The team of researchers at Stanford University looked at each other in astonishment. They were examining brain scans taken from two groups of Californian teenagers, and expected to find some differences between them. But the ones that emerged were much, much bigger than anybody could have anticipated.¹

The first group were studied before the pandemic and the second after Covid-19 hit. What the scientists found was that physical changes to the brain that normally take place during adolescence had speeded up in the second group. These included the thinning of the cortexThe outer layer of an organ, especially the brain. and growth of the amygdalaAn almond-shaped structure in the middle of the brain that is thought to hold some of our oldest instincts, such as the "fight or flight" impulse. and hippocampusA ridge of grey matter tissue in the brain associated with memory. .  

"Brain age difference was about three years - we hadn't expected that large an increase given that the lockdown was less than a year [long]," says the head of the project, Professor Ian Gotlib.

The original purpose of the study was to find how stress early in life affects teenagers' mental health. The second group reported stronger symptoms of depressionLow mood that affects someone's daily life, and can last for weeks or months. and anxietyA feeling of unease or worry. It can be mild or very severe. than the first.

But it is hard to know what to make of these results. Did the faster ageing of the teenagers' brains contribute to their mental stress? Was it even a bad thing?

"In older adults, these brain changes are often association with reduced cognitiveRelating to the processes of thinking and reasoning. functioning," says Ian Gotlib. "It's not clear yet what they mean in adolescents."

Another expert, Professor Michael Thomas from Birkbeck, University of London, suggests that the changes could be beneficial - but also that they might be temporary:

"These data can't tell us whether negative long-term outcomes are inevitable, or whether the plasticity of the brain will allow this generation to bounce back."

We should not be surprised by this uncertainty. According to Professor Pankaj Sah of Queensland University, "We know more about space than we know about the brain. There's many, many connections in the brain - probably more than there are stars in the Milky WayThe galaxy that contains our solar system. It has the structure of a disc, but looks to us like a band because we are inside it.."

Among the questions yet to be answered are why we dream and why some people suffer from dementiaA syndrome associated with memory loss and other declining brain functions.  in old age but not others. Hardest of all is the mystery of the conscious mind: how we know that we exist.

The problem, says psychiatrist Thomas Insel, is that the brain is so much more complex than any other part of the body, and scientists have not had the tools they need to study it.

A breakthrough may be near, thanks to advances in scanning and initiatives like the EU's Human Brain Project. This aims to create a full-scale computer model encapsulating everything known about the brain. In the US, the government has invested $100m (£80m) in its own BRAIN project.

The expense and human cost of conditions such as Alzheimer's disease have been a major spur.

But according to Professor Morten Overgaard of Aarhus University, there are some things we may never discover: "The question of consciousness and why we have subjective experience seems to be just as mysterious now, even with all this progress, as it was 100 years ago."

Yes: People are unable to agree what the mind is, let alone how it works. The brain is so astonishingly complex that there is no way humans will ever be able to understand it completely.

No: There have been many things that people once thought impossible to explain, including light and life. Thanks to advances in technology it is only a matter of time before we can map the brain fully.

Or... The mind is an illusion, so there is no point in trying to investigate it. Only physical things have an existence: therefore although the brain is real, our thoughts and feelings are not.