Unlike other regions of the brain with famous names like the hippocampus and the amygdala, the claustrum has not yet built its reputation in our cultural imagination.
Resembling a thin cherry leaf, it sits deep within the brain, cloistered between the putamen and the insula. From there, it fulfills a key relational function: it is the most connected node of the human nervous system, a crucial link in the dense network of neurons that is the brain. Just as the corridors of a monastery lead to the vast courtyard surrounded by arcades, all the cerebral paths converge in the cloister.
a communication node
Neurons in our brain emerge from long “arms” (axons) through which they communicate remotely with other colleagues. Communication between distant areas is essential for brain function. In addition, uncoordinated brain communication is the cause of neurodegenerative and psychiatric diseases.
In this context, the Cloister’s enormous ability to spread its messages to remote areas makes it an extraordinary candidate to lead the brain orchestra.
This enclave surprised Francis Crick himself, Nobel Prize for the discovery of the structure of DNA, who in the last years of his research said he was fascinated by the ability of interconnection of the cloister. He went on to suggest that the cloister was the cerebral core of consciousness.
Investigating this proposition empirically, it was observed that a sudden change in the activity of the cloister generated a loss of consciousness that could reverse when the disturbance ceases.
Other studies have suggested that the cloister is responsible for integrating information from different senses, such as sight and hearing, thus helping to build a coherent and unified perception.
A third proposition suggests that the cloister filters relevant information from the outside world, thus guiding our attention where it is needed.
Despite the multiple proposals on the work of the cloister, so far the only consensus is that its main function is still unknown to us.
Do the wounds of the cloister give clues?
On the other hand, we know the hippocampus as the guardian of memory and the amygdala as the cradle of emotions and, in particular, of fear. And we have it so clearly from the study of patients with injuries specific to these areas.
Probably the best-known proper name in neuroscience is that of Henry Molaison (HM), the patient whose hippocampus was removed to treat his devastating epilepsy. Although HM stopped having seizures after the procedure, in turn lost the ability to create new memories. HM’s memory cut off at the age of 27 before the operation and he could not remember a single day spent trying to recover, first from his operation and then from his amnesia.
Other popular initials in neuroscience are those of the patient SM, who, as a result of Urbach-Wiethe disease, lost not only her amygdala, but also her ability to sense fear. After his illness, Spider-Man was mysteriously able to pet spiders and kiss snakes without hesitation, as well as watch movies like “The Silence of the Lambs” without batting an eyelid.
But what about the cloister? Can we understand what this is for by studying patients without a cloister? Do they, perhaps, perceive the world in a disintegrated and disorganized way? The thin and elongated shape of the claustrum, as well as its confinement between adjacent structures, make specific lesions of this area infrequent. However, a recent study attempted to document the cases that, to date, described patients with lesions of the claustrum.
Contrary to the telltale symptomatology of HM and SM, patients with claustral lesions suffer from disparate and heterogeneous symptoms. More than one suffers from Cotard syndrome, a neuropsychiatric condition that affects just over a hundred people worldwide. It consists of developing the belief of being dead, of being decomposed or of having lost internal organs.
Patients with a damaged cloister also present with delusions with religious and amorous themes. In addition, many patients with claustral lesions experience alterations in sleep patterns, disorientation, and spontaneous loss of consciousness. Altered neural activity in the claustrum also appears to generate pain and burning sensations.
In mammals, birds and reptiles
Despite the efforts of this study, the main function of the cloister still eludes us. However, we know that this structure has been preserved throughout evolution, indicating that it must contribute to a vital function. In fact, all mammals have a claustrum, and equivalents also exist in birds and reptiles.
The common point of all these diversified evolutionary claustra is their abundant connectivity with other areas of the brain. Thanks to this vital property, the cloister could have appeared as a coordinating structure for ancient fundamental and evolutionary processes, such as sleep, in species such as reptiles and birds. In mammals, the connectivity of the claustrum and its location as a node of several brain networks could have been exploited by other brain systems, for example those that regulate cognitive processes such as perception and attention.
Perhaps because of the protection afforded by its enclosure and its dense connections to other areas, the cloister lends itself to being used by various brain systems to perform various functions. This would explain why it manifests itself through disparate empirical observations that are difficult to interpret.
As the multifunctionality of the claustrum becomes clearer, the apparent monofunctionality of other brain areas such as the hippocampus and amygdala begins to be questioned.
This article was a finalist in the 2nd edition of the youth disclosure contest organized by the Lilly Foundation and The Conversation Spain.