Human brain’s high energy demands linked with more complex cognitive circuitry, not just bigger size

Findings have implications for human evolution.

AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE (AAAS)—The human brain has exceptionally high metabolic demands compared with other species, accounting for roughly 20% of the body’s energy consumption. Now, brain scans from 30 people suggest that the neuromodulator activity involved in cognitive functions such as reading and memory formation may contribute most to these high energetic costs. The findings shed new light on brain evolution, indicating that human cognition may have emerged in part from the evolutionary expansion of specific brain networks – not just from a larger brain size. Common theories on human evolution propose that humans’ high cognitive capacity is largely due to an adaptive increase in brain size. However, research has shown that the scale of human brain structure is not unique among mammals, with some species having larger brains, higher brain-to-body mass ratios, or more neurons. Recent studies suggest that neuromodulator activity – dynamic regulation of neurons via chemicals such as dopamine and serotonin – may have influenced the evolution of human cognition and behavior over time. However, it has been unclear how these processes and their energy demands vary across the human brain. To investigate, Gabriel Castrillon and colleagues employed a correlative neuroimaging approach to analyze the distribution of energy usage and signaling in the brains of 30 participants, using data from positron emission tomography and functional magnetic resonance imaging. The researchers measured energetic costs for signaling activities across functional networks of the brain, finding that frontoparietal networks – which have expanded most in human brain evolution – demanded around 67% more energy than sensorimotor networks per gram of tissue. Regions that exhibited more regulation from neuromodulators, such as serotonin, dopamine, and noradrenaline, required the most energy. “Our findings suggest that the evolution of human cognition may have emerged not only from an overall larger brain but particularly by the development of slow-acting neuromodulator circuits,” Castrillon et al. write.


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