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by Conor Feehly
for
Quanta Magazine reveal our brain's effort to keep us alive and the evolutionary constraints that sculpted our most complex organ...
All you want to do is put your feet up and zone out to whatever is on television.
Though the inactivity may feel like a well-earned rest, your brain is not just chilling. In fact, it is using nearly as much energy as it did during your stressful activity, according to recent research.
Sharna Jamadar, a neuroscientist at Monash University in Australia, and her colleagues reviewed research from her lab and others around the world to estimate the metabolic cost of cognition - that is, how much energy it takes to power the human brain.
Surprisingly, they concluded that effortful, goal-directed tasks use only 5% more energy than restful brain activity. In other words, we use our brain just a small fraction more when engaging in focused cognition than when the engine is idling.
It often feels as though we allocate our mental energy through strenuous attention and focus.
But the new research builds on a growing understanding that the majority of the brain's function goes to maintenance.
While many neuroscientists have historically focused on active, outward cognition, such as attention, problem-solving, working memory and decision-making, it's becoming clear that beneath the surface, our background processing is a hidden hive of activity.
Our brains regulate our bodies' key physiological systems, allocating resources where they're needed as we consciously and subconsciously react to the demands of our ever-changing environments.
The neuroscientist Sharna Jamadar compiled data from simultaneous MRI and PET imaging
to estimate the
brain's energy use.
The brain is not purely a cognition machine, but an object sculpted by evolution - and therefore constrained by the tight energy budget of a biological system.
Thinking may make you feel tired, then, not because you are out of energy, but because you have evolved to preserve resources.
This study of neural metabolism, when tied to research on the dynamics of the brain's electrical firing, points to the competing evolutionary forces that explain the limitations, scope and efficiencies of our cognitive capabilities.
The Cost of a Predictive Engine
The human brain is incredibly expensive to run.
At roughly 2% of body weight, the organ gorges on 20% of our body's energetic resources.
For infants, that number is closer to 50%.
The brain is not purely a cognition machine, but an object sculpted by evolution...
The brain's energy comes in the form of the molecule adenosine triphosphate (ATP), which cells make from glucose and oxygen.
A tremendous expanse of thin capillaries:
Once synthesized within cells, ATP powers communication between neurons, which enact the brain's functions.
Neurons carry electrical signals to their synapses, which allow the cells to exchange molecular messages; the strength of a signal determines whether they will release molecules (or "fire").
If they do, that molecular signal determines whether the next neuron will pass on the message, and so on.
Maintaining what are known as membrane potentials - stable voltages across a neuron's membrane that ensure that the cell is primed to fire when called upon - is known to account for at least half of the brain's total energy budget.
Measuring ATP directly in the human brain is highly invasive.
So, for their paper, Jamadar's lab reviewed studies, including their own findings, that used other estimates of energy use - glucose consumption, measured by positron-emission tomography (PET), and blood flow, measured by functional magnetic resonance imaging (fMRI) - to track differences in how the brain uses energy during active tasks and rest.
When performed simultaneously, PET and fMRI can provide complementary information on how glucose is being consumed by the brain, Jamadar said.
It's not a complete measure of the brain's energy use because neural tissues can also convert some amino acids into ATP, but the vast majority of the brain's ATP is produced by glucose metabolism.
Jamadar's analysis showed that a brain performing active tasks consumes just 5% more energy compared to a resting brain.
When we are engaged in an effortful, goal-directed task, such as studying a bus schedule in a new city, neuronal firing rates increase in the relevant brain regions or networks - in that example, visual and language processing regions.
This accounts for that extra 5%; the remaining 95% goes to the brain's base metabolic load.
The neuroscientist Jordan Theriault at Northeastern University believes that the brain is a prediction engine
that is always
planning for what comes next.
Researchers don't know precisely how that load is allocated, but over the past few decades, they have clarified what the brain is doing in the background.
Much of that signal is from the default mode network, which operates while we're resting or otherwise not engaged in apparent activity.
Additionally, beneath the iceberg of awareness, our brains are keeping track of the mosaic of physical variables - body temperature, blood glucose level, heart rate, respiration, and so on - that must remain stable, in a state known as homeostasis, to keep us alive.
If any of them stray too far, things can get bad pretty quickly.
Theriault speculates that most of the brain's base metabolic load goes toward prediction.
To achieve its homeostatic goals, the brain needs to always be planning for what comes next - building a sophisticated model of the environment and how changes might affect the body's biological systems.
Prediction, rather than reaction, Theriault said, allows the brain to dole out resources to the body efficiently.
The Brain's Evolutionary Constraints
A 5% increased energy demand during active thought may not sound like much, but in the context of the entire body and the energy-hungry brain, it can add up.
And when you consider the strict energetic constraints our ancestors had to deal with, weariness at the end of a taxing day suddenly makes a lot more sense.
The neuroscientist Zahid Padamsey has studied how the availability of energy (or food)
has influenced the
evolution of the brain.
The modern world, in which calories are relatively abundant for many people, contrasts starkly with the conditions of scarcity that Homo sapiens evolved in.
That 5% increase in burn rate, over 20 days of persistent, active, task-related focus, can amount to a whole day's worth of cognitive energy.
If food is hard to come by, it could mean the difference between life and death.
In fact, the brain has built-in systems to prevent overexertion.
To better understand these energetic constraints, in 2023 Padamsey summarized research on certain peculiarities of electrical signaling that indicate an evolutionary tendency toward energy efficiency.
For one thing, you might imagine that the faster you transmit information, the better. But the brain's optimal transmission rate is far lower than might be expected.
Theoretically, the top speed for a neuron to feasibly fire and send information to its neighbor is 500 hertz.
However, if neurons actually fired at 500 hertz, the system would become completely overwhelmed. The optimal information rate - the fastest rate at which neurons can still distinguish messages from their neighbors - is half that, or 250 hertz.
Our neurons, however, have an average firing rate of 4 hertz, 50 to 60 times less than what is optimal for information transmission.
What's more, many synaptic transmissions fail:
That's because we didn't evolve to maximize total information sent.
Sending the maximum amount of information for as little energy as possible (bits per ATP), the optimal neuronal firing rate is under 10 hertz.
Evolutionarily, the large, sophisticated human brain offered an unprecedented level of behavioral complexity - at a great energetic cost.
This negotiation, between the flexibility and innovation of a large brain and the energetic constraints of a biological system, defines the dynamics of how our brain transmits information, the mental fatigue we feel after periods of concentration, and the ongoing work our brain does to keep us alive.
That it does so much within its limitations is rather astonishing.
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