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A peek behind the science curtain

Bethany Brookshire



Even brain images can be biased

Study samples that are too rich and too well-educated may give a biased picture of brain development

MRI images of brains

Brain scan studies of large groups of people can tell us things about what the “average” brain looks like. But when the sample itself isn’t average, are the brains?

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An astonishing number of things that scientists know about brains and behavior are based on small groups of highly educated, mostly white people between the ages of 18 and 21. In other words, those conclusions are based on college students.

College students make a convenient study population when you’re a researcher at a university. It makes for a biased sample, but one that’s still useful for some types of studies. It would be easy to think that for studies of, say, how the typical brain develops, a brain is just a brain, no matter who’s skull its resting in. A biased sample shouldn’t really matter, right?

Wrong. Studies heavy in rich, well-educated brains may provide a picture of brain development that’s inaccurate for the American population at large, a recent study found. The results provide a strong argument for scientists to pay more attention to who, exactly, they’re studying in their brain imaging experiments.   

It’s “a solid piece of evidence showing that those of us in neuroimaging need to do a better job thinking about our sample, where it’s coming from and who we can generalize our findings to,” says Christopher Monk, who studies psychology and neuroscience at the University of Michigan in Ann Arbor.

The new study is an example of what happens when epidemiology experiments — studies of patterns in health and disease — crash into studies of brain imaging. “In epidemiology we think about sample composition a lot,” notes Kaja LeWinn, an epidemiologist at the University of California in San Francisco. Who is in the study, where they live and what they do is crucial to finding out how disease patterns spread and what contributes to good health.  But in conversations with her colleagues in psychiatry about brain imaging, LeWinn realized they weren’t thinking very much about whose brains they were looking at. Particularly when studying healthy populations, she says, there was an idea that “a brain is a brain is a brain.”

But that’s a dangerous assumption. “The brain does not exist in a vacuum, destined to follow some predetermined developmental pathway without any deviation,” LeWinn says. “Quite the opposite, our brains, especially in early life, are exquisitely sensitive to environmental cues, and these cues shape how we develop.” She wondered whether the sampling used in brain imaging studies might affect the results scientists were seeing.

To find out, LeWinn and her colleagues turned to the Pediatric Imaging, Neurocognition and Genetics — or PING — study. “It’s probably the best study we have of pediatric brain imaging,” she says.

Conducted across eight cities (including San Diego, New York and Honolulu), the study included more than 1,000 children from ages of 3 to 20. It recorded information about the children’s genetics, mental development and emotional function. And of course, it contains lots of images of their brains. The goal was to gain a comprehensive set of data on how children’s brain develop over time.

The PING database is large, well-organized and free for any scientists to look at. LeWinn and her colleagues examined the dataset for the race, sex, parental education and household income of its participants.

The end sample of 1,162 brains was a bit more diverse than the U.S. population. According to the 2010 census, the U.S. population is about 70 percent white, 14 percent black and 7.5 percent Hispanic. By contrast, the racial breakdown of the PING study was 42 percent white, 10 percent black and 24 percent Hispanic, with a larger percentage of “other” or mixed-race participants.  

“It was more diverse. That’s not common,” LeWinn says. This could be because the study sites were in large cities with diverse populations, she notes.

The PING study participants weren’t like the average American in other ways as well. The children were from richer households than Americans in general, and their parents were more highly educated. While only 11 percent of Americans have a post-college education, 35 percent of the PING study’s children had parents who had attended graduate school.

So LeWinn and her colleagues set out to make the data in the PING study look more like the data from the U.S. population as a whole. They applied sample weights to the brain imaging data, giving more weight to the brains of kids with poorer, less educated families, and adding additional weights to match the racial demographics of the United States.

In the newly weighted data, LeWinn and her group noticed that children’s brains matured more quickly. The cortex of the brain reached a peak surface area 2.4 years earlier than the original data would have suggested.  Some brains areas — such as the amygdala, an area associated with emotional processing — appeared to reach maturity a full four years faster. “Low socioeconomic status is associated with faster brain development, so that’s one potential explanation,” LeWinn notes. The group reported their findings October 12 in Nature Communications.

