Mysteries of the Teenage Brain

Teenagers and young adults think and act differently from grownups. CU scientist Marie Banich is helping us see why. 

Marie Banich vividly remembers the day she first saw a human brain up close.

It was a Friday morning in 1983. She was standing in a frigid basement morgue, one of two graduate students among a group of University of Chicago medical students. The air was thick with the odor of formaldehyde. Her instructor was headed straight for her with a pale, lesion-riddled slice of gray matter from a cadaver cupped in his hands: "What do you think this person died from?" he asked, pushing the specimen closer.

"And then the world started to go dark," said Banich, who raced to a nearby sink.

She was mortified, but returned for the following week’s brain cutting lesson, lured by pure fascination with the then-little-understood 3-pound organ.

In adolescence, the human brain is still a work in progress." 

She wanted to know more: What made her mom a leftie? Why was her dad so good at building things? And what was going on inside her own brain to enable her to, in her early 20s, grasp things she couldn’t as a teen?

"I was fascinated early on with this idea that variations in the structure and function of the brain actually shape the way we experience our world," she said recently over coffee at a North Boulder café.

Now a professor at CU, Banich literally wrote the book on cognitive neuroscience: The fourth edition of her 1997 textbook Cognitive Neuroscience comes out later this year. And, as a pioneer in the use of functional magnetic resonance imaging (fMRI) brain imaging in adolescents, she’s helped shed light on why teenagers and young adults think — and act — differently from adults. This year, she and other CU researchers joined a landmark multi-center study that will follow 10,000 children from age nine for 10 years to explore precisely how their brain changes and what factors shape the change.

"Over the years, we’ve learned that adolescence is an incredibly important time for brain development, when the brain is particularly malleable and receptive to its environment," said Banich, of CU Boulder's Institute of Cognitive Science and Department of Psychology and Neuroscience. "But we only have broad brush strokes. We need a clearer picture."

Mapping the Teen Brain

Early brain research focused on diseased or injured brains. Neuroscientists would pore over the brain of someone who had died with cognitive deficits, looking for lesions as clues to which regions controlled what functions.

The 1990s advent of fMRI, which measures blood flow as a proxy for brain activity, allowed researchers to study healthy, living subjects and look beyond structure to see how the brain functioned. And because fMRI was painless, it could be used on minors.

Banich was hired at CU in 2000 in part because of her expertise in fMRI techniques, then relatively new. She worked with colleagues at CU Anschutz to get a neuroimaging program off the ground.

At the time, scientists thought the teen brain had fully developed because it was about the same weight and shape as an adult brain. But Banich’s work has shown otherwise.

"If you look at basic cognitive abilities, they are pretty mature by age 16," she said. "But if you look at the systems that allow you to control and coordinate your behavior — what some might refer to as your free will — that is all still developing."

The frontal lobes, which serve an executive function, and the white matter, which helps different regions talk to each other, continue to refine well into early adulthood, for example.

Meanwhile, the unfinished teen brain interacts with the world uniquely.

In a series of studies in the early 2000s funded by the MacArthur Foundation, Banich and colleagues showed that teens lack a mature ability to conceptualize the future, have less self-control, are less organized in their decision making, are more vulnerable to peer pressure — and are much more sensitive to reward than to punishment. By the early-to-mid 20s, these traits tend to fade.

Said Banich, "In adolescence, the carrot is huge and the stick is practically invisible."

Work by her and others helped influence the U.S. Supreme Court’s 2005 decision to abolish the death penalty for juveniles and 2010 decision to eliminate, for them, most life sentences without parole.

Banich’s early writings also inspired a new generation of neuroscientists.

"She was a role model," said UC Berkeley psychology professor Silvia Bunge, who read Banich’s early work as a graduate student and recently discussed a new Banich paper with her students. "She was part of the first generation of researchers doing brain imaging research to study human cognition, and she has moved mountains to advance this field."

Pruning the Gray Matter 

At a time when teens are biologically hardwired for risky behavior, their brains may also be especially sensitive to the consequences of it.

As Banich explains it, the teen brain is like a bush that’s leafing out in springtime, then selectively pruning itself to suit its environment. Exposure to drugs or trauma, for example, may stunt that natural development.

In one 2016 study of 466 adults who had used marijuana, Banich found that the earlier they started, the less developed their white matter, which plays a key role in helping different parts of the brain communicate. Those subjects reporting early or frequent drug use also showed alterations in the shape of certain brain regions — one associated with reward, another with memory.

Just what this means for behavior or cognition is still to be determined, but Banich said it suggests that "if you start early and use marijuana a lot, there are likely to be lasting effects on the brain."

In contrast, the experience of exercise or intellectual stimulation might fuel positive brain development, she believes.

A self-described nerd who grew up feeling out of place in a "stereotypical New Jersey town where the guys wanted to be football players and the girls wanted to be cheerleaders," Banich credits her parents for encouraging her to travel and to go to college.

"Looking back on it I think I was fortunate to end up in a really beautiful environment for my brain in my late teens and early 20s — it was the right stimulus at the right time."

Future Frontiers

In 2012, Banich founded the Intermountain Neuroimaging Consortium at CU Boulder, bringing together neuroscientists from across the region to study everything from addiction to pain to learning and memory. They all rely on fMRI as a tool. Instead of examining one slice of brain at a time, they can measure 40 slices, each divided into a 64 by 64 grid, every half second for a half hour during one scan of a healthy human.

Currently, the center is one of 21 test sites nationwide for the federally supported Adolescent Brain Cognitive Development (ABCD) study. It has already begun testing teenagers.

Banich, who serves as co-principal investigator with CU professor John Hewitt, director of the Institute for Behavioral Genetics, hopes the study will fill in some of the many remaining gaps in our understanding of the teenage brain. Is there a phase when the brain is most sensitive to the effects of drugs? Of excess screen time? How great a role does genetics play?

In her rare spare time, Banich travels — sightseeing in Italy, hiking in Sedona — to "press the reset button" on a brain she knows is prone to obsessing about work.

She also cares for her mother, who recently had a stroke, and pursues other projects aimed at divining the neurological underpinnings of addiction, which afflicted one close family member, and mental illness, which drove another to suicide.

"All of these life experiences have really humbled me and made me realize the limits to what I know about the brain," she said. "There’s still so much work to be done." 

Illustration by Dan Page; Photo courtesy Marie Banich