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Olfactory tubercle

The olfactory tubercle (OT), also known as the tuberculum olfactorium, is a basal forebrain structure that serves as a key component of the mammalian olfactory system, integrating sensory olfactory inputs with reward, motivation, and behavioral processing.^[1] It is present in all studied mammals, including rodents, primates, and humans, where it appears as a ventral bulge along the base of the forebrain, posterior to the olfactory peduncle and anterior to the optic chiasm, bordered laterally by the olfactory tract and medially by the midline.^[2] Characterized by a tri-laminar organization—comprising a superficial molecular layer (Layer I), a dense cellular layer (Layer II), and a multiform layer (Layer III)—the OT also contains distinctive islands of Calleja, which are clusters of densely packed GABAergic granule cells.^[1] As part of the ventral striatum, the OT translates odor perception into goal-directed actions, contributing to odor discrimination, valence encoding, and motivated behaviors such as reward-seeking and social chemosignal responses.^[2] Structurally, the OT exhibits regional variations across species: it forms a prominent elliptical bulge in rodents and carnivores, with well-defined laminar hills and valleys, while in humans and primates, it is less conspicuous, often embedded between the uncus of the hippocampus and the medial forebrain bundle.^[1] Diverse neuronal populations populate its layers, including medium spiny neurons in Layer III that express dopamine receptors, pyramidal-like cells, and interneurons, enabling complex signal processing.^[2] The islands of Calleja, particularly prominent in the anterior OT, add to its heterogeneity by providing inhibitory modulation through GABAergic projections.^[1] These architectural features support the OT's role in both primary olfactory relay and higher-order integration, distinguishing it from adjacent structures like the piriform cortex.^[2] Functionally, the OT receives direct monosynaptic inputs from the olfactory bulb's tufted and mitral cells primarily to Layer I, as well as association fibers from the piriform cortex targeting Layers II and III, allowing it to process and refine odor information beyond initial sensory detection.^[1] Its efferent projections, mainly via medium spiny neurons, target reward-related nuclei such as the ventral pallidum, nucleus accumbens, and ventral tegmental area, linking olfaction to limbic and motivational circuits.^[2] This connectivity positions the OT at the interface of sensory and affective processing, where it modulates odor hedonics (pleasantness), facilitates multisensory integration (e.g., olfactory-auditory cues), and influences behaviors like appetitive approach or aversion.^[1] In behavioral contexts, disruptions in OT function have been implicated in altered reward processing, such as in addiction models involving psychostimulants.^[2] Historically, the OT was first described in 1896 by anatomist Albert von Kölliker, with early studies emphasizing its olfactory role, though its striatal affiliations were clarified in the mid-20th century by researchers like Lennart Heimer.^[1] Contemporary neuroscience continues to explore its elusive functions in humans, where imaging studies reveal connections to the orbitofrontal cortex and hypothalamus, underscoring its conserved yet evolutionarily adapted contributions to olfactory-driven decision-making.^[2]

Structure

Gross Anatomy

The olfactory tubercle is situated in the basal forebrain, positioned anterior to the optic chiasm and posterior to the olfactory peduncle, forming part of both the ventral striatum and the olfactory cortex.^[3] In rodents, it appears as a distinct structure on the ventral surface of the forebrain, bounded laterally by the lateral olfactory tract, medially by the midline ridge, and anteriorly by the olfactory bulb projections.^[4] This location underscores its role in integrating olfactory information at the interface of sensory and limbic systems.^[5] The structure exhibits a tubercle-like, ellipsoidal bulge with a characteristic trilaminar organization. The superficial molecular layer (layer I) consists primarily of afferent fibers and sparse neuropil, overlying the dense cell layer (layer II), which contains packed neuronal somata arranged in a gyrating pattern of hills and valleys.^[4] Deeper, the multiform layer (layer III) incorporates fiber bundles and embedded clusters known as islands of Calleja, which are dense aggregations of granule cells extending rostrocaudally.^[3] This layered configuration gives the olfactory tubercle its undulating, tubular appearance in sagittal and coronal views.^[4] In terms of size, rendering it a prominent feature visible during gross dissection of the rodent brain.^[3] However, its dimensions and distinctiveness vary comparatively across species; it occupies a larger relative volume in macrosmatic animals such as rats and dogs, where enhanced olfaction correlates with a more pronounced basal forebrain bulge and clear lamination.^[5] In contrast, microsmatic species like primates and humans exhibit a reduced and less distinct structure, with diminished lamination and challenges in delineation due to evolutionary minimization of olfactory reliance.^[3]

