“The human brain has 100 billion neurons, each neuron connected to 10 thousand other neurons. Sitting on your shoulders is the most complicated object in the known universe,” observes physicist Michio Kaku. The neocortex, observed Carl Sagan is where “matter is transformed into consciousness.” Located deep in the brain’s center, the subcortex, the most evolutionarily ancient part of our brain, processes everything from our basic senses to long-term memories.
“Most Perfectly Organized Part”
Noble Prize laureate Roger Penrose suggest that the human brain and its cerebral cortex, the folded gray matter that covers the first couple of millimeters of the outer brain like wrapping paper, is more complex than our Milky Way Galaxy. “If you look at the entire physical cosmos,” Penrose says, our brains are a tiny, tiny part of it. But they’re the most perfectly organized part.”
New research by the University of Melbourne has revealed a new brain atlas of the human subcortex, an astoundingly complex hierarchical structure and 27 new subcortical regions.
Subcortex –Uncharted Terra Incognita
Since the 1th century, brain cartographers have mostly focused on mapping the territories, regions and networks of the outermost layer of the human brain, known as the cerebral cortex, which has left the subcortex as an uncharted terra incognita. As a result of this scarcity of subcortical atlases, many attempts to derive a wiring diagram for the brain, known as the human connectome, often exclude the subcortex.
Dysfunction of the subcortex is associated with numerous brain and mental health disorders, including Huntington’s and Parkinson’s diseases, schizophrenia and depression. Selecting the best location for focal therapies targeting the subcortex – like deep brain stimulation (DBS) – requires detailed subcortical atlases to enable accurate targeting, which involves implanting electrodes into a patient’s brain and electrically stimulating subcortical targets to treat the symptoms of Parkinson’s disease, epilepsy and several neuropsychiatric disorders.
27 New Territories of the Subcortex
The University of Melbourne team used high-resolution functional magnetic resonance imaging (MRI) scans of more than 1000 people to map the most detailed subcortical atlas to date, discovering 27 new territories of the human subcortex, each demarcated by distinct borders and associated with a distinct function. The new atlas reveals an astoundingly complex organizational architecture that stretches across four hierarchical levels.
Amazingly, the borders separating some of the areas shifted when people were asked to engage in cognitively demanding tasks during the brain scan, suggesting that these territories and regions of the subcortex are dynamic and can reorganize depending on an individual’s actions and thoughts.
The atlas, mapping the functional areas of one of the remaining unchartered territories of the human brain, was made possible by the high-quality brain scans provided by the Human Connectome Project, was initially mapped using brain scans from a 3 Tesla MRI scanner – this is the type of scanner typically found in most hospital radiology departments.
Specific regions and territories of the subcortex are connected to the rest of the brain and form brain-wide networks.
The Central Gatekeeper
Specific regions and territories of the subcortex are connected to the rest of the brain and form brain-wide networks. The subcortex is the brain’s central gatekeeper, modulating input and output information between the outer layers of the brain and the rest of the body.Our atlas reveals how specific regions and territories of the subcortex are connected to the rest of the brain and form brain-wide networks that orchestrate everything from cognitive function to sensory and motor processing.
While the subcortex is spatially distant from the outer layers of the brain, we found that each of the 27 newly-discovered subcortical territories display patterns of activity that are highly synchronized with specific parts of the cerebral cortex.
Image Credits: Top of page, Shutterstock License; Human Brian, The Glass Brain Project