Many disparate conditions such as depression, phobias, and attention deficit hyperactivity disorder (ADHD) may have the same underlying cause: a delay in “pruning,” a process in which unnecessary connections between brain cells disappear.
The finding is one of the largest brain-scanning studies conducted in adolescents and has been confirmed in several other data sets, including people of other ages. “We have been able to show that these different [conditions] All are related to an underlying neurobiological factor,” says Barbara Sahakian at Cambridge University.
The results lend weight to a controversial recent idea in neuroscience that many different conditions share a common cause – a concept known as the P factor.
Until now, this was based primarily on the fact that many people have more than one condition or that different conditions can be diagnosed at different times in their lives, as well as from DNA studies. Turns out that a single set of gene variants predisposes people to being older than many. conditions.
Now Sahakian and colleagues are proposing a neurobiological basis for the P factor, which they call the “neuropsychopathological (NP) factor.”
The researchers conducted their first analysis using a Current set of brain scanning images for nearly 2000 adolescents 14, where there was also information about their possible associated symptoms.
Participants or their parents completed questionnaires to generate scores for symptoms of mental health conditions such as depression, anxiety, eating disorders and phobias. They were also scored for ADHD and assessed for signs of autism symptoms.
People who had the highest scores for these conditions, indicating stronger symptoms, were also more likely to have a greater density of tissue in the prefrontal cortex, the outer region of the brain at the front of the head. The researchers say this indicates a lack or delay in pruning in their brains.
Pruning is a mysterious process that begins in childhood, though increases dramatically in adolescence. This is thought to be because synapses – the connections between brain cells – that are not used a lot are lost. “As you’re learning and your brain is becoming more efficient, you get rid of the extra synapses you don’t want,” Sahakian says.
It was not possible to measure differences in pruning delay or tissue density in this study, nor is it known whether those who had greater tissue density in the prefrontal cortex at this stage catch up with the rest of the population. The latter, at least to some extent, Sahakian says.
The finding was confirmed in four other sets of brain scanning data, including a second collection of brain scans and symptom scores for nearly 2000 adolescents, and two studies that included people in their 20s. This indicates that the lack of brain pruning is related to brain function even in adulthood.
Pruning deficiency has previously been proposed as a cause of schizophrenia, ADHD, and autism, but not other conditions.
robert plomin at King’s College London, who have been involved in genetic studies supporting the idea of an AP factor, say the newly proposed explanation is plausible as a contributor, but may not be the whole story. “You’re looking for something that is a general mechanism,” he says.