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Claustrum

The claustrum is a thin, irregular sheet of gray matter neurons located in the telencephalon of mammalian brains, positioned between the insular cortex and the putamen, with extensive reciprocal connections to nearly all regions of the neocortex as well as subcortical structures such as the thalamus, striatum, and limbic system.^[1] This highly conserved structure, present in most mammals including humans and rodents, spans the rostral half of the telencephalon and exhibits a complex three-dimensional morphology divided into anterior-posterior and dorso-ventral subdivisions based on gene expression patterns and connectivity profiles.^[1] Its enigmatic nature stems from its small size and elusive boundaries, which have historically complicated precise delineation in neuroimaging and histological studies.^[2] Anatomically, the claustrum forms a paired, bilateral band-like structure hidden beneath the white matter of the neocortex, bordered by the external capsule laterally and the extreme capsule medially, and it integrates sensory, motor, and cognitive information through its vast, topographically organized projections.^[1] In rodents, for instance, it extends from the forceps minor anteriorly to the level of the anterior commissure posteriorly, with subdivisions like the dorsal claustrum linking to sensory cortices and the ventral portion connecting to limbic areas.^[1] Human claustrum, visible on high-resolution MRI, mirrors this organization but is more elongated and integrated with the basal ganglia, influencing its role in interhemispheric communication via bilateral projections.^[2] Neurotransmitter systems, including glutamate for excitatory signaling and GABA for inhibition, facilitate its interactions, with gap junctions potentially enabling rapid synchronization across distant brain regions.^[1] Functionally, the claustrum is implicated in coordinating cortical activity, particularly in processes related to consciousness, attention, and sensory integration, though its exact mechanisms remain under investigation.^[2] Pioneering hypotheses by Crick and Koch (2005) posited it as a central hub for binding disparate sensory inputs into unified percepts, supported by evidence of its role in regulating slow-wave oscillations during wakefulness and sleep.^[2] Recent studies show that optogenetic activation of claustral neurons can suppress neocortical activity, mimicking anesthesia-induced loss of consciousness, while electrical stimulation in humans disrupts awareness without affecting motor functions.^[2] It also contributes to salience detection and decision-making, with place and object cells identified in rodent claustrum akin to those in the hippocampus, suggesting involvement in spatial navigation and memory retrieval.^[1] Disruptions, as seen in conditions like epilepsy or psychedelic-induced states, further highlight its modulation of cortical excitability and global brain synchronization.^[2]

Anatomy

Location and gross structure

The claustrum is a thin, sheet-like nucleus of gray matter positioned between the insular cortex laterally and the putamen medially.^[3] It lies deep within the white matter, embedded between the extreme capsule (separating it from the insula) and the external capsule (separating it from the putamen).^[4] In humans, the claustrum forms an elongated, irregular sheet that extends rostrocaudally for approximately 30–40 mm, from the level of the temporal lobe to the frontal lobe, with a thickness varying from about 1 to 3 mm.^[5] This structure occupies roughly 0.25% of the cerebral cortex volume and exhibits slight asymmetry, with the right claustrum typically larger (around 829 mm³) than the left (around 706 mm³).^[3] The claustrum was first described in 1809 by the German anatomist Johann Christian Reil, who highlighted its enclosed, "cloistered" position amid surrounding white matter tracts—deriving its name from the Latin claustrum, meaning a barrier or enclosure.^[6] Earlier observations existed, but Reil's account emphasized its distinct anatomical isolation.^[7] On magnetic resonance imaging (MRI), the claustrum appears as a subtle hyperintense band on T2-weighted sequences due to its gray matter composition, but its visualization remains challenging because of its thin profile and proximity to adjacent white matter structures, often requiring high-resolution scans for accurate delineation.^[8]

Connections

The claustrum exhibits extensive bidirectional connections with nearly all cortical regions, establishing it as a key integrative structure in the brain. These reciprocal links include sensory areas such as visual and auditory cortices, motor cortices, prefrontal regions involved in executive function, and limbic structures like the cingulate gyrus.^[9]^[1] Projections from the claustrum to the cortex are predominantly ipsilateral, though weaker contralateral connections exist via the corpus callosum.^[9] Subcortically, the claustrum receives strong inputs from and sends outputs to the thalamus, facilitating relay of sensory and associative information.^[1] It also maintains robust connections with the basal ganglia, particularly the striatum, and brainstem nuclei including the superior colliculus, supporting multimodal integration.^[9]^[1] Claustral projections to the cortex demonstrate layer-specific targeting, with dense innervation primarily in layer 4, where synapses form on spines of pyramidal neurons, consistent with roles in relaying sensory inputs.^[10] Terminals are also prominent in deeper layers 5 and 6, particularly in associative cortical areas, targeting pyramidal cells and interneurons.^[11]^[12] In humans, claustral connectivity shows regional asymmetry, with denser projections to frontal and temporal lobes compared to the occipital lobe.^[13] Diffusion tensor imaging (DTI) studies reveal the strongest fiber tract densities to frontal cortices, followed by temporal and parietal regions, with sparser links to occipital areas.^[13] Recent diffusion-weighted imaging (DWI) and tractography analyses from 2023 to 2025 have further mapped these pathways in large human cohorts, confirming the claustrum's status as a "connector hub" with reciprocal links to over 100 cortical subregions based on high-resolution tractography data.^[14]^[5] These studies highlight its highest connectivity density per unit volume among subcortical structures, underscoring its role in linking diverse brain networks.^[14]^[15]

