In sign language communication, people use their hands as well as the mouth - but now a study has tried to understand whether the two are used together or separately.
Researchers David P. Vinson and colleagues at University College London found that the hand and lip movements are separate in the signer's brain, not part of the same sign.
The team recruited both deaf and hearing signers, all of whom grew up signing with deaf parents.
"We noticed that there were quite a few cases where the hands and the mouth seemed to be doing something different," said Vinson.
They found participants making mistakes-signing and mouthing "banana" when the picture was an apple, for example.
But when they were translating English words, the hands made the same kind of mistakes, but the lips didn't. This suggests that the lip movement isn't part of the sign.
This means that in essence, they were reading out English words and pronouncing them out loud at the same time - like processing two languages at the same time.
This study appears in Psychological Science, a journal of the Association for Psychological Science.