Cellular and Neurochemical Features

The olfactory tubercle (OT) is predominantly composed of medium spiny neurons (MSNs), which constitute approximately 90-95% of its neuronal population and are analogous to those in the striatum. These GABAergic projection neurons feature large somata and extensively branched, spine-rich dendrites that receive inputs across the structure's layers. MSNs express dopamine D1 and D2 receptors, with D1 receptors primarily on direct-pathway neurons and D2 on indirect-pathway neurons, enabling modulation by dopaminergic afferents.^[2]^[6] In addition to MSNs, the OT contains distinct populations of granule cells, particularly within the islands of Calleja (ICj), which are dense clusters of small, GABAergic neurons located mainly in the multiform layer. These granule cells have short, sparse processes and are interspersed with larger cells bearing longer dendrites, contributing to the OT's unique compartmentalization. Interneurons, comprising a smaller fraction, include cholinergic neurons expressing choline acetyltransferase and somatostatin-positive cells, which provide local inhibition and modulation.^[2]^[6]^[7] The neurochemical profile of the OT is characterized by high dopamine concentrations derived from projections of the nigrostriatal and mesolimbic pathways, particularly from the ventral tegmental area, with dense dopamine transporter immunoreactivity facilitating rapid signaling. It is enriched in opioid systems, including mu (MOR), delta (DOR), and kappa (KOR) receptors, as well as enkephalins expressed in D2-MSNs and dynorphins in D1-MSNs; substance P immunoreactivity is prominent in fibers innervating layers I and II. Moderate levels of acetylcholine arise from cholinergic interneurons and basal forebrain projections, while serotonin inputs from raphe nuclei express receptors such as 5-HT2A and 5-HT6, supporting sensory and motivational integration.^[6]^[8]^[6] Structurally, the OT exhibits a trilaminar organization: the molecular layer (layer I) contains sparse axons and receives primary olfactory inputs; the dense cell layer (layer II) is packed with MSN somata and shows high expression of opioid peptides and dopamine receptors; and the multiform layer (layer III) includes fiber tracts, ICj granule cells, and projection neurons. This layering underscores the OT's striatal-like properties, sharing MSNs and patch/matrix compartments with the nucleus accumbens, yet distinguished by its olfactory-specific granule cells in the ICj.^[2]^[7]^[6]

Development

The olfactory tubercle in mammals originates from progenitor cells in the lateral ganglionic eminence (LGE), particularly its ventral subdivision (vLGE), as well as contributions from the rostromedial telencephalic wall. These progenitors express key transcription factors such as Dlx1/2, which regulate the differentiation of GABAergic neurons, and Gsh2, which patterns the LGE to direct striatal and olfactory histogenesis.^[9]^[10]^[11] In rodents, a model species for studying telencephalic development, neurogenesis of the olfactory tubercle primarily occurs during an embryonic window from E11 to E15, generating over 90% of its neurons in a lateral-to-medial gradient. Projection neurons, including pyramidal-like cells in layer II, are produced first from the vLGE via a ventral migratory stream, followed by interneurons such as granule cells that populate deeper layers. Layer formation, comprising the molecular (layer I), pyramidal (layer II), and polymorphic (layer III) strata, emerges through postnatal cell rearrangements, largely complete by postnatal day (P) 5–10. Full structural maturation, including dense packing and initial circuit refinement, is achieved by approximately P21 (postnatal week 4).^[9] Human olfactory tubercle development follows a similar ontogenetic pattern scaled to longer gestation, with neurogenesis peaking between gestational weeks 8 and 20, though the structure exhibits reduced overall size relative to other mammals due to evolutionary diminution of the olfactory system. Ventral patterning of the telencephalon, including LGE specification, is influenced by Sonic Hedgehog (SHH) signaling from midline sources, which promotes Gsh2 expression and ventral progenitor identity. Postnatal refinements, such as synaptic pruning to eliminate excess connections and progressive myelination of axons in layer III, extend into adolescence, supporting circuit stabilization.^[11]^[12] Developmentally, the olfactory tubercle is conserved across mammals, reflecting its ancient role in olfactory processing, but exhibits species-specific expansions in olfactory-specialized lineages such as rodents, where it forms a prominent basal forebrain bulge, compared to the smaller, less distinct structure in primates including humans.^[13]