Microanatomy and cell types

The claustrum exhibits a distinctive histological organization characterized by densely packed neurons with sparse myelinated fibers internally, forming a thin sheet of gray matter between the insular cortex and striatum.^[16] This dense neuronal packing is evident in Nissl stains, where the claustrum appears as a compact cluster of somata lacking the laminar architecture typical of neocortex.^[7] Myelinated fibers are minimal within the core but more abundant in the dorsal portion, creating a ventral region with virtually no myelination, as revealed by myelin basic protein staining in rodents.^[16] The structure is subdivided into anterior-dorsal, posterior-dorsal, and ventral regions, with the anterior-dorsal linking to somatosensory and motor cortices, posterior-dorsal to visual areas, and ventral to auditory, olfactory, and limbic structures based on differential connectivity patterns.^[17] The primary cellular constituents of the claustrum are excitatory pyramidal neurons, which constitute approximately 80-90% of the neuronal population and are glutamatergic in nature.^[18] These spiny projection neurons, often termed type I cells, feature dendrites covered in spines and project bidirectionally to cortical targets.^[7] A smaller fraction, about 10-15%, comprises GABAergic interneurons that provide local inhibition and lack spines on their dendrites, classified as type II cells.^[19] This excitatory-inhibitory ratio mirrors that of neocortical layers, supporting the claustrum's role in integrating cortical signals.^[20] Neurochemically, the claustrum displays elevated levels of calcium-binding proteins, including parvalbumin and calbindin, which are expressed in both projection neurons and interneurons to regulate intracellular calcium dynamics.^[21] Parvalbumin immunoreactivity forms a dense plexus throughout the neuropil, particularly in the dorsal claustrum, while calbindin staining reveals a central gap in some species, highlighting regional heterogeneity.^[16] Neuropeptides such as somatostatin are also prominent, often colocalizing with calbindin in inhibitory neurons, and contribute to modulating local circuitry.^[22] Cholinergic inputs, though present, are sparse and primarily arrive from basal forebrain nuclei.^[7] At the ultrastructural level, claustral synapses exhibit a balanced mix of excitatory and inhibitory connections, with most being asymmetric (excitatory) and targeting dendritic spines of pyramidal neurons.^[23] Electron microscopy studies indicate a high synaptic density within the claustrum, comparable to that of cortical layers, underscoring its capacity for dense information processing.^[10] Principal neurons rarely synapse directly with one another, favoring instead connections onto interneurons and extrinsic afferents.^[23] Developmentally, the claustrum originates from progenitors in the ganglionic eminence of the ventral telencephalon for inhibitory neurons and from pallial progenitors for excitatory neurons, with neurons migrating tangentially and radially during early gestation to their final positions. In humans and other mammals, this process culminates in the formation of the sheet-like structure by mid-gestation, establishing the claustrum's mature cytoarchitecture.^[24]^[25]^[20]

Physiology and Function

Role in attention and alertness

The claustrum plays a key role in regulating alertness by modulating transitions between sleep and wakefulness through its projections to the anterior cingulate cortex. In mice, optogenetic and chemogenetic manipulation of claustral neurons projecting to the anterior cingulate reveals that elevated claustral activity promotes unresponsiveness during non-rapid eye movement sleep, reducing awakenings from sensory stimuli such as sounds, while lower activity heightens sensory engagement and cortical arousal during wakefulness.^[26] This bidirectional control acts as a gatekeeper, adjusting the brain's responsiveness to external inputs across arousal states to optimize behavioral adaptation.^[27] In attentional switching, the claustrum functions as a coordinator, synchronizing cortical networks to facilitate focused, task-oriented behavior. Optogenetic stimulation of claustral projections to the frontal cortex in mice enhances impulse control during reward-seeking tasks, increasing the probability of terminating consumption after initial engagement and thereby promoting more deliberate, goal-directed actions over impulsive ones.^[28] This mechanism underscores the claustrum's capacity to restrict maladaptive persistence, aligning cortical activity for efficient attention shifts in dynamic environments. The claustrum contributes to sensory gating by inhibiting irrelevant stimuli, thereby enabling selective attention. Selective inhibition of claustral projection neurons in mice impairs resilience to auditory distractors in visual tasks, leading to performance drops of up to 40% and increased response latencies, as these neurons suppress distractor representations in the auditory cortex by 25-30%.^[29] Although primarily excitatory, a subset of these projections includes GABAergic elements that may facilitate this feedforward inhibition, filtering noise to prioritize salient sensory information. Claustral activity generates neural oscillations, including gamma-band frequencies (30-80 Hz), which support attentional binding across sensory modalities. Hypotheses based on claustral connectivity suggest it detects and integrates synchronous axonal inputs to produce these oscillations, aiding in the temporal coordination essential for binding features into coherent percepts during attention.^[30] In humans, EEG studies of claustral lesions, often associated with status epilepticus, show correlated abnormalities such as disrupted oscillatory patterns alongside cognitive impairments, including attention deficits, linking claustral integrity to sustained attentional processing.^[31] Recent fMRI findings from 2025 indicate that the human claustrum initiates cognitive control networks tailored to externally driven tasks, dynamically engaging frontoparietal regions to meet environmental demands.^[32] This initiation role highlights the claustrum's position at the nexus of sensory input and executive function, ensuring rapid network reconfiguration for alert, attentive states.

