The claustrum (Latin, meaning "to close" or "to shut") is a thin sheet of neurons and supporting glial cells in the brain, that connects to the cerebral cortex and subcortical regions including the amygdala, hippocampus and thalamus.

[1] It is considered to be the most densely connected structure in the brain, and thus hypothesized to allow for the integration of various cortical inputs such as vision, sound and touch, into one experience.

[4][5] Other hypotheses suggest that the claustrum plays a role in salience processing, to direct attention towards the most behaviorally relevant stimuli amongst the background noise.

[9][8] Through interhemispheric connections, the claustrum is believed to play a role in synchronizing activity in widely separated, but functionally related, parts of the brain such as between frontal eye fields and the visual cortex.

[18] More specifically, electrophysiological studies show extensive connections to thalamic nuclei and the basal ganglia, while isotopological reports have linked the claustrum with the prefrontal, frontal, parietal, temporal and occipital cortices.

[13] Altogether, these findings leave the claustrum as the most highly connected structure per regional volume in the brain and suggest that it may serve as a hub to coordinate activity of cerebral circuits.

[13][17][24] In summary, the cortical and subcortical connectivity of the claustrum implies that it is most involved with processing sensory information, as well as the physical and emotional state of an animal.

Moreover, the claustrum possesses a distinct topological organization for each sensory modality as well as the dense connectivity it shares with frontal cortices.

[13] Within the claustrum, local connectivity is dominated by feed-forward disynaptic inhibition wherein parvalbumin-expressing interneurons suppress the activity of nearby projection neurons.

[28] Combined, these two circuits suggest that the claustrum is capable of performing local transformations of diverse input information from across the brain.

[4] Excitatory cell types in the claustrum consist of two main classes which differentially project to cortical and subcortical brain regions.

[31] Finally, many studies show that the claustrum is best distinguished structurally by its prominent plexus of parvalbumin-positive fibers formed by parvalbumin-expressing inhibitory cell types.

[5] In recent studies, the use of myelin basic protein (MBP) and retrogradely traveling cholera toxin have additionally been used as effective methods of identifying the claustrum.

Crick and Koch suggest that the claustrum has a role similar to that of a conductor within an orchestra as it attempts to co-ordinate the function of all connections.

The claustrum has been confirmed to be reciprocally connected to the prefrontal cortex, visual, auditory, sensory, and motor regions respectively.

It has also been suggested that it operates in the opposite context; that through divisive normalization the claustrum may implement resistance to certain input modalities to prevent “distraction”.

The claustrum, in order to facilitate consciousness, would need to integrate various sensory and motor modalities from various parts of the cortex.

A functional magnetic resonance imaging (fMRI) scan looks at oxygenated blood levels in the brain as a way of observing the activity of specific cortical areas.

The claustrum is situated anatomically at the confluence of a large number of white-matter tracts used to connect different parts of the cortex.

Gap junctions have been shown to exist between aspiny (lacking dendrite projections) interneurons of the claustrum – suggesting a role in its ability to synchronize these modalities as input is received.

For example, projections to motor and oculomotor areas would assist with gaze movement to direct attention to new stimuli by increasing the firing frequency of claustral neurons.

Stimulation of the left claustrum in humans has produced "a complete arrest of volitional behavior, unresponsiveness, and amnesia without negative motor symptoms, or mere aphasia" suggesting the involvement in consciousness.

[14] Furthermore, MRI studies have shown that increased signal intensity within the claustrum has been associated with status epilepticus – a condition in which epileptic seizures follow one another without recovery of consciousness between events.

[14] Another study looking at the symptomology of schizophrenia established that the severity of delusions was associated with decreased grey matter volume of the left claustrum; postulating that correlations exist between the structure and positive symptoms seen in this psychiatric disorder.

[45] Damage to the claustrum may mimic various common diseases or mental disorders; delayed development of the structure appears to be linked to autism.

[39] The authors state that 'The claustrum is a coordinator of global slow-wave activity, and it is so exciting that we are getting closer to linking specific brain connections and actions with the ultimate puzzle of consciousness.'

[47] A team of investigators led by neuroscientists at Beth Israel Deaconess Medical Center has identified lesions in the claustrum as the likely origin of parkinsonism across different conditions.

The mapping of the 29 lesions – which were located in different regions of the brain – revealed that connectivity to the claustrum was the single most sensitive and specific marker of lesion-induced parkinsonism.

[49] In animals, through tract tracing, findings have shown that the claustrum has extensive connections throughout the cortex with sensory and motor regions along with the hippocampus.

In cats, high-frequency stimulation (HFS) of the claustrum can alter motor activity, induce autonomic changes, and precipitate an “inactivation syndrome” described as “decreased awareness".