During the COVID-19 pandemic, governments around the world implemented restrictive measures -- such as stay-at-home orders and school closures -- to mitigate the spread of the respiratory illness. It's been well-documented that this disruption of daily routines and social activities had a negative impact on the mental health of adolescents.

According to Report, which cites Science Daily, adolescence, the period of transition between childhood and adulthood, is marked by dramatic changes in emotional, behavioral and social development. It's also a time when a sense of self-identity, self-confidence and self-control are developed. The pandemic reduced social interaction for teenagers and led to documented reports of anxiety, depression and stress, especially for girls.

New research from the University of Washington, published online September 9 in the Proceedings of the National Academy of Sciences, found the pandemic also resulted in unusually accelerated brain maturation in adolescents. This maturation was more pronounced in girls. When measured in terms of the number of years of accelerated brain development, the mean acceleration was 4.2 years in females and 1.4 years in males.

"We think of the COVID-19 pandemic as a health crisis," said Patricia Kuhl, senior author and co-director of the UW Institute for Learning & Brain Sciences (I-LABS), "but we know that it produced other profound changes in our lives, especially for teenagers."

Brain maturation is measured by the thickness of the cerebral cortex, the outer layer of tissue in the brain. The cerebral cortex naturally thins with age, even in teens. Chronic stress and adversity are known to accelerate cortical thinning, which is associated with an increased risk for the development of neuropsychiatric and behavioral disorders. Many of these disorders, such as anxiety and depression, often emerge during adolescence -- with females at a higher risk.

The UW research began in 2018 as a longitudinal study of 160 teens between 9 and 17 years, with the original objective of evaluating changes in brain structure during typical adolescence.

The cohort was slated to return in 2020, but the pandemic delayed the repeat tests until 2021. By then, the original intent to study typical teen development was no longer viable.

"Once the pandemic was underway, we started to think about which brain measures would allow us to estimate what the pandemic lockdown had done to the brain," said Neva Corrigan, lead author and research scientist at I-LABS. "What did it mean for our teens to be at home rather than in their social groups -- not at school, not playing sports, not hanging out?"

Using the original 2018 data, researchers created a model of expected cortical thinning during the teen years. They then re-examined the brains of the adolescents, over 80% of whom returned for the second set of measurements. The teens' brains showed a general effect of accelerated thinning across adolescence, but this was much more pronounced in females. The cortical thinning effects in females were seen all over the brain, in all lobes and both hemispheres. In males, the effects were only seen in the visual cortex.