Involvement in consciousness

The Crick and Koch hypothesis posits that the claustrum serves as a neural synchronizer, binding disparate sensory inputs across cortical regions into a unified conscious percept through coordinated rhythmic activity and potential gap junction-mediated communication.^[33] This model emphasizes the claustrum's extensive bidirectional connections with sensory, motor, and association cortices, enabling it to orchestrate the temporal alignment of neural oscillations essential for conscious integration.^[2] Subsequent updates to the hypothesis, incorporating two decades of anatomical and physiological data, refine this view by highlighting the claustrum's role in facilitating rather than solely generating consciousness, with evidence from optogenetic and lesion studies supporting its involvement in modulating cortical synchrony during wakeful states.^[2] Building on this, the interface model frames the claustrum as a critical bridge between subcortical arousal systems in the brainstem and higher-order cortical processes underlying awareness, potentially linking awakening to perceptual integration.^[2] A 2023 review synthesizes evidence that the claustrum coordinates these elements, acting as a hub for salience detection and attentional gating that contributes to the emergence of conscious content without being indispensable for basic wakefulness.^[2] This perspective aligns with global workspace theory, wherein the claustrum may amplify and broadcast relevant neural signals across distributed cortical networks, enhancing the accessibility of information to conscious processing during tasks requiring perceptual binding.^[2] Lesion and stimulation studies provide clinical support for these roles. In a seminal 2014 case, electrical stimulation of the claustrum in an epileptic patient induced reversible loss of awareness, characterized by behavioral arrest and unresponsiveness, suggesting its direct involvement in maintaining conscious state. More recent 2024 case reports document transient claustral hypoactivity, often linked to epileptic or inflammatory events, correlating with episodes of altered consciousness such as confusion and disorientation, further implicating the structure in state transitions.^[34] Functional imaging corroborates these findings; positron emission tomography (PET) scans reveal heightened claustral metabolic activity during conscious perception tasks, such as binocular rivalry, where it correlates with the resolution of ambiguous stimuli into reportable awareness. Recent syntheses from 2023 to 2025 emphasize the claustrum's facilitative, non-essential function in consciousness, countering earlier notions of it as the singular "seat" of awareness by demonstrating that while disruptions impair integration, consciousness persists in its absence through compensatory cortical mechanisms.^[2] This updated understanding positions the claustrum as a modulator of conscious binding, particularly in contexts demanding rapid sensory synthesis, rather than a prerequisite for all conscious experience.^[35]

Potential roles in memory and cognitive control

Emerging research has implicated the claustrum in memory acquisition and consolidation, particularly through its interactions with hippocampal circuits. In a 2024 study using mice, pharmacological inhibition of the claustrum immediately after training in an inhibitory avoidance task—a form of fear conditioning—significantly impaired long-term memory formation, as evidenced by reduced performance in retention tests (p < .0001), while inhibition 3 hours post-training had no effect. This time-sensitive role suggests the claustrum contributes to the consolidation of fear-based memories into stable long-term representations. Concurrently, increased c-Fos expression in the claustrum and the CA1 region of the dorsal hippocampus following training (p < .01 and p < .001, respectively) highlights the involvement of claustral-hippocampal projections in this process.^[35] The claustrum also appears essential for memory reconsolidation, the process by which reactivated memories are updated and restabilized. In the same 2024 mouse study, claustral inhibition via lidocaine infusion shortly after memory reactivation in the inhibitory avoidance task disrupted reconsolidation, leading to persistent memory deficits observable up to 14 days later (p < .05), whereas inhibition 48 hours post-reactivation showed no impairment. These findings indicate that claustral activity is required for the plasticity underlying memory updating after retrieval, potentially via its dense projections to memory-related structures like the hippocampus.^[35] In the domain of cognitive control, recent investigations point to the claustrum as a key initiator of large-scale brain network states that support executive functions. A 2025 human fMRI study demonstrated that claustral activity precedes and orchestrates network reconfiguration for both externally driven sensory tasks and internally driven processes, such as decision-making and error monitoring, establishing its role in flexible cognitive control across task demands. This aligns with the claustrum's position within thalamo-cortical loops that facilitate network switching.^[32] Furthermore, the claustrum modulates prefrontal cortex dynamics to enable adaptive cognition, including working memory maintenance. Resting-state fMRI analysis from 2024 revealed strong functional connectivity between the claustrum and the frontoparietal network (FPN), including prefrontal regions, which is associated with working memory and task switching. Complementing this, a 2024 study in rodents showed that disrupting claustral activity impairs the persistent neural representations necessary for holding information in working memory, underscoring its contribution to prefrontal-dependent flexible behavior.^[5]^[36]

