Our brains take rhythmic snapshots of the world as we walk – and we never knew

For decades, psychology departments around the world have studied human behaviour in darkened laboratories that restrict natural movement.

Our new study, published today in Nature Communications, challenges the wisdom of this approach. With the help of virtual reality (VR), we have revealed previously hidden aspects of perception that happen during a simple everyday action – walking.

We found the rhythmic movement of walking changes how sensitive we are to the surrounding environment. With every step we take, our perception cycles through “good” and “bad” phases.

This means your smooth, continuous experience of an afternoon stroll is deceptive. Instead, it’s as if your brain takes rhythmic snapshots of the world – and they are synchronised with the rhythm of your footfall.

The next step in studies of human perception

In psychology, the study of visual perception refers to how our brains use information from our eyes to create our experience of the world.

Typical psychology experiments that investigate visual perception involve darkened laboratory rooms where participants are asked to sit motionless in front of a computer screen.

Often, their heads will be fixed in position with a chin rest, and they will be asked to respond to any changes they might see on the screen.

This approach has been invaluable in building our knowledge of human perception, and the foundations of how our brains make sense of the world. But these scenarios are a far cry from how we experience the world every day.

This means we might not be able to generalise the results we discover in these highly restricted settings to the real world. It would be a bit like trying to understand fish behaviour, but only by studying fish in an aquarium.

Instead, we went out on a limb. Motivated by the fact our brains have evolved to support action, we set out to test vision during walking – one of our most frequent and everyday behaviours.

Doing tests in a lab isn’t quite the same as seeing and interacting with things in the real world. sirtravelalot/Shutterstock

A walk in a (virtual) forest

Our key innovation was to use a wireless VR environment to test vision continuously while walking.

Several previous studies have examined the effects of light exercise on perception, but used treadmills or exercise bikes. While these methods are better than sitting still, they don’t match the ways we naturally move through the world.

Instead, we simulated an open forest. Our participants were free to roam, yet unknown to them, we were carefully tracking their head movement with every step they took.

Participants walked in a virtual forest while trying to detect brief visual ‘flashes’ in the moving white circle.

We tracked head movement because as you walk, your head bobs up and down. Your head is lowest when both feet are on the ground and highest when swinging your leg in-between steps. We used these changes in head height to mark the phases of each participant’s “step-cycle”.

Participants also completed our visual task while they walked, which required looking for brief visual “flashes” they needed to detect as quickly as possible.

By aligning performance on our visual task to the phases of the step-cycle, we found visual perception was not consistent.

Instead, it oscillated like the ripples of a pond, cycling through good and bad periods with every step. We found that depending on the phases of their step-cycle, participants were more likely to sense changes in their environment, had faster reaction times, and were more likely to make decisions.

Oscillations in nature, oscillations in vision

Oscillations in vision have been shown before, but this is the first time they have been linked to walking.

Our key new finding is these oscillations slowed or increased to match the rhythm of a person’s step-cycle. On average, perception was best when swinging between steps, but the timing of these rhythms varied between participants. This new link between the body and mind offers clues as to how our brains coordinate perception and action during everyday behaviour.

Next, we want to investigate how these rhythms impact different populations. For example, certain psychiatric disorders can lead to people having abnormalities in their gait.

There are further questions we want to answer: are slips and falls more common for those with stronger oscillations in vision? Do similar oscillations occur for our perception of sound? What is the optimal timing for presenting information and responding to it when a person is moving?

Our findings also hint at broader questions about the nature of perception itself. How does the brain stitch together these rhythms in perception to give us our seamless experience of an evening stroll?

These questions were once the domain of philosophers, but we may be able to answer them, as we combine technology with action to better understand natural behaviour.The Conversation

Matthew Davidson, Postdoctoral research fellow, lecturer, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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How exercise increases brain volume — and may slow memory decline

Exercising for 25 minutes a week, or less than four minutes a day, could help to bulk up our brains and improve our ability to think as we grow older. A new study, which involved scanning the brains of more than 10,000 healthy men and women from ages 18 to 97, found that those who walked, swam, cycled or otherwise worked out moderately for 25 minutes a week had bigger brains than those who didn’t, whatever their ages.

Bigger brains typically mean healthier brains.