Connectivity

Afferent Inputs

The olfactory tubercle (OT) receives primary sensory input through direct monosynaptic projections from the mitral and tufted cells of the main olfactory bulb, traveling via the lateral olfactory tract to terminate predominantly in layer Ia of the OT.^[1] These projections form the core olfactory pathway, enabling rapid transmission of odor information to the ventral striatum.^[14] Limbic and reward-related afferents provide dense modulatory innervation to the OT. Dopaminergic fibers arise primarily from the ventral tegmental area (VTA) and, to a lesser extent, the substantia nigra pars compacta, targeting medium spiny neurons across OT layers and influencing reward processing.^[15] Noradrenergic inputs originate from the locus coeruleus, modulating state-dependent olfactory responses, while serotonergic projections from the raphe nuclei contribute to sensory gating and emotional valence integration.^[1] Multisensory afferents to the OT are sparser but diverse, supporting integration beyond olfaction. Projections from the prefrontal cortex convey signals facilitating associative learning,^[14] whereas gustatory inputs from the nucleus of the solitary tract (NTS) enable convergence with taste-related visceral signals.^[16] Amygdala and basal forebrain structures, including the substantia innominata, provide additional limbic inputs that modulate motivational aspects of sensory processing.^[14] Rodent tract-tracing studies quantify afferent contributions to OT medium spiny neurons, revealing that approximately 50% of monosynaptic inputs originate from olfactory regions like the piriform cortex and olfactory bulb, with the remainder distributed among modulatory (e.g., ~2% from midbrain including VTA) and limbic sources such as the amygdala and basal forebrain.^[14] Recent investigations (2020–2025) highlight dynamic aspects of these inputs, particularly enhanced phasic dopamine release from VTA terminals to the OT during odor-reward pairings, which drives synaptic strengthening through long-term potentiation-like mechanisms to reinforce stimulus-reward associations.^[17]

Efferent Outputs and Plasticity

The olfactory tubercle (OT) primarily exerts its influence through GABAergic projections originating from medium spiny neurons (MSNs), the principal output neurons of this ventral striatal structure. These MSNs, comprising both D1- and D2-receptor expressing subtypes, send dense efferents to the nucleus accumbens (NAc) shell, ventral pallidum (VP), and substantia nigra pars reticulata (SNr), thereby integrating olfactory signals into reward and motor circuits.^[18] The projections to the NAc shell facilitate coordination within the ventral striatum, while those to the VP and SNr contribute to downstream basal ganglia processing, with the VP serving as a key relay for motivated behaviors.^[19] In contrast, granule cells within the OT, particularly those clustered in the islands of Calleja, provide local inhibitory projections that modulate intraregional activity, refining sensory processing without extensive long-range dissemination. Limbic targets of OT efferents include bidirectional connections with the basolateral amygdala and prefrontal cortex, enabling emotional modulation of olfactory-driven responses. These pathways allow the OT to convey valence-tagged olfactory information to amygdalar circuits for fear or reward association and to prefrontal areas for executive control.^[18] Additionally, reciprocal olfactory outputs project back to the piriform cortex, supporting feedback refinement of odor representation and integration with higher-order processing.^[19] For motor and behavioral functions, OT influences locomotion indirectly through the basal ganglia via the globus pallidus, where VP projections gate action selection based on olfactory cues.^[18] Plasticity in OT efferents is evident in mechanisms such as phasic dopamine release from VTA afferents, which promotes adaptive circuit changes during odor-reward learning.^[19]^[17] Experience-dependent plasticity, such as in odor-reward learning, drives structural remodeling, as demonstrated in recent connectivity studies using viral tracing in rodents.^[20] These changes underlie strengthened links following associative training, enhancing reward salience of odors.