Empirical evidence from animal and human studies

In animal models, optogenetic silencing of claustrum neurons projecting to the anterior cingulate cortex in mice has been shown to reduce sensory responsiveness and alertness during goal-directed behaviors, with heightened claustral activity corresponding to restricted engagement in sleep states.^[26] Similarly, optogenetic modulation of claustral axons in the prefrontal cortex normalizes neuronal responsiveness and enhances neural variability, thereby influencing wakefulness and behavioral engagement levels.^[37] Calcium imaging techniques, including fiber photometry and two-photon microscopy, have revealed claustral bursts synchronized with attention tasks, such as cross-modal sensory selection, where neuronal activity in the anterior claustrum reflects movement planning rather than direct sensory input.^[26]^[38] Lesion experiments in rodents, including pre-2023 knife-cut disruptions of claustral pathways, demonstrate impairments in sensory-motor integration, with affected animals exhibiting deficits in coordinating visual and auditory cues for navigation and response.^[39] More recent viral tracing studies in mice (2025) using retrograde and anterograde approaches confirm these network effects, showing that claustral projections to the anterior cingulate cortex bidirectionally regulate cognitive processing, with disruptions leading to altered drug-seeking behaviors and motivational responses.^[15] These tracing methods highlight the claustrum's role in integrating inputs from widespread cortical sources, supporting its function in multisensory convergence.^[40] In human studies, intracranial recordings from epilepsy patients have captured claustral activity linked to consciousness, with spikes in the structure preceding alterations in awareness during seizures, as observed in electrocorticography data from 2023 cases.^[2] Functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) analyses from 2024-2025 cohorts quantify claustral-cortical synchrony during cognitive tasks, revealing strong bidirectional connectivity with prefrontal and parietal regions that initiates network states for attention and task-switching. For instance, resting-state fMRI shows the claustrum's effective connectivity in modulating triple networks (salience, executive, default mode), with peak synchrony during externally driven cognitive demands.^[41] Post-2023 evidence further supports the claustrum's initiation of these networks, as DTI tractography in large cohorts (n>100) demonstrates robust in vivo detection of its projections.^[42]^[14]

Clinical and Pathological Significance

Neurological disorders

The claustrum has been implicated in epilepsy as a potential hub for seizure propagation, particularly in focal epilepsies originating from limbic structures. The ventral claustrum, with its dense connections to the hippocampus, amygdala, and motor cortex, may facilitate the generalization of seizures from temporal lobe origins to broader cortical networks, acting as a critical node in epileptic circuits.^[43] Recent neuroimaging evidence, including EEG-fMRI studies, demonstrates claustral activation during interictal epileptiform discharges in patients with temporal lobe epilepsy, suggesting underlying hyperexcitability in this region.^[43] The "claustrum sign"—a hyperintense signal on T2/FLAIR MRI—frequently appears in status epilepticus cases, indicating inflammatory changes and network dysfunction that promote seizure spread, particularly in refractory forms such as FIRES and NORSE. In parkinsonism, dopaminergic denervation significantly impacts the claustrum, leading to reduced dopamine and noradrenaline levels that disrupt its role in multisensory integration and network coordination.^[44] Postmortem examinations reveal alpha-synuclein inclusions, including Lewy bodies and neurites, in the claustrum of Parkinson's disease (PD) patients, present in approximately 75% of non-demented cases and nearly all with dementia, often alongside astrocytic pathology unique to PD.^[45] These inclusions correlate with motor symptom severity through broader network disruptions, as functional connectivity studies show altered claustral interactions with basal ganglia circuits in PD, exacerbating bradykinesia and rigidity.^[46] Stroke-induced lesions to the claustrum can disrupt its relay functions between sensory and motor areas, resulting in motor and language deficits. Isolated claustral infarctions, though rare, have been associated with contralateral hemiparesis, ataxia, and mild dysarthria or aphasia due to impaired thalamocortical projections. For instance, a documented case of left claustral ischemia presented with ataxic gait, dizziness, paresthesia in the left half of the face, tongue, and head, and decreased hearing in the left ear, highlighting the structure's contribution to sensorimotor integration.^[47] Emerging 2025 evidence points to claustral modulation as a therapeutic avenue in PD via deep brain stimulation (DBS), particularly targeting subthalamic nucleus pathways that influence claustral axons. Stimulation of these projections normalizes neuronal responsiveness in PD models, enhancing cognitive-motor integration by restoring variability in sensorimotor networks and improving dual-task performance.^[45] This approach, informed by claustrum's connections to basal ganglia, shows promise in alleviating both motor and executive function deficits beyond traditional DBS targets.^[48]

