupling dominated in deep layers in HC and Rew conditions, TS resulted in the recruitment of superficial layer neurons to respiration. These results jointly suggest that respiration dynamically entrains prefrontal neuronal activity depending on the behavioral state.SIGNIFICANCE STATEMENT The mPFC, through its extensive connections (e.g., to the amygdala, the striatum, serotoninergic and dopaminergic nuclei), flexibly regulates cognitive behaviors. Impairment of prefrontal functions can lead to disease states, such as depression, addiction, or anxiety disorders. Deciphering the complex regulation of PFC activity during defined behavioral states is thus an essential challenge. Here, we investigated the role of a prefrontal slow oscillation that has recently attracted rising interest, the respiration rhythm, in modulating prefrontal neurons during distinct behavioral states. We show that prefrontal neuronal activity is differently entrained by the respiration rhythm in a cell type- and behavior-dependent manner. These results provide first insight into the complex modulation of prefrontal activity patterns by rhythmic breathing.

Frequency-dependent centrifugal modulation of the activity of different classes of mitral and tufted cells in olfactory bulb

Mitral/tufted cells (M/TCs), the principal output neuron classes form complex circuits with bulbar neurons and long-range centrifugal circuits with higher processing areas such as the horizontal limb of the diagonal band of Broca (HDB). The precise excitability of output neurons is sculpted by local inhibitory circuits. Here, light-gated cation channel channelrhodopsin-2 (ChR2) was expressed in HDB GABAergic neurons to investigate the short-term plasticity of evoked postsynaptic currents/potentials of HDB input to all classes of M/TCs and effects on firing in the acute slice preparation. Activation of the HDB directly inhibited all classes of output neurons exhibiting frequency-dependent short-term depression of evoked inhibitory postsynaptic current (eIPSC)/potential (eIPSP), resulting in decreased inhibition of responses to olfactory nerve input as a function of input frequency. In contrast, activation of an indirect circuit of HDB→interneurons→M/TCs induced frequency-dependent disinhibition, resulting in short-term facilitation of evoked excitatory postsynaptic current (eEPSC) eliciting a burst or cluster of spiking in M/TCs. The facilitatory effects of elevated HDB input frequency were strongest on deeper output neurons (deep tufted and mitral cells) and negligible on peripheral output neurons (external and superficial tufted cells). Taken together, GABAergic HDB activation generates frequency-dependent regulation that differentially affects the excitability and responses across the five classes of M/TCs. This regulation may help maintain the precise balance between inhibition and excitation of neuronal circuits across the populations of output neurons in the face of changes in an animal sniffing rate, putatively to enhance and sharpen the tuning specificity of individual or classes of M/TCs to odors.NEW & NOTEWORTHY Neuronal circuits in the olfactory bulb closely modulate olfactory bulb output activity. Activation of GABAergic circuits from the HDB to the olfactory bulb has both direct and indirect action differentially across the five classes of M/TC bulbar output neurons. The net effect enhances the excitability of deeper output neurons as HDB frequency increases, altering the relative inhibition-excitation balance of output circuits. We hypothesize that this sharpens the tuning specificity of classes of M/TCs to odors during sensory processing.

Active antennal movements in Drosophila can tune wind encoding

Insects use their antennae to smell odors,1,2 detect auditory cues,3,4 and sense mechanosensory stimuli such as wind5 and objects,6,7,8 frequently by combining sensory processing with active movements. Genetic access to antennal motor systems would therefore provide a powerful tool for dissecting the circuit mechanisms underlying active sensing, but little is known about how the most genetically tractable insect, Drosophila melanogaster, moves its antennae. Here, we use deep learning to measure how tethered Drosophila move their antennae in the presence of sensory stimuli and identify genetic reagents for controlling antennal movement. We find that flies perform both slow adaptive movements and fast flicking movements in response to wind-induced deflections, but not the attractive odor apple cider vinegar. Next, we describe four muscles in the first antennal segment that control antennal movements and identify genetic driver lines that provide access to two groups of antennal motor neurons and an antennal muscle. Through optogenetic inactivation, we provide evidence that antennal motor neurons contribute to active movements with different time courses. Finally, we show that activation of antennal motor neurons and muscles can adjust the gain and acuity of wind direction encoding by antennal displacement. Together, our experiments provide insight into the neural control of antennal movement and suggest that active antennal positioning in Drosophila may tune the precision of wind encoding.

Artificial Olfactory Signal Modulation for Detection in Changing Environments

Animals have evolved to sense in complex environments through both modulation behavior including sniffing as well as sophisticated neural processing including memory and neuromodulation. Here, we explore thermal modulation of chemically diverse sensor arrays, where response patterns are based on partitioning of odorants across the array. The differential response patterns contain information about the chemical nature of the odorant for identification. By transitioning away from well-defined concentration modulation, traditionally used in the field, to thermal modulation, it is possible to capture both diagnostic patterns as well as intensity information in complex environments. This performance is demonstrated with carbon-black based, chemically diverse sensor arrays, that are thermally modulated with light at 25 mHz exposed to different analytes of varying concentrations.

An intrinsic oscillator underlies visual navigation in ants

Many insects display lateral oscillations while moving, but how these oscillations are produced and participate in visual navigation remains unclear. Here, we show that visually navigating ants continuously display regular lateral oscillations coupled with variations of forward speed that strongly optimize the distance covered while simultaneously enabling them to scan left and right directions. This pattern of movement is produced endogenously and conserved across navigational contexts in two phylogenetically distant ant species. Moreover, the oscillations' amplitude can be modulated by both innate or learnt visual cues to adjust the exploration/exploitation balance to the current need. This lower-level motor pattern thus drastically reduces the degree of freedom needed for higher-level strategies to control behavior. The observed dynamical signature readily emerges from a simple neural circuit model of the insect's conserved pre-motor area known as the lateral accessory lobe, offering a surprisingly simple but effective neural control and endorsing oscillation as a core, ancestral way of moving in insects.

The development of sniffing

Sniffing is a commonly displayed behavior in rodents, yet how this important behavior adjusts throughout development to meet the sensory demands of the animals has remained largely unexplored. In this issue of Chemical Senses, Boulanger-Bertolus et al. investigates the ontogeny of odor-evoked sniffing through a longitudinal study of rats engaged in several olfactory paradigms from infancy to adulthood. The results of this study yield a cohesive picture of sniffing behavior across three developmental stages, while also providing direct comparisons within subjects between these timepoints. As we discuss herein, these results advance the field in relation to existing literature on the development of odor-evoked sniffing behavior in several important ways.

Changes in spontaneous and odorant-induced single-unit activity of mitral/tufted neurons of the rat olfactory bulb during xylazine-tiletamine-zolazepam anesthesia

The nature and severity of mitral/tufted (M/T) cells reactions to odorants presented in anesthesia depend on various factors, and, above all, the nature and concentration of the odor, anesthesia, and the functional state of the olfactory bulb (OB). Compared to wakefulness, under anesthesia, the intensity of OB M/T cells responses to the odorants presented increases. However, the influence of anesthesia dynamics on the intensity of such responses has not been studied. To address this problem in rats, the activity of M/T cells and the local field potentials (LFP) in OB were recorded in the course of xylazine-tiletamine-zolazepam (XTZ) anesthesia. It has been shown that in the course of the anesthesia, the average frequency of background and odorant-induced single-unit activity of M/T cells increases, while the dominant frequency value of LFP in the gamma frequency range (90–170 Hz), on the contrary, decreases. The observed effects are assumed to be associated with changes in the functional state of the OB and systems for processing olfactory information in anesthesia.