Unfortunately, this study can’t tell scientists if children’s brains actually are maturing faster than we think they are. The weighted sample isn’t a representation of what average brain development looks like in the United States. Instead, it’s just closer to what it might look like. “I would like to see this replicated in an actual sample of people who do represent the population,” says Kate Mills, a cognitive neuroscientist at the University of Oregon in Eugene.

But brain development wasn’t the point. Instead, the point is to show that when there’s a bias in the sample of participants in a brain imaging study, the data are biased, too. Even a large sample may not provide an accurate picture of brain development — if that sample has biases of its own.

It’s a strong argument for an unbiased sample, no matter the type of study. “It’s illustrating the impact of sample composition on these measures,” Mills says. “It’s not something we can disregard anymore.” She’s optimistic that change is nigh. “The datasets being collected now [in brain imaging studies] are already taking this more seriously.”

But it can be difficult to get study volunteers who represent a particular population. “A representative sample is expensive and challenging,” Monk notes. For his own recent brain imaging work, Monk has teamed up with a large existing project to get a larger sample, but even then, he says, “it’s still questionable whether or not the sample can be made representative.” People may not respond to the call. Volunteers may not show up. But unless scientists put in the extra legwork to make sure those people are accounted for, our picture of how human brains work won’t apply to everyone.

Health,, Nutrition

Low-fiber diets make gut microbes poop out

By Bethany Brookshire 3:14pm, January 15, 2016
A low-fiber diet makes for low bacterial diversity in mice. A new study shows those mice can then pass a denuded microbiome on to their offspring.
Health,, Physiology

High-intensity interval training has great gains — and pain

By Bethany Brookshire 4:25pm, January 5, 2016
Intense spurts of activity followed by brief rest can improve heart health, blood glucose and muscle endurance. But some question if the pain of HIIT workouts will impede the popularity.
Science & Society,, Psychology

In science, a lack of replication shouldn’t kill your reputation

By Bethany Brookshire 5:34pm, December 22, 2015
The proof is science is when a study is replicated. When it’s not, do scientists suffer? A new study says researchers may overestimate the negative effects.
Psychology,, Science & Society,, Neuroscience

Caffeine gives cocaine an addictive boost

By Bethany Brookshire 12:52pm, November 17, 2015
Not only is it popular to “cut” cocaine with caffeine, the combination might be more addictive.
Psychology,, Science & Society

No, cheese is not just like crack

By Bethany Brookshire 8:24am, October 30, 2015
Recent news reports claimed that a study shows cheese is addictive. But the facts behind the research show cheese and crack have little in common.
Psychology,, Science & Society

Views on bias can be biased

By Bethany Brookshire 7:00am, October 26, 2015
When presented with a study showing bias against women, male scientists are more inclined to nitpick the results. But a little intervention can go a long way toward gender equality in science.

That familiar feeling comes from deep in the brain

By Bethany Brookshire 4:29pm, October 13, 2015
Knowing what’s new and what we’ve seen before is at the base of memory. A new study shows that with a flash of light, scientists can change the firing of brain cells, and make the old new again.

How a fat hormone might make us born to run

By Bethany Brookshire 10:58am, September 25, 2015
Many runners finish long races in a euphoric mood. The underpinnings of this runner’s high may involve many chemicals, including the fat hormone leptin.
Clinical Trials,, Biomedicine,, Health

With flibanserin approval, a complicated drug takes the spotlight

By Bethany Brookshire 4:48pm, September 3, 2015
The Food and Drug Administration has approved the first drug to increase women’s sexual desire. But whether the benefits outweigh the side effects depends on who you ask.
Neuroscience,, Nutrition

The need to feed and eating for pleasure are inextricably linked

By Bethany Brookshire 4:44pm, August 27, 2015
Scientists used to think that the hunger and the pleasure from food could be easily distinguished. But new results show these systems are inextricably intertwined.
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