Psychiatric conditions

The claustrum has been implicated in several psychiatric conditions, particularly those involving disruptions in cognition, emotion regulation, and salience attribution, such as schizophrenia and anxiety disorders. In schizophrenia, structural imaging studies consistently reveal reduced claustral volume, which correlates with core symptomatic features. A 2024 investigation using high-resolution MRI demonstrated that individuals with schizophrenia exhibit significantly lower claustrum volumes compared to healthy controls, with these reductions mediating impairments in attentional performance. ^[49] This finding aligns with a 2025 morphometric analysis indicating that claustral volumes in schizophrenia patients are more than 10% smaller than in controls, specifically linking smaller claustrum size to the severity of negative symptoms like social withdrawal and blunted affect. The claustrum's role in schizophrenia extends to functional connectivity deficits, particularly in salience processing, where it integrates sensory and cognitive signals to prioritize relevant stimuli. Disruptions in claustral-prefrontal cortex connections contribute to aberrant salience attribution, a hallmark of psychotic symptoms. In anxiety and stress-related disorders, the claustrum exhibits hyperactivation during acute stress responses, amplifying emotional reactivity. Rodent models from 2024 research show claustral hyperactivity in states mimicking post-traumatic stress disorder (PTSD), such as after exposure to fear-conditioning paradigms, where anterior claustral lesions mitigate exaggerated fear responses and anxiety-like behaviors. ^[50] Fine-regional mapping in these models further reveals that specific claustral subregions regulate anxiety susceptibility, with optogenetic activation enhancing avoidance behaviors. ^[51] Neurochemical imbalances, including dysregulated GABAergic signaling within the claustrum, underlie attentional lapses observed in anxiety, as reduced inhibitory tone disrupts the claustrum's gating of prefrontal inputs during heightened arousal. ^[52] These findings highlight the claustrum's potential as a therapeutic target for modulating stress-induced psychiatric symptoms.

Consciousness disorders and lesions

Lesions to the claustrum have been implicated in various syndromes involving disruptions to conscious states, including prolonged loss of consciousness and reversible unawareness. In a study of 171 patients with penetrating traumatic brain injuries, claustrum damage was significantly associated with the duration, but not the frequency, of loss of consciousness, suggesting a role in sustaining rather than initiating awareness.^[53] A systematic review of 38 human case studies further documented loss of consciousness in 8 instances following claustral lesions, often alongside seizures or altered mental states resembling locked-in-like unresponsiveness.^[31] Bilateral claustrum lesions, as reported in a 1996 case of severe transitory encephalopathy in a pediatric patient, led to recurrent seizures, psychotic symptoms, and temporary sensory losses, with full reversal upon lesion resolution after five weeks.^[54] In patients with disorders of consciousness, claustrum dysfunction correlates with impaired awareness levels. A 2023 resting-state fMRI study of 104 patients in vegetative (n=80) or minimally conscious states (n=24) revealed reduced functional connectivity involving the claustrum and anterior insula compared to healthy controls, with lower connectivity in vegetative states versus minimally conscious states, indicating a potential modulatory influence on recovery of awareness.^[55] Although direct atrophy was not quantified, these network alterations highlight the claustrum's integration in cortical circuits essential for conscious processing. Surgical interventions near the claustrum, such as insular resections for epilepsy, carry risks of transient consciousness lapses due to the structure's anatomical proximity and connectivity. Intraoperative electrical stimulation adjacent to the claustrum has induced reversible disruptions in awareness, mimicking lesion effects and underscoring surgical hazards in this region.^[56] Unilateral claustrum resection in low-grade glioma cases has shown high rates of functional recovery, with minimal persistent deficits in consciousness. Recovery from claustral lesions often involves neuroplasticity through compensatory connections in remaining neural networks. In cases of unilateral damage, the contralateral claustrum and associated pathways facilitate partial restoration of conscious functions, as evidenced by neurobehavioral improvements uncorrelated with lesion size alone.^[31] A 2025 retrospective cohort study of 20 pediatric patients with claustrum sign—hyperintense MRI lesions indicating potential neuroinflammation—found impaired consciousness in 70%, yet resolution in 89% of follow-up scans (median 53 days post-onset), with better outcomes in non-FIRES etiologies, supporting a modulatory rather than causal role in awareness disorders.^[57] Recent lesion studies from 2023 to 2025 emphasize this non-causal modulation, aligning with theories of the claustrum's integrative function in consciousness.^[2]

