Researchers have found just where you lost your train of thought.
They've seen what's happening in the brain at the moment we get startled and lose our train of thought, and they've turned up a link between that just-lost thought and one of the classic symptoms of Parkinson's disease.
It might come when someone interrupts you, and you forget what you were saying. Or when a loud noise startles you.
"An unexpected event appears to clear out what you were thinking," said Adam Aron, a neuroscientist at the University of California, San Diego, who led the research.
Their experiment seems to show the brain engages a physical stopping order that interrupts the train of thought.
"We know what the electrical signals look like when somebody has to stop a movement," Aron told NBC News.
"The radically new idea is that just as the brain's stopping mechanism is involved in stopping what we're doing with our bodies it might also be responsible for interrupting and flushing out our thoughts," Aron said.
"We are providing a neural mechanism by which that happens," he added. "The same stopping system that gives you that kind of jolt when you are getting out of the elevator, and someone else is in your way and you have to stop, that same stopping system is stopping your train of thought."
The team focused on one part of the brain's stopping system called the subthalamic nucleus.
They got volunteers to put on an electrode cap and take on a computer-based memory task. First, they tested to see if a surprise could make people lose concentration.
They showed them boring strings of consonants and said they'd be shown another string and would have to decide quickly if it was identical to the first. A simple tone preceded the test part of the experiment.
The volunteers had to keep the first string of consonants in mind as they compared it to the second.
Sometimes, the researchers played the sound of a bird singing instead of the tone. It did make their 21 volunteers either slow down or make errors, they reported in the journal Nature Communications.
Then they got 22 different volunteers to do the test with the electrode cap on. And they got seven volunteers with Parkinson's disease to do a similar test, but they'd had surgery to implant the electrodes to treat their symptoms.
The electrodes could read the brain activity precisely in the Parkinson's patients, while the caps gave a rough idea of brain activity in the student volunteers.
The more the subthalamic nucleus was engaged by the startling sound, the more likely the volunteers were to make mistakes, the team found.
"We've shown that unexpected, or surprising, events recruit the same brain system we use to actively stop our actions, which, in turn, appears to influence the degree to which such surprising events affect our ongoing trains of thought," said cognitive neurologist Jan Wessel, who worked on the study and who is now at the University of Iowa.
The subthalamic nucleus of the brain is also involved in some particular symptoms of Parkinson's — the inability to change focus easily, and the inability to initiate motion. Parkinson's patients sometimes find they "freeze' because their brain somehow doesn't tell their legs to move, for instance.
Deep brain stimulation, in which electrodes are implanted in the brain, is meant to treat these and other symptoms.
And Parkinson's patients have also found they sometimes become over-focused and cannot change gears in their thoughts, Aron said.
If the same thing is happening in a healthy brain, it might mean the system itself is universal, the researchers said.
For instance, it may be what happens when people must make a "broad stop" — for example, to avoid colliding with someone else suddenly.
"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," Wessel said. That might offer a way to treat depression or post-traumatic stress disorder.
"We don't want to stretch it too far to make big claims about treating anything," Aron added. "This is highly speculative, but it could be fruitful to explore if the subthalamic nucleus is more readily triggered in ADHD."
It might be possible to somehow train people to overcome whatever's making them so easily distracted, he said.