The differences were most pronounced in parts of the brain involved with thinking and memory, which often shrink as we age, contributing to risks for cognitive decline and dementia.

“This is an exciting finding and gives us more fuel for the idea that being physically active can help maintain brain volume across the life span,” said David Raichlen, a professor of biological sciences and anthropology at the University of Southern California. He studies brain health but was not involved with the new study.

The results have practical implications, too, about which types of exercise seem best for our brain health and how little of that exercise we may really need.

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Little exercise, big brain

“We wondered, if we chose a very low threshold of exercise what would we see?” said Cyrus A. Raji, an associate professor of radiology and neurology at Washington University in St. Louis, who led the new study.

He and his colleagues were well aware that exercise is good for brains, especially as we age. Physically active older people are far less likely than the sedentary to develop Alzheimer’s disease or other types of memory loss and cognitive decline.

But he also knew that few people in the real world exercise much. “You hear that you need 10,000 steps a day,” he said, “or 150 minutes a week. But it’s very hard to reach” those goals.

Would less – even far less – exercise still help to build healthier brains, he and his colleagues wondered?

What about, for instance, 25 minutes of exercise a week, a sixth of the 150 minutes recommended in most formal exercise guidelines?

“It seemed an achievable amount for most people,” Raji said. But would it show effects on brains?

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10,125 brain scans

He and his colleagues turned to existing brain scans for 10,125 mostly healthy adults of all ages who’d come to the university medical center for diagnostic tests. Beforehand, these patients had provided information about their medical histories and how often and strenuously they’d exercised during the past two weeks.

The researchers divided them into those who’d exercised for at least 25 minutes a week and those who hadn’t.

Then, with the aid of artificial intelligence, they began comparing scans and exercise habits, looking for differences in brain volume, or how much space a brain and its constituent parts fill. More volume is generally desirable.

A clear pattern quickly emerged. Men and women, of any age, who exercised for at least 25 minutes a week showed mostly greater brain volume than those who didn’t. The differences weren’t huge but were significant, Raji said, especially when the researchers looked deeper inside the organ.

There, they found that exercisers possessed greater volume in every type of brain tissue, including grey matter, made up of neurons, and white matter, the brain’s wiring infrastructure, which supports and connects the thinking cells.

More granularly, the exercisers tended to have a larger hippocampus, a portion of the brain essential for memory and thinking. It usually shrinks and shrivels as we age, affecting our ability to reason and recall.

They also showed larger frontal, parietal and occipital lobes, which, together, signal a healthy, robust brain.

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Moderate exercise was best for brains

“It was surprising and encouraging” to see such widespread effects in the brains of people who were exercising so little, Raji said.

Of course, this study was associational, meaning it showed links between exercise and brain health, but not that exercise necessarily caused the improvements. So it’s possible other lifestyle factors or genetics were at play, or that people with big brains just happened to like exercise. But given the number of scans and the wide age range, Raji believes the effects of exercise on people’s brains were real and direct and would help to maintain our ability to think well as we grow older.

Exactly how exercise might be altering brains is impossible to say from this study. But Raji and his colleagues believe exercise reduces inflammation in the brain and also encourages the release of various neurochemicals that promote the creation of new brain cells and blood vessels.

In effect, exercise seems to help build and bank a “structural brain reserve,” he said, a buffer of extra cells and matter that could protect us somewhat from the otherwise inevitable decline in brain size and function that occurs as we age. Our brains may still shrink and sputter over the years. But, if we exercise, this slow fall starts from a higher baseline.

Perhaps best of all, the most effective exercise in the study was also relatively gentle. People who said they exercised moderately, meaning they could still chat as they worked out, wound up with somewhat greater brain volume than those who exercised more vigorously, such as by swift running.

But the numbers of vigorous exercisers were quite small, making comparisons suspect, Raji said, and their brain volume was still larger than among those who rarely, if ever, exercised at all.

Overall, any exercise of any type and in even small amounts is likely to be “a very good idea” for brain health, he said.Raichlen agrees. “Studies like this continue to provide strong evidence that moving your body even a small amount may have an impact on brain health, and that it is never too early, or too late, to start.”How exercise increases brain volume — and may slow memory declineImage Link Flickr
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