She likens the feeling to seeing "someone scurrying in the distance" or "tuning into footsteps behind you".
"For some people, you don't tune it out again. Your brain is continually monitoring."
In noisy, hectic modern life, it's not very useful, she says.
And if you keep getting triggered, the frustration and anger can build.
For some people, it's strangers' habits that are most irksome, while for others, it's loved ones.
One common way people try to manage the condition is by avoiding looking at fidgeting or by distracting themselves, Dr Gregory says.
Others may try to avoid people entirely, as much as they can.
If there is only one isolated visual trigger - such as hair twirling - the expert says it is sometimes possible to use reframing therapy to help the person view the situation in a more positive way.
"You might look at it deliberately and create a new backstory for why someone is doing that movement."
That can help reduce the anger and anxiety, she says.
"A lot of people feel really embarrased or ashamed that they get such strong reactions," Dr Gregory adds.
"That, itself, can be a problem because suppressing your emotions can intensify them and make them worse."