People are oblivious to distractions when focusing on a specific task because the human brain has a 'limit' on how much information it can process at once

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The human brain has a limit on how much information it can process at once due to a finite energy supply, a new study reveals.  

UK neuroscientists say that energy supply to the brain remains constant and can't exceed an upper limit, however challenging a task is. 

But as the brain uses more energy in processing the task at hand, less energy is supplied to processing outside our immediate focus, they say. 

This results in what's known as 'inattentional blindness' – when stimuli that's available in plain sight doesn't register, even if it's valuable to us.

This can help explain why we are sometimes unable to concentrate on what our family members are telling us while we're playing video games or watching TV. 

As the brain uses more energy in processing what we attend to, less energy is supplied to processing outside our focus, researchers found. This can help explain why we struggle to hear our family members while watching TV or why we fail to notice important emails

It also shows how dangerous it can be to divert our attention away from driving to focus on distractions, such as smartphones. 

Researchers used broadband near-infrared spectroscopy to measure brain metabolism levels while participants performed a computerised task. 

The brain allocates less energy to the nerve cells, known as neurons, that respond to information outside the focus of our attention as the task gets harder, they found. 

The brain constantly uses around 20 per cent of our metabolic energy, even while we're resting our mind during sleep.

This constant but limited supply of energy does not increase when there is more for our mind to process, however.   

'It takes a lot of energy to run the human brain,' said study author Professor Nilli Lavie at University College London. 

'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 prioritising 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.'  

Focusing our attention on a particular task or thought can change how the brain allocates its limited energy, the researchers claim. 

As the brain uses more energy in processing what we attend to, less energy is supplied to processing outside our attention focus.         

In experiments, researchers used near-infrared spectroscopy, a non-invasive brain imaging method that measures cellular metabolism, to measure changes in brain activity. 

The method measures the oxidation levels of an enzyme involved in energy metabolism in brain cells' mitochondria – the energy generators that power each cell's biochemical reactions. 

With this method, they were able to see how much energy brain regions use as people focus attention on a task, and how that changes when the task becomes more mentally demanding.  

The study highlights how dangerous it can be to divert our attention away from driving to focus on distractions, such as mobile phones

The technique was used to measure brain metabolism in different regions of the visual cortex in the brains of 18 people of both sexes as they carried out visual search tasks.

The tasks at hand, which were either defined as 'complex' or 'simple', was known as the 'attended' stimuli.

These tasks were sometimes run in parallel with a visual distraction that was irrelevant to the task – known as the 'unattended' stimuli.

They found levels of cellular metabolism in brain areas that responded to the task at hand rose as the task became more complex.

These increases were directly mirrored with reduced cellular metabolism levels in areas responding to unattended stimuli.

This 'push-pull pattern' was closely synchronised, showing a trade-off of limited energy supply between attended and unattended brain processing. 

'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.'

Professor Lavie said the phenomenon is in play during our day-to-day lives – especially during the coronavirus lockdown as we've been trying to juggle multiple tasks at once.  

'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,' she said. 

'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 has been published in Journal of Neuroscience.