Not as Dumb as You Thought: The Brainstem 

This piece, by Onno Berkan, was published on 10/22/24. The original text, by Hansen et al., was published by Nature Neuroscience on 10/16/24.

A new McGill study sheds new light on how the brainstem interacts with the cerebral cortex. Using extra powerful fMRIs, researchers mapped connections between 58 brainstem nuclei and various regions of the cerebral cortex.

Researchers found that certain areas in the brainstem act as "hubs," showing widespread connectivity with the cerebral cortex. These hubs are involved in motor coordination, balance, neurotransmitter systems, and more, relaying messages.

Next, cortical phenomena were found to be reflected in brainstem: aspects of cortical function, including brain rhythms, cognitive processes, and the brain's hierarchical organization were traced back to specific connections with the brainstem.

The researchers also identified groups of brainstem nuclei that connect to cortical networks involved in memory, sensation, movement, and emotion. This was found to suggest the brainstem's widespread involvement in various cognitive and behavioral functions.

Cortical regions were found to follow an anterior-posterior gradient in their connection strength to the brainstem, with anterior regions showing stronger connections. This pattern likely allows for a “front-to-back” organization in the brain, with the anterior cingulate cortex (very important in decision making!) acting as a primary hub. This gradient suggests that the brainstem has a differential influence on cortical functions, with potentially greater impact on higher-order cognitive and emotional processes associated with anterior brain regions.

Lastly, the study revealed that the well-known sensory-association axis in the cortex is reflected in brainstem connectivity patterns. This axis suggests that there is a gradient of functional specialization across the cerebral cortex, ranging from primary sensory and motor areas to higher-order association areas, and the study suggests that this hierarchy of cortical function may emerge from connectivity patterns with the brainstem.

The researchers also noted that the brainstem is more functionally connected with the cortex than with itself, which aligns with the fact that major white matter tracts in the brainstem project to regions outside the brainstem.

To conclude, this study extends our understanding of how the human brain operates as an integrated system, highlighting the crucial role of the brainstem in shaping cortical function. It opens up new avenues for research into brain development, aging, and various neurological disorders.

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