One of the many characteristics that set mammals and reptiles apart is the claustrum structure exclusive to the mammalian brain. The claustrum is a thin sheet hidden deep under the neocortex, that receives sensory feedback from all regions of the cortex. Known for facilitating functions ranging from decision-making to the consciousness, the claustrum is hypothesized to aid in the unity of perception in a human’s conscious state (i.e. integrating visual, auditory, and other cues). A team of researchers have found evidence of this classic  mammalian brain structure in reptiles. And it may play a large role during sleep.

Professor Gilles Laurent and his team of researchers at the Max Planck Institute for Brain Research discovered a structure similar to the claustrum, in the Australian bearded dragon Pogona vitticeps, and a freshwater turtle. The finding was a by-product of the team’s ongoing investigation of the dragon’s brain activity during sleep. Comparison with RNA sequencing information from mice helped identify the tiny region to be homologous, or of similar origin and structure, to the mammalian claustrum. Viral neuronal tracing (releasing a virus to trace pathways and connections within the nervous system) showed this claustrum’s wide connectivity to different regions of the forebrain.

Exploring the claustrum’s role during dragon’s sleep, the team found that electro-shocking the claustrum restricted sharp-wave ripples (which appear during a state of restfulness) in slow-wave sleep (the deepest phase of non-rapid eye movement sleep). The shocks, however, had no impact on the sleep rhythm, so the claustrum is not responsible for the generation of sleep waves themselves, but controls some activity during non-REM sleep. Yet, viral tracing showed the dragon’s claustrum projects widely to the forebrain and receives sensory inputs from mid- and hind-brain, further establishing the claustrum’s connections to brain regions known for their involvement in mammalian sleep activity.

This discovery is the first evidence suggesting the existence of a reptilian claustrum, thereby hinting at its presence in the common ancestor of the classes of vertebrates. However, does the claustrum’s involvement during sleep indicate it affects higher cognitive processes as well? Beyond the claustrum, are there more undiscovered similarities between the mammalian and the reptilian brain?

Managing Correspondent: Rhea Grover

Original science article: A claustrum in reptiles and its role in slow-wave sleep, in Nature

Original press article: “Hidden away: An enigmatic mammalian brain area revealed in reptiles”, in Phys.org

Image courtesy: Needpix, Wikipedia, Wikimedia Commons