Function

Sensory Integration

The olfactory tubercle (OT) functions as a key site for multisensory convergence, where direct inputs from the olfactory bulb integrate with visual signals from retinal ganglion cells and auditory signals from the auditory cortex, thereby facilitating enhanced odor localization and source identification.^[1]^[21]^[22] This integration allows the OT to contextualize olfactory information with environmental cues, supporting perceptual binding in complex sensory scenes. For instance, anatomical projections from retinal ganglion cells contribute visual modulatory influences, while auditory cortical fibers enable temporal alignment of sounds with odors.^[1] Beyond primary odor processing, the OT plays a non-olfactory role by processing contextual aspects of odors through modulatory inputs from other sensory modalities. Visual cues, for example, can modulate OT neuronal firing during odor discrimination tasks in rodents, altering response patterns to incorporate environmental context without relying solely on olfactory signals.^[23] This modulation enhances the salience of odor-related decisions, as seen in behavioral paradigms where paired sensory inputs improve discrimination accuracy.^[16] Electrophysiological recordings in rodents provide direct evidence of cross-modal responses in OT neurons. Approximately 29% of OT single units exhibit supraadditive or suppressive interactions when odors are paired with auditory tones, with combined stimuli often increasing firing rates beyond unimodal responses alone (e.g., odor-evoked firing enhanced by auditory input).^[24] These responses occur early in the olfactory processing stream, underscoring the OT's capacity for rapid sensory fusion.^[22] Recent human neuroimaging studies using fMRI have revealed OT activation during pleasant odor-visual pairings, such as lavender scents combined with visual cues, even when odors are absent post-association, indicating a role in odor-guided attention independent of direct sniffing or primary olfaction.^[25] In these paradigms, visual stimuli alone elicit BOLD signals in the OT proportional to prior odor intensity, highlighting its involvement in associative perceptual binding.^[25] Despite these functions, the OT is not a primary olfactory relay, serving instead as a secondary processor downstream from the olfactory bulb and piriform cortex. Lesion models in rodents demonstrate that OT damage leads to deficits in sensory integration, resulting in altered perceptual responsiveness and illusions in multisensory odor tasks, such as impaired source attribution.^[1]

Reward and Motivation

The olfactory tubercle (OT) serves as a critical node in the mesolimbic dopaminergic pathway, where dopamine release from ventral tegmental area (VTA) afferents encodes associations between odors and rewards.^[26] Activation of the VTA-OT dopaminergic pathway during rewarding behaviors, such as sucrose licking, increases firing rates in OT neurons, thereby linking olfactory cues to hedonic value.^[27] This dopamine-mediated mechanism enables the OT to integrate sensory olfactory inputs with motivational signals, facilitating odor-reward learning without requiring extensive conditioning.^[26] In drug reinforcement, the OT acts as a primary site for cocaine self-administration in rats, with animals exhibiting robust intake into the anteromedial region during 90-minute sessions limited to 60 infusions.^[28] Mu-opioid receptor activation within the OT enhances hedonic "liking" responses to sweet tastes, potentially through projections to downstream reward areas like the ventral pallidum, amplifying pleasure from palatable stimuli.^[29] This opioid modulation underscores the OT's role in processing the sensory-affective components of reinforcement. The OT modulates the motivational salience of odors by assigning incentive value, particularly enhancing drive for food-seeking in states of hunger.^[27] A 2021 study demonstrated that the OT processes natural rewards from pleasant odors, such as citronellol, without prior learning, as evidenced by increased neural activity and conditioned place preference in mice.^[30] Human functional MRI scans corroborate this, showing OT activation surges correlated with subjective ratings of odor attractiveness during exposure to rewarding olfactory stimuli.^[30] Recent optogenetic studies (2020–2025) confirm the OT's involvement in odor-guided eating motivation; for instance, stimulating D1 receptor-expressing neurons in the anteromedial OT elicits place preference, promoting approach behaviors toward food-related odors.^[27] Specific subregions, including the islands of Calleja, function as hedonic hotspots that amplify pleasure responses, potentially via opioid signaling to heighten the affective impact of rewarding scents.^[29]

