Some people have a voice inside their head that never stops. Mine was that way until I started meditating. I didn’t believe that it was me thinking, but I didn’t know until I could do things without a constant internal monologue.
There are people who almost never talk to themselves in their heads. They have to talk to other people about their thoughts in order to process them. And one of the first tenets of speed reading is stop saying the words in your head and just read.
We have good quality research proving that we do, especially from the deaf community.