Effects of psychedelics and pharmacological modulation

Psilocybin, a classic serotonergic psychedelic, induces significant alterations in claustral activity, as evidenced by functional magnetic resonance imaging (fMRI) studies from 2020. These investigations reveal acute desynchronization within the claustrum, characterized by decreased amplitude of low-frequency fluctuations (ALFF) and reduced blood-oxygen-level-dependent (BOLD) signal variance in both left (p=0.013 for ALFF, p=0.011 for variance) and right (p=0.023 for ALFF, p=0.021 for variance) claustrum regions following administration of 10 mg/70 kg psilocybin.^[58] Such desynchronization disrupts the claustrum's functional connectivity with key networks, including reduced links to the default mode network (DMN) and auditory network, while increasing connectivity with the fronto-parietal task control (FPTC) network on the right side and decreasing it on the left.^[58] This pattern of claustral desynchronization correlates with subjective psychedelic experiences, particularly ego dissolution, where participants report a loss of self-boundaries and heightened perceptual fluidity, mirroring broader reductions in DMN integrity observed across psychedelic states.^[59] Serotonergic modulation plays a pivotal role in these effects, with the claustrum exhibiting high expression of 5-HT2A and 5-HT2C receptors on glutamatergic neurons, as confirmed by quantitative PCR (qPCR) in rat models (N=16).^[60] Activation of these receptors, particularly through psychedelics like 2,5-dimethoxy-4-iodoamphetamine (DOI), enhances excitatory postsynaptic currents (EPSCs) in claustrum neurons projecting to the anterior cingulate cortex (ACC), thereby mediating hallucinogenic experiences such as altered perception and sensory integration.^[60] While 5-HT2A agonism is traditionally linked to hallucinogenesis, recent evidence highlights 5-HT2C receptors in reversing inhibitory synaptic effects in the claustrum-ACC circuit, suggesting a nuanced serotonergic mechanism underlying the structure's role in psychedelic-induced behavioral responses.^[60] These receptor-mediated changes contribute to the claustrum's central involvement in the perceptual distortions characteristic of hallucinogenic states.^[61] The therapeutic potential of pharmacological modulation targeting the claustrum has emerged in recent trials, particularly with low-dose psilocybin. A 2025 preclinical study demonstrated that psychedelics like DOI reverse the polarity of long-term synaptic plasticity in cortical-projecting claustrum neurons, promoting net long-term potentiation (LTP) and enhancing claustrocortical circuit efficacy, which aligns with neuroplasticity mechanisms for treating depression.^[62] In human contexts, low-dose psilocybin (10 mg/70 kg) increases claustral connectivity with task-positive networks like the FPTC, potentially countering DMN hyperconnectivity associated with depressive rumination, as supported by ongoing clinical evaluations of psilocybin for treatment-resistant depression.^[58] A 2025 bioRxiv preprint further indicates that psilocybin modulates effective connectivity between the claustrum and triple networks (DMN, FPTC, salience), with changes correlating to subjective psychedelic effects and suggesting implications for sustained antidepressant outcomes through rewired claustral circuits.^[63] Beyond serotonergics, other agents like ketamine exert dissociative effects potentially involving claustral NMDA receptor blockade. As a non-competitive NMDA antagonist, ketamine induces rapid antidepressant actions and dissociation at subanesthetic doses, with emerging 2025 research linking such effects to broader neuroplasticity in circuits akin to those modulated by psychedelics in the claustrum, though direct claustral NMDA-specific impacts remain under investigation.^[62] This modulation may contribute to ketamine's disruption of sensory integration, paralleling claustral roles in attention and perception. Recent 2024-2025 research has expanded understanding of these interactions, including psilocybin's suppression of claustral excitability via 5-HT1B receptors, which depresses signaling in claustrum neurons and further disrupts functional connectivity with cortical networks.^[64] These findings highlight the claustrum as a key hub for psychedelic mechanisms, with implications for novel pharmacological interventions in perceptual and cognitive disorders.^[65]

