Brain maturation mediates cognitive improvement in teens

Cortical thinning and reduced sleep slow wave activity (SWA), both markers of brain maturation in adolescence, facilitate age-related improvements in cognitive function, according to a Singapore study.

“These findings affirm prior work showing neural refinement for learning efficiency still taking place in late adolescence,” the researchers said.

A total of 109 adolescents (aged 15–19 years, 49 boys) were enrolled and subjected to magnetic resonance imaging (MRI) and polysomnography (PSG). Over 2 months, participants also accomplished a battery of cognitive tests designed to assess their executive function, speed of processing and working memory, sustained attention, and nonverbal intelligence.  

Of the cognitive functions assessed, nonverbal intelligence (r, 0.24; p=0.01) and speed of processing (r, 0.33; p=0.001) improved significantly with age. No such effect was reported for sustained attention, working memory, or executive function (r, <0.04; p>0.72). [Sleep 2021;doi:10.1093/sleep/zsab206]

Likewise, after controlling for sex, there were significant and inverse partial correlations between age and cortical thickness for the left (r, –0.32; p=0.001) and right (r, –0.32; p=0.001) hemispheres, suggesting that cortical thinning occurred with increasing age.

Further analysis per lobe showed that such an interaction was strongest in the parietal (left hemisphere: r, –0.34; p<0.001; right hemisphere: r, –0.36; p<0.001) and temporal (left hemisphere: r, –0.46; p<0.001; right hemisphere: r, –0.43; p<0.001) regions and was less pronounced in the left hemisphere occipital region (r, –0.20; p=0.04).

The researchers also reported that SWA was associated negatively with age (r, –0.35; p<0.001) and positively with cortical thickness measures.

To assess whether developmental brain changes affected age-related cognitive improvements, the researchers performed regression analysis and used serial medication models. They found that cortical thickness in the left middle (b, 0.141, 95 percent confidence interval [CI], 0.0057–0.3478] and right middle (b, 0.2078, 95 percent CI, 0.0275–0.5245) temporal regions exerted an indirect effect on improvements in speed of processing.

Similarly, cortical thickness in the left middle (b, 0.0317, 95 percent CI, 0.0003–0.0888) and right middle (b, 0.0313, 95 percent CI, 0.0024–0.0833) temporal regions had a significant and indirect effect on nonverbal intelligence.

Notably, both cortical thickness and SWA were found to be full mediators of age-related cognitive improvements, as direct effects ceased to be significant after stepwise inclusion of cortical thickness and SWA into the model. Moreover, the proportion of variance explained by the models applied significantly increased regardless of whether SWA or cortical thickness was incorporated first.

“In summary, our findings show that age-related improvements in cognition during mid-late adolescence are mediated by cortical thinning and reductions in slow wave activity, particularly in the middle/superior temporal regions—which undergo the most maturational change in this age range,” the researchers said.

“Future work should investigate multimodal associations of brain, sleep, and cognitive development longitudinally and explore the impact of brain insults or sleep habit changes during specific developmental periods,” they added.