Reading involves motor signals to the speech system that do not