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The brain devotes its limited energy to what we focus on

The human brain can only process so much information at once, which is due to a constant yet limited energy supply, according to expert at University College London

Using a brain imaging technique to measure cell metabolism, the researchers found that the brain dedicates more energy to what we pay attention to and focus on.

Meanwhile, less energy is allocated for processing information outside of our range of focus.

Study senior author Nilli Lavie is a professor in the UCL Institute of Cognitive Neuroscience.

“It takes a lot of energy to run the human brain. We know that the brain constantly uses around 20% of our metabolic energy, even while we rest our mind, and yet it’s widely believed that this constant but limited supply of energy does not increase when there is more for our mind to process,” said Professor Lavie.

“If there’s a hard limit on energy supply to the brain, we suspected that the brain may handle challenging tasks by diverting energy away from other functions, and prioritizing the focus of our attention.”

“Our findings suggest that the brain does indeed allocate less energy to the neurons that respond to information outside the focus of our attention when our task becomes harder. This explains why we experience inattentional blindness and deafness even to critical information that we really want to be aware of.”

A team of cognitive neuroscientists and biomedical engineers used a non-invasive optical imaging method to measure cerebral metabolism. This allowed the experts to investigate how much energy is used by the brain as people focus attention on a task, and how this energy level may change when a task becomes more mentally demanding. 

“In this way, we have managed to connect people’s experience of brain overload to what’s going on inside their neurons, as high energy demands for one purpose are balanced out by reduced energy use related to any other purpose,” said Professor Lavie. 

“If we try to process too much information we may feel the strain of overload because of the hard limit on our brain capacity.”

The team measured the oxidation levels of an enzyme that is involved in energy metabolism in the mitochondria of brain cells located in different regions of the visual cortex. 

This technique was performed on 18 participants as they executed visual search tasks that were either complex or simple. The individuals were sometimes presented with a visual distraction that was irrelevant to the task.


The experts observed elevated cellular metabolism in the brain areas that were responding to the task, and the energy dedicated to the task increased as it became more challenging. This activity directly corresponded with reduced cellular metabolism levels in brain areas responding to irrelevant stimuli.

According to the researchers, the push-pull pattern was closely synchronized, showing a trade-off of limited energy supply between attended and unattended processing.

“By using our in-house developed broadband near-infrared spectroscopy, an optical brain monitoring technology we developed at UCL, we were better able to measure an enzyme in the mitochondria (the power factory of the cells) that plays an integral part in metabolism,” said study co-author Professor Ilias Tachtsidis.

“During recent months, we’ve heard from a lot of people who say they’re feeling overwhelmed, with constant news updates and new challenges to overcome,” said study first author Merit Bruckmaier.

“When your brain is at capacity, you are likely to fail to process some information. You might not even notice an important email come in because your child was speaking to you, or you might miss the oven timer go off because you received an unexpected work call. Our findings may explain these often-frustrating experiences of inattentional blindness or deafness.”

The study is published in the Journal of Neuroscience.

By Chrissy Sexton, Staff Writer


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