Behavioral Roles

The olfactory tubercle (OT) plays a key role in modulating locomotion and exploratory behaviors, particularly in response to novel olfactory stimuli. Bilateral lesions of the OT, often combined with nucleus accumbens damage using 6-hydroxydopamine, significantly reduce locomotor activity in non-deprived rats, impairing novelty-induced exploration by disrupting dopaminergic signaling in ventral striatal circuits. Activation of D1 dopamine receptor-expressing neurons in the medial OT promotes forward locomotion and approach behaviors toward reward-associated odors, facilitating odor-guided navigation in enriched environments.^[31]^[32] In social and reproductive contexts, the OT integrates pheromonal odors with social cues to influence mate preference and copulatory efficiency. Lesions of the OT in male rats markedly decrease mounting and intromission behaviors, reducing overall sexual performance by impairing the processing of olfactory incentives essential for reproductive motivation. In females, silencing medial OT neurons via designer receptors abolishes innate preferences for male pheromones, such as urinary odors, highlighting the structure's role in odor-driven social attraction.^[33]^[34] The OT mediates odor-guided feeding and ingestive behaviors by evaluating food-related odors in relation to internal states. According to a proposed model, during satiety, activation of D2 receptor-expressing neurons in the anteromedial OT and D1 neurons in the lateral OT may suppress intake by diminishing the motivational value of appetitive odors, preventing overeating. Conversely, in states of hunger, enhanced OT activity promotes consumption, as D1 neurons in the anteromedial OT amplify hedonic responses to food cues. A 2020 review emphasizes these subtype-specific roles, with D2 neurons particularly inhibiting excessive feeding through aversion-like mechanisms.^[16] A 2025 study suggests that feeding-induced suppression in the olfactory cortex, potentially involving the OT, reduces satiation signals, prolonging intake during binge episodes.^[35] The OT contributes to arousal modulation, enhancing attention and vigilance during olfactory exploration. Hyperactivity in OT circuits, driven by dopaminergic inputs, correlates with increased exploratory drive in response to salient odors, supporting adaptive vigilance without pathological over-arousal.^[16] Recent evidence indicates the OT regulates motivated olfactory search, accelerating odor-tracking in reward contexts through orexin-1 receptor signaling in the anteromedial domain. In mice associating neutral odors with food rewards, OT activation facilitates faster approach and discrimination, converting neutral cues to attractive ones for efficient foraging.^[36]

