During adolescence, the structure and function of our brains undergo change; still, can this time in our lives be described as a period of greater brain plasticity? “No,” say University College London researchers after reviewing a series of neuroscience studies. However, they believe memory, social stress, and drug use are processed differently during this phase than in other life stages.

Unusually, their investigation also revealed that, as we enter middle age, we remember more from our adolescent years than either before or after this time in our lives. 

What exactly is plasticity?

"Plasticity refers to the brain's ability to adapt to environmental influences," Delia Fuhrman, Institute Of Cognitive Neroscience, told Medical Daily. "We can differentiate between experience-dependent and experience-expectant plasticity."

Throughout our lives, our brains retain an ability to learn and this is due to what scientists call "experience-dependent plasticity." Plasticity describes the nervous system’s ability to adapt to experiences, physiological changes, and demands from the environment. More simply, new experiences create fresh connections within the brain... and this is plasticity.

During sensitive periods, the plasticity of our brains is "experience-expectant" — knowing, so to speak, it will encounter a rush of stimuli, the brain is primed for the experience. Infancy, as you might expect, is a sensitive period.

During this sensitive period, for example, our visual system primes for the eye receiving a wealth of visual input. But what if the brain is deprived of this new information because, say, a baby is born with congenital cataracts? In such a case, the brain, specifically the visual cortex and other visual system features, will not develop as they should. If the cataracts are removed during the sensitive period, the visual system will quickly catch up; if removed after the period closes, though, the brain simply cannot adjust as quickly and may never recover fully.

"During early childhood it is particularly easy to learn a language," Fuhrmann suggests another example. "An infant's brain 'expects' to be exposed to linguistic input, making this time of life a sensitive period for language characterized by experience-expectant plasticity."

While the sensitive period during infancy is clear, neuroscientists wonder if the brain has a second sensitive period during adolescence?

Memory First 

To answer this question, the UCL scientists examined the most recent neuroscientific research, exploring “three areas of adolescent development that are proposed to be characterized by heightened plasticity: memory, social processing, and the effects of drug use.”

Immediately, the researchers discovered memory tests in different cultures show a “reminiscence bump.” Beginning at age 35, we are more likely to remember autobiographical events occurring between ages 10 and 30, while details of our life before or after this period are more likely to be forgotten. Our memories of music, books, films, and public events are also stronger during adolescence. At the same time, higher-order working memory abilities, which recruit frontal brain regions still in development, continue to improve during the early teens even though working memory reaches maturity in childhood.

This enhanced ability to form memories combined with a newfound ability to construct complex working memory might indicate a sensitive period, the researchers concluded, but much more evidence is needed to verify their claim.

Next, the UCL team explored studies indicating social stress and social exclusion have a greater impact during the teen years. With many mental illnesses beginning during the teens and early adulthood, the research team noted stress may be a trigger for these disorders. Teens are slower to forget frightening or negative memories, which suggests another vulnerability.

“Heightened plasticity may not only result in increased opportunities for development, but also in increased vulnerabilities,” wrote the authors. 

Adolescence is a time of risky health behaviors, including experiments with alcohol and drugs. The UCL authors argue this indicates young teens are particularly susceptible to peer influence, a form of social stress, as well as stronger effects from drug use. Meanwhile, studies on rodents prove that marijuana sensitivity increases during the adolescent years, while human investigations suggest cannabis use during the early teens results in decreased IQ and gray matter atrophy.

Analyzing their findings all together, the researchers speculate the adolescent brain processes memory, social stress, and drug use differently than in other life stages, yet “little conclusive evidence” verifies these findings in humans. Ultimately, they call for “more studies” to fill in any knowledge gaps. While the teen years generally represent a unique opportunity for enhanced brain plasticity, it may be the case that missed developmental opportunities can be made up later, which means this is not a sensitive period, by definition.

Source: Fuhrmann D, Knoll LJ, Blakemore SJ. Adolescence as a Sensitive Period in Brain Development. Trends in Cognitive Sciences. 2015.

Note: Direct quotes from Delia Fuhrman were added to this article after publication.