Role in pain processing

Recent research has revealed the claustrum's activation in response to acute painful stimuli and pain-predictive cues. Functional magnetic resonance imaging (fMRI) studies in healthy human subjects demonstrate increased blood-oxygen-level-dependent (BOLD) signals in the claustrum at the onset of noxious heat stimulation, with significant left claustrum activation peaking approximately 32.5 seconds after stimulus onset (p-FDR = 0.023) and right claustrum activity greater during pain compared to warm non-painful conditions (p-FDR = 0.004). Additionally, the left claustrum responds to auditory cues predicting impending pain (p-FDR = 0.030), highlighting its role in anticipatory pain processing. In rodent models, claustrum neurons projecting to the anterior cingulate cortex (ACC) show enhanced activity, marked by increased c-Fos expression, specifically following mechanical noxious stimuli but not innocuous touch, with a bimodal response pattern including an initial peak and a later rebound 10-20 minutes post-stimulation.^[66] The claustrum contributes to network modulation in chronic pain by driving aberrant cognitive processing, with fMRI evidence indicating involvement in loops that amplify pain salience. In patients with chronic migraine, pathological right claustrum hyperactivity during acute pain and cognitive conflict tasks exceeds that in controls (p = 0.008), correlating with disrupted functional connectivity to the posterior inferolateral dorsolateral prefrontal cortex (piDLPFC). This hyperactivity is supported by structural connections via the superior thalamic radiation, a white matter tract linking the claustrum to thalamic and prefrontal regions, which facilitates excitatory signaling and enhances salience attribution to painful inputs. Such mechanisms underlie cognitive impairments in chronic pain, where claustral overactivation perpetuates heightened pain perception through these integrated circuits.^[66] The claustrum may exert an inhibitory role in pain gating, potentially mediated by GABAergic interneurons within its circuitry. Chemogenetic suppression of claustro-ACC projections in rodents acutely attenuates mechanical allodynia in inflammatory pain models, as measured by increased von Frey filament withdrawal thresholds, and impairs the formation of pain-associated memories in conditioned place aversion paradigms. This suppression highlights a feedforward inhibitory mechanism, where claustral modulation dampens ACC hyperactivity to alleviate nociceptive behaviors, suggesting therapeutic potential for targeting these pathways in pain disorders.^[66]

Comparative Anatomy and Function

In rodents

The claustrum in rodents, such as mice and rats, forms a thin, sheet-like aggregation of neurons situated beneath the insular cortex and adjacent to the putamen, exhibiting a more compact structure compared to the elongated form observed in humans. This rodent claustrum is proportionally larger relative to the overall cortical volume, comprising a distinct dorsal claustrum and often studied in conjunction with the adjacent dorsal endopiriform nucleus as a functional complex. Anatomical studies in mice highlight its laminar organization, with neurons displaying diverse morphologies including pyramidal-like and multipolar cells, enabling precise targeting in experimental paradigms.^[21]^[67]^[61] Connectivity of the rodent claustrum is characterized by dense, reciprocal projections to widespread cortical regions, including the prefrontal, somatosensory, and entorhinal cortices, as well as strong links to thalamic nuclei that facilitate sensory integration. Viral tracing in mice has revealed topographic organization in these claustro-cortical pathways, with anterior claustrum preferentially connecting to frontal areas and posterior regions linking to temporal lobes. Subcortical inputs, including from the thalamus and basal ganglia, support its role in modulating cortical activity, though rodent-specific patterns emphasize compact, efficient wiring suited to smaller brain sizes. Recent anterograde tracing studies in rats confirm these dense thalamic-cortical interfaces, underscoring the claustrum's position as a hub for inter-regional communication.^[68]^[69]^[61] Functional investigations in rodents have illuminated the claustrum's involvement in memory processes, with 2024 pharmacological and lesion studies in mice demonstrating its necessity for the acquisition, consolidation, and reconsolidation of long-term memories in inhibitory avoidance tasks. Optogenetic manipulation in rats has further revealed that silencing claustrum projections to the entorhinal cortex impairs contextual fear learning and retrieval, indicating a role in associating environmental cues with aversive outcomes. In pain processing, recent work from 2023-2025 highlights the claustrum's modulation of nociceptive behavior via projections to the anterior cingulate cortex; for instance, optogenetic inhibition of this pathway in mice reduces allodynia in acute pain models, while chronic pain states disrupt claustro-cingulate signaling, leading to heightened sensitivity. Evidence from 2023 imaging and electrophysiology in mice also links claustral activity to sleep-wake regulation, where it suppresses cortical ignition during non-rapid eye movement sleep to promote restful states, paralleling its role in attentional gating during wakefulness.^[35]^[70]^[71]^[72] Rodents offer experimental advantages for claustrum research due to their small size, which permits high-resolution in vivo imaging techniques like two-photon microscopy and optogenetics for circuit-level manipulations not feasible in larger animals. These models have enabled seminal 2023-2025 studies on mouse and rat claustra, providing mechanistic insights into memory and pain that inform broader neural principles, though structural parallels to the human claustrum suggest conserved functions in cognitive integration.^[73]^[74]^[75]

