All roads of the brain lead to the cloister

Unlike other areas of the brain with famous names like the hippocampus and the amygdala, the claustrum has not yet built its reputation in our cultural imagination.

Looking like a fine cherry leaf, it is found deep in the brain, cloistered between the putamen and the insula. From there, it performs a key relational function: it is the most connected node in 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 wide courtyard surrounded by arches, all the cerebral paths converge in the cloister.

a communication node

From the neurons of our brain emerge long “arms” (axons) through which they communicate with other colleagues from a distance. Communication between remote areas is essential for brain function. Furthermore, uncoordinated brain communication is the cause of neurodegenerative and psychiatric diseases.

In this context, the enormous capacity of the cloister to spread its messages to distant areas makes it an extraordinary candidate for conductor of the orchestra of the brain.

This enclave surprised Francis Crick himself, Nobel Prize winner for the discovery of the structure of DNA, who in the last years of his research claimed to be fascinated by the interconnection capacity of the cloister. He went on to suggest that the cloister was the cerebral nucleus of consciousness.

When empirically investigating this proposal, it was observed that suddenly altering 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 cohesive and unified perception.

A third proposal suggests that the cloister filters relevant information from the outside world, thus guiding our attention where it is required.

Despite the multiple proposals on the work of the cloister, until now, the only consensus is that its main function is still unknown to us.

Do the injuries of the cloister give clues?

In contrast, 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 clear from the study of patients suffering from specific injuries to these areas.

The best-known proper name in neuroscience is probably that of Henry Molaison (HM), the patient whose hippocampus was removed to cure his devastating epilepsy. Although HM stopped having epileptic seizures after the intervention, in turn lost the ability to create new memories. HM’s memory stopped at the age of 27 before the operation and she could not remember a single day that she spent trying to recover, first from her operation and then from her amnesia.

Other popular initials in neuroscience are those of patient SM, who, as a consequence of Urbach-Wiethe disease, lost not only her amygdala, but also her ability to feel fear. After her illness, she SM was mysteriously capable of petting spiders and kissing snakes without hesitation, as well as watching movies like “Silence of the Lambs” without batting an eye.

But what about the cloister? Can we understand what it is for by studying patients without a cloister? Do they perceive, perhaps, the world in a disintegrated and disorganized way? The fine and elongated shape of the claustrum, as well as its confinement between adjacent structures, make specific lesions in this area infrequent. However, a recent study tried to document the cases that, to this day, have described patients with lesions of the claustrum.

Unlike the revealing symptomatology of HM and SM, patients with claustral lesions suffer from disparate and heterogeneous symptoms. More than one suffers from Cotard’s syndrome, a neuropsychiatric condition that affects just over a hundred people in the world. It consists of developing the belief of having died, being decomposed or having lost internal organs.

Patients with a damaged cloister also show delusions with religious and love themes. In addition, several patients with claustral lesions show alterations in their sleep patterns, disorientation, and spontaneous loss of consciousness. Altering the neuronal activity of the claustrum also seems to generate sensations of pain and burning.

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, which indicates that it must contribute to some vital function. In fact, all mammals have a claustrum, and counterparts to it exist in birds and reptiles as well.

Common to all these evolutionarily diverse claustra is their abundant connectivity with other brain areas. Thanks to this vital property, the cloister could have emerged as a coordination structure for fundamental and evolutionarily ancient 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 multiple 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. That would explain why it manifests itself in disparate empirical observations that are difficult to interpret.

While the multifunctionality of the claustrum becomes clearer, the apparent monofunctionality of other brain areas such as the hippocampus and the amygdala begins to be questioned.

This article was a finalist in the 2nd edition of the youth dissemination contest organized by the Lilly Foundation and The Conversation Spain.