Clinical Significance

Associated Neurological and Psychiatric Disorders

The olfactory tubercle (OT) plays a significant role in addiction and substance use disorders, particularly through its involvement in dopamine-mediated reward processing. Rodent studies demonstrate that the OT exhibits hypersensitivity to psychostimulants such as cocaine and amphetamines, supporting intracranial self-administration of these drugs directly into the OT, which underscores its contribution to drug-seeking behavior.^[37] In cocaine users, positron emission tomography (PET) imaging reveals altered dopamine signaling in the ventral striatum, with reduced D2 receptor binding that correlates with prolonged use and craving intensity.^[38] In schizophrenia, postmortem examinations indicate structural abnormalities in the OT, including reduced dendritic arborization in spiny neurons and increased glial fibrillary acidic protein (GFAP) expression suggestive of inflammation, along with volume reductions in affected regions compared to controls.^[39]^[40] These changes are associated with olfactory dysfunction, reflecting disrupted sensory and reward integration.^[41] Depression and mood disorders involve diminished OT function, particularly in patients exhibiting anhedonia, where functional magnetic resonance imaging (fMRI) shows decreased activation in the OT during reward anticipation tasks.^[42] A 2023 review highlights the OT's role in deficits of emotion-reward integration, linking its dopaminergic dysregulation to core symptoms of major depressive disorder. In Parkinson's disease, progressive dopaminergic denervation in the OT contributes to both olfactory loss and motivational apathy, exacerbating non-motor symptoms; a 2025 meta-analysis of studies, including those from 2022-2024, reports that subthalamic nucleus deep brain stimulation (DBS) can improve odor discrimination but not thresholds in responsive patients.^[43]^[44] Recent research from 2020-2025 positions the OT as a promising therapeutic target for neuropsychiatric disorders, with optogenetic studies in rodent models of depression demonstrating that selective modulation of OT D3 dopamine receptor-expressing neurons bidirectionally regulates depression-like behaviors and restores reward sensitivity.^[42]^[45] As of 2025, emerging studies also explore OT involvement in autism spectrum disorders, particularly in social chemosignaling deficits.^[46]

Olfactory and Sensory Dysfunctions

The olfactory tubercle (OT) plays a critical role in olfactory processing, and its dysfunction contributes to anosmia (complete loss of smell) and hyposmia (reduced smell sensitivity), particularly in neurodegenerative conditions like Parkinson's disease (PD) and Alzheimer's disease (AD). In PD, olfactory impairment affects 50-90% of patients, often preceding motor symptoms, with alpha-synuclein pathology observed in the OT alongside other olfactory regions, leading to deficits in odor identification and discrimination.^[47] Postmortem analyses reveal reduced granule cell density in the OT's islands of Calleja, clusters of densely packed neurons essential for olfactory signal modulation, with age-related declines exacerbated in PD and AD, correlating with hyposmia severity. In AD, similar OT involvement manifests as early hyposmia in up to 90% of cases, linked to tau propagation through olfactory circuits including the OT, impairing odor memory and detection.^[48] Multisensory integration disorders further highlight OT dysfunction, where impaired processing leads to cross-modal deficits in combining olfactory cues with visual or auditory inputs. In schizophrenia, patients exhibit olfactory identification deficits, with OT's role in multisensory convergence disrupted, resulting in heightened errors in odor-guided tasks such as associating smells with visual stimuli; functional imaging shows altered OT activation during these integrations.^[49]^[1] This contributes to broader sensory processing anomalies, where OT-mediated valence encoding of odors fails to align with other modalities, amplifying perceptual mismatches observed in up to 40-60% of affected individuals.^[1] Post-traumatic and neurodegenerative insults often disrupt OT inputs, causing persistent olfactory loss. Following head injury or traumatic brain injury (TBI), shearing forces damage olfactory pathways, including projections to the OT, leading to anosmia in 10-30% of severe cases; structural MRI reveals reduced connectivity in OT-linked networks, with incomplete recovery in chronic phases.^[50] In COVID-19-related anosmia, prevalent from 2020-2025, studies detect SARS-CoV-2 RNA in the OT and associated inflammation, causing temporary hyposmia in over 50% of infected individuals, though most recover within months due to resolved neuroinflammation.^[51] Diagnostic approaches leverage OT imaging for prognostic insights. MRI-based volumetry of the OT and connected olfactory structures predicts olfactory recovery post-injury or infection, with volumes below normative ranges indicating poorer outcomes; functional MRI further assesses OT activation during odor tasks to gauge integration deficits.^[52] Therapeutic interventions target OT plasticity to mitigate these dysfunctions. Olfactory training, involving repeated exposure to diverse odors, promotes structural and functional recovery by enhancing synaptic plasticity in the OT and broader olfactory cortex, improving hyposmia scores by 20-50% in post-viral and post-traumatic cases over 3-6 months.^[53] This approach leverages the OT's capacity for experience-dependent reorganization, as evidenced by increased regional connectivity in training responders.^[54]