In cats and other carnivores

In cats, the claustrum presents as an elongated, sheet-like nucleus positioned between the external capsule and extreme capsule, displaying pronounced anterior-posterior gradients in its morphology and cellular density. Comparative anatomical analyses indicate that the cat's claustrum features a long thin stem with a dorsal enlargement, distinguishing it from more compact forms in other mammals. A 2025 study using electrolytic lesions and electron microscopy confirmed that the dorsal claustrum receives separate and concurrent inputs from frontal and occipital cortices, with abundant degenerative boutons observed post-lesion.^[76]^[77]^[78] Histological examinations, particularly those employing Golgi impregnation techniques prior to 2023, have identified a layered organization in the dorsal claustrum, comprising diverse neuronal populations such as pyramidal-like projection neurons and local interneurons with spiny and aspiny morphologies.^[79] Functional investigations in cats highlight the claustrum's involvement in visuomotor integration, as evidenced by classic lesion studies from the 1970s and 1980s. These experiments demonstrated that targeted ablations of the claustrum produce behavioral deficits akin to neglect syndromes, including contralateral inattention to visual cues and impaired orienting responses, which disrupt coordinated sensory-motor behaviors essential for predation.^[80] Stimulation and recording studies further support this role, revealing that claustral activity modulates cortical responses in visual areas, facilitating the integration of sensory inputs for motor output.^[81] Connectivity patterns in the feline claustrum underscore its integration with sensory processing networks, featuring dense reciprocal projections to visual cortical areas such as the lateral suprasylvian sulcus and somatosensory regions in the parietal cortex. These links enable rapid relay and synchronization of multimodal sensory information, potentially underpinning predatory attention by enhancing stimulus salience during hunting.^[82]^[83] Recent research since 2020 on the claustrum in carnivores has provided some new empirical anatomical data specific to cats, though functional studies remain limited. Emerging evidence draws parallels to pain processing pathways observed in other mammals, suggesting the claustrum may contribute to nociceptive modulation in feline models.^[20] This relative scarcity of post-2020 functional studies highlights a research gap, leaving much of the foundational knowledge on feline claustrum reliant on pre-1990 histological and lesion data.^[80]

In primates

In non-human primates, the claustrum exhibits a more expanded structure relative to rodents, with a higher volume-to-cortical-volume ratio that supports broader functional integration across the neocortex, approaching the extensive cortical coverage observed in humans.^[21] This expansion is evident in species like macaques and marmosets, where the claustrum occupies a thin sheet-like position between the external and extreme capsules, facilitating topographic connections to diverse cortical regions.^[84] Recent high-resolution MRI studies have advanced mapping of the primate claustrum; for instance, a 2023 multimodal MRI atlas of the marmoset brain delineates subcortical structures including the claustrum using T2-weighted imaging and diffusion metrics, revealing distinct signal intensities that differ from those in macaques due to variations in iron distribution.^[85] Complementing this, a 2025 single-nucleus RNA sequencing atlas of the macaque claustrum identifies 48 cell types and provides a 3D connectivity model, highlighting its modular organization into projection-selective zones that interface with widespread cortical areas.^[84] Functionally, the primate claustrum contributes to social cognition and attention by integrating multisensory social cues. In a 2024 functional MRI study of awake macaque monkeys, the claustrum showed robust activation during tasks involving congruent audio-visual social stimuli, such as coo calls paired with mutual grooming scenes, indicating its role in associating stimuli based on semantic meaning rather than low-level sensory features.^[86] This activation pattern extended to unimodal visual social cues like faces and aggressive expressions, suggesting the claustrum modulates attention toward salient social information by linking sensory inputs to limbic contexts.^[86] Such responses align with the claustrum's involvement in perceptual binding, where it supports decisions on stimulus relevance during joint attention-like scenarios.^[86] The claustrum's connectivity in primates features extensive projections to prefrontal regions, underpinning executive control mechanisms comparable to those in humans. Tracing studies reveal strong ipsilateral links from claustral zones to prefrontal areas, including the anterior cingulate cortex, enabling synchronization of cortical networks for goal-directed behavior.^[84] These connections form part of a cortico-basal ganglia circuit, where claustral neurons provide modulatory input to prefrontal executive functions, such as conflict monitoring and decision-making.^[87] A 2023 model posits that the claustrum instantiates task-positive networks by amplifying frontal signals and coordinating targeted cortical areas, supported by functional imaging data from macaques showing claustral engagement during cognitively demanding tasks.^[88] Electrophysiological correlates in primates hint at the claustrum's role in consciousness-related processes, such as perceptual awareness. Although direct recordings remain limited, 2024-2025 imaging and connectivity data from macaques demonstrate claustral activity tied to perceptual decisions in social contexts, where activation patterns predict integration of multimodal inputs into unified percepts.^[86] This builds on broader evidence of claustral-prefrontal interactions facilitating conscious access to sensory information, paralleling human findings on its orchestration of awareness.^[84] Recent studies from 2023 to 2025 have updated understanding of the primate claustrum in cognitive control, emphasizing its modular projections for integrating sensory, motor, and executive domains in non-human primates like macaques.^[84] These advances, including detailed atlases and functional mappings, reveal how claustral subregions support adaptive behaviors, such as attentional shifts and social decision-making, without overlapping basic sensory processing seen in other species.^[88]

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