The claustrum (Latin, meaning "to close" or "to shut") is a thin, bilateral collection of neurons and supporting glial cells, that connects to cortical (e.g., the pre-frontal cortex) and subcortical regions (e.g., the thalamus) of the brain. It is located between the insula laterally and the putamen medially, separated by the extreme and external capsules respectively. The blood supply to the claustrum is fulfilled via the middle cerebral artery. It is considered to be the most densely connected structure in the brain, allowing for integration of various cortical inputs (e.g., colour, sound and touch) into one experience rather than singular events. The claustrum is difficult to study given the limited number of individuals with claustral lesions and the poor resolution of neuroimaging.

The claustrum is made up of various cell types differing in size, shape and neurochemical composition. Five cell types exist, and a majority of these cells resemble pyramidal neurons found in the cortex. Within the claustrum, there is no organization of cell types compared to the cortex, and the somas of the cells can be a pyramidal, fusiform or circular shape. The principal cell type found in the claustrum is type 1 cells, which are large cells covered in spiny dendrites.

The claustrum usually connects to the cortex in an ipsilateral manner; however, the few that travel contralaterally are considerably weaker than the former. The claustrum acts as a conductor for inputs from the cortical regions so these respective areas do not become unsynchronized. Without the claustrum, one could respond to stimuli that are familiar to the individual but not to complex events. Additionally, the claustrum is essential in combining sensory and motor modalities so that various anatomical patterns are present. One of the proposed functions of the claustrum is to differentiate between relevant and irrelevant information so that the latter can be ignored. Cortical components of consciousness include the fronto-parietal cortex, cingulate and precuneus. Due to the claustrum's widespread connectivity to these areas, it is suggested that it may play a role in both attention and consciousness. The neural networks that mediate sustained attention and consciousness implicate numerous cortical areas, many of which overlap in connectivity with the claustrum. Previous clinical reports suggest that conscious processes are lateralized to the left hemisphere in humans.