Indian elephants apparently have larger brains than we do. Mice have a higher brain-to-body mass ratio, and long-finned pilot whales have more neurons. So what makes humans—and more specifically, the human brain—different?
As far as organs are concerned, the energy consumed by the human brain is undoubtedly huge - nearly 50 grams of sugar, or 12 pieces of sugar, are consumed every day. This has one of the highest energy requirements relative to body metabolism of any known species. But what consumes this energy? If the size and number of neurons in the human brain match primate predictions, and the energy consumed per neuron is comparable to other mammals, then the energy consumption of the human brain should not be unique.
The cost of signaling
A team of neuroscientists speculates that the amount of signaling within the human brain may be responsible for its increased energy needs. One result of this is that areas of the brain that are more tightly connected and have more signaling will use more energy.
To test their hypothesis, the scientists first imaged the brains of 30 healthy, right-handed volunteers aged 20 to 50 years old. Imaging was performed at two different institutions, and the researchers used the imaging to correlate the energy expenditure (measured by glucose metabolism) of specific brain regions with their signaling and connectivity levels. They found that in all 30 brains, energy expenditure and signaling occurred simultaneously. But certain areas stand out. Signaling pathways in certain areas of the cerebral cortex (the front part of the brain) require nearly 70% more energy than signaling pathways in sensorimotor areas.
The frontal cortex is one of the most rapidly expanding regions during human evolution. Robert Sapolsky believes: "The most important role of the prefrontal cortex is to make difficult decisions in the face of temptation-delayed gratification, long-term planning, impulse control, emotional regulation. The prefrontal cortex is critical for allowing you to do the right thing on the more difficult things. This is a problem that humans must constantly face. Energy-wise, this requires a huge cost."
Strengthening regulation is also the key to cognition
Energy is required not only for signaling, but also for the regulation of signaling to ensure that signaling occurs at the appropriate level and at the appropriate time.
The researchers used the Allen Brain Atlas to study gene activity in the frontal cortex. They found increased activity of neuromodulators and their receptors. The authors note: "The human brain spends too much energy on long-term regulation of (fast) neurotransmitters with (slow) neuromodulators such as serotonin, dopamine, or norepinephrine. There are also endogenous opiates. This effect is more about setting the tone of overall excitability than delivering individual bits of information."
After linking energy use to signaling and slow-acting neuromodulation in the cerebral cortex, the last thing the scientists did was investigate a neural network project that maps cognitive functions to brain regions. It was found that the energy-consuming, highly connected, highly regulated, and evolutionarily expanded parts of the cerebral cortex are those involved in complex functions such as memory processing, reading, and cognitive inhibition. This supports their view that "an expensive signaling architecture is dedicated to human cognition."
Science Advances, 2023. DOI:10.1126/sciadv.adi7632