Derailed train of thought? Blame brain's stopping system

Did you forget what it was you were about to say as the phone rang? According to the researchers, the same brain system that is involved in interrupting or stopping movement in our bodies also interrupts cognition and derails your train of thought.

The findings may give insights into Parkinson's disease, says neuroscientist Adam Aron from University of California-San Diego.

The disease can cause muscle tremors as well as slowed-down movement and facial expression.

Parkinson's patients may also present as the "opposite of distractible", often with a thought stream so stable that it can seem hard to interrupt.

Along with collaborators at Oxford University and first author Jan Wessel in the Aron Lab, Aron found that the same brain system that is implicated in "over-stopping" motor activity in these patients might be also be keeping them over-focused.

The current study focuses particularly on one part of the brain's stopping system -- the subthalamic nucleus (STN).

This is a small lens-shaped cluster of densely packed neurons in the midbrain.

The study analysed signals from the scalp in 20 healthy subjects as well as signals from electrode implants in the STN of seven people with Parkinson's disease.

The STN is the main target for therapeutic deep brain stimulation in Parkinson's disease.

The results, published in the journal Nature Communications, show that unexpected events manifest the same brain signature as outright stopping of the body.

They also recruit the STN.

The more the STN was engaged -- or the more that part of the brain responded to the unexpected sound -- the more it affected the subjects' working memory and the more they lost hold of what they were trying to keep in mind.

A possible future line of investigation, Aron said, is to see if the STN and associated circuitry plays a role in conditions characterised by distractibility, like Attention Deficit Hyperactivity Disorder (ADHD).

"It might also be potentially interesting to see if this system could be engaged deliberately - and actively used to interrupt intrusive thoughts or unwanted memories," said Wessel.