History

Early Anatomical Descriptions

The olfactory tubercle was first described in 1896 by Swiss anatomist Rudolf Albert von Kölliker during dissections of mammalian brains, particularly in rats, where he identified it as a tubular prominence adjacent to the olfactory bulb and tract, emphasizing its role in the ventral basal forebrain.^[1] This initial characterization highlighted its distinct morphology and position within the olfactory pathway, distinguishing it from surrounding structures like the anterior perforated substance.^[55] In the early 1900s, Spanish neuroscientist Santiago Ramón y Cajal advanced the understanding of the olfactory cortex's microstructure using Golgi staining methods, revealing a layered organization with pyramidal cells predominant in layer II and semilunar cells contributing to dendritic arborizations.^[56] These observations established the cortical-like histology of olfactory processing networks, including regions adjacent to the tubercle. Concurrently, American comparative anatomist C. Judson Herrick, in studies from the 1910s, compared mammalian olfactory regions to homologous areas in amphibian forebrains, such as those in Necturus, noting conserved features in olfactory tract terminations and evolutionary continuity across vertebrates. Lesion and ablation experiments in rats during the 1940s and 1950s targeting ventral forebrain areas demonstrated deficits in olfactory discrimination and motivated behaviors following damage to the tubercle, supporting its classification as a component of the ventral striatum. Nomenclature refined in the 1960s through fiber tract degeneration analysis by Lennart Heimer clarified the tubercle's boundaries, distinguishing it from the broader "anterior perforated substance" in primates and rodents by tracing olfactory and striatal projections.^[57] Early characterizations were constrained by light microscopy and Nissl or Golgi stains, which delineated gross layers and cell types but overlooked neurochemical diversity, such as dopaminergic innervation, limiting insights into its integrative functions until later techniques emerged.^[1]

Modern Experimental and Neuroimaging Studies

During the electrophysiology era from the 1970s to the 2010s, extracellular recordings in animal models revealed that olfactory tubercle (OT) neurons respond robustly to olfactory stimuli and integrate multisensory information, including auditory cues. For instance, studies by Wilson and colleagues demonstrated that single units in the rat OT exhibit odor-evoked activity, with a significant proportion showing cross-modal responses to tones, highlighting the region's role in sensory convergence.^[22] These findings established the OT as a site for early processing of odor-reward associations through phasic firing patterns observed during reward-related behaviors. Advancements in neuroimaging from the 2000s onward utilized fMRI and PET to demonstrate OT activation in humans during reward tasks involving pleasant odors. Early fMRI studies showed that the OT reflects odor attractiveness, with increased activity correlating to hedonic processing in the ventral striatum. In the 2010s, diffusion tensor imaging (DTI) further mapped OT connectivity, revealing structural links to reward circuits like the nucleus accumbens and prefrontal areas, which underpin motivational responses to olfactory cues. Molecular and genetic tools in the 2010s to 2025 enabled precise manipulation of OT circuits. Optogenetic studies, such as those silencing D1 dopamine receptor-expressing neurons in the OT, demonstrated that these cells are critical for odor-reward associations, with inhibition disrupting seeking behaviors in cocaine-conditioned paradigms.^[58] Recent breakthroughs from 2020 to 2025 have refined OT connectivity and function. Tract-tracing techniques in 2024 confirmed OT integration with prefrontal processes through connections to reward and executive networks.^[59] A 2025 review highlighted the OT as a central hub in emotion-reward networks, linking dysfunction to neuropsychiatric conditions like addiction and depression.^[46] These methodological shifts, from invasive lesion studies to non-invasive imaging and genetic tools, have facilitated safer, higher-resolution investigations, paving the way for clinical applications in olfactory-reward disorders.

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