A generation ago, parents worried about the effects of TV; before that, it was radio.

Now, the concern is "screen time”, a catchall term for the amount of time that children, especially preteens and teenagers, spend interacting with TVs, computers, smartphones, digital pads and video games.

This age group draws particular attention because screen immersion rises sharply during adolescence, and because brain development accelerates then, too, as neural networks are pruned and consolidated in the transition to adulthood.

On Sunday evening, CBS’ "60 Minutes” reported on early results from the ABCD Study (for Adolescent Brain Cognitive Development), a $300 million project financed by the National Institutes of Health. The study aims to reveal how brain development is affected by a range of experiences, including substance use, concussions and screen time.

As part of an exposé on screen time, "60 Minutes” reported that heavy screen use was associated with lower scores on some aptitude tests, and to accelerated "cortical thinning” — a natural process — in some children. But the data is preliminary and it is unclear whether the effects are lasting or even meaningful.

Does screen addiction change the brain?

Yes, but so does every other activity that children engage in: sleep, homework, playing soccer, arguing, growing up in poverty, reading, vaping behind the school. The adolescent brain continually changes, or "rewires” itself, in response to daily experience and that adaptation continues into the early to mid-20s.

What scientists want to learn is whether screen time, at some threshold, causes any measurable differences in adolescent brain structure or function, and whether those differences are meaningful. Do they cause attention deficits, mood problems, or delays in reading or problem-solving ability?

Have any such brain differences been found?

Not convincingly. More than 100 scientific reports and surveys have studied screen habits and well-being in young people, looking for emotional and behavioural differences, as well as changes in attitude, such as in body image.

In 2014, scientists from Queen’s University Belfast reviewed 43 of the best designed such studies. The studies found that social networking allows people to broaden their circle of social contacts in ways that could be both good and bad, for instance by exposing young people to abusive content.

The review’s authors concluded that there was "an absence of robust causal research regarding the impact of social media on the mental well-being of young people.” In short: Results have been mixed and sometimes contradictory.

Psychologists have also examined whether playing violent video games is connected to aggressive behaviour. More than 200 such studies have been carried out; some researchers found links, others have not.

One challenge in studying this and other aspects of screen time is identifying the direction of causality: Do children who play a lot of violent video games become more aggressive as a result, or were they drawn to such content because they were more aggressive from the start?

Even if scientists found strong evidence of a single, measurable effect — if, say, three hours of daily screen time was associated with a heightened risk of being diagnosed with ADHD — such a clear association would not necessarily suggest there were any consistent, measurable differences in brain structure.

Individual variation is the rule in brain development. The size of specific brain regions such as the prefrontal cortex, the rate at which those regions edit and consolidate their networks, and the variations in these parameters from person to person make it very difficult to interpret findings. To address such obstacles, scientists need huge numbers of research subjects and a far better understanding of the brain.

Isn’t that what the NIH study aims to address?

Yes. The ongoing ABCD study expects to follow 11,800 children through adolescence, with annual magnetic resonance imaging, to see if changes in the brain are linked to behaviour or health.

The study began in 2013, recruiting 21 academic research centres, and initially focused on the effects of drug and alcohol use on the adolescent brain. Since then, the project has expanded and now includes other targets such as the effects of brain injury, screen time, genetics and an array of "other environmental factors.”

The recently published paper covered by "60 Minutes” provided an early glimpse of the anticipated results. A research team, based at the University of California, San Diego, analysed brain scans from more than 4,500 preteens and correlated those with the children’s amount of screen time (as reported by the children themselves in questionnaires) and their scores on language and thinking tests. The findings were a mixed bag.

Some heavy screen users showed cortical thinning at younger ages than expected; but this thinning is part of natural brain maturation, and scientists do not know what that difference means. Some heavy users scored below the curve on aptitude tests, others performed well.

But the accuracy of self-reported screen time estimates is hard to ascertain. And the association between small differences in brain structure and how people actually behave is even more vague. As a result, researchers effectively are multiplying one uncertain relationship by another and need to make statistical adjustments.

Clear conclusions are extremely hard to come by and complicated by the fact that a brain scan is no more than a snapshot in time: a year from now, some of the observed relationships could be reversed.

The authors acknowledge as much.

"This diversity of findings provides an important public health message, that screen media activity is not simply bad for the brain or bad for brain-related functioning,” they concluded.

In other words, the measured effects may be good, or, more likely, not meaningful at all, until further research demonstrates otherwise.

But surely screen addiction is somehow bad for the brain?

It is probably both bad and good for the brain, depending on the individual and his or her viewing habits. Many people who are socially isolated, as a result of abuse, personal quirks or developmental differences such as Asperger’s syndrome, establish social networks through their screens that would be impossible to find in person.

Disentangling negative consequences to physical brain development from positive ones will be enormously difficult, given the many other factors potentially in play: the effects of marijuana use, drinking, vaping, genetic differences, changes at home or school, and the entire emotional storm of adolescence.

Most parents are probably already aware of the biggest downside of screen time: the extent to which it can displace other childhood experiences, including sleep, climbing over fences, designing elaborate practical jokes and getting into trouble.

Indeed, many parents — maybe most — watched hours of TV a day themselves as youngsters. Their experiences may be more similar to their children’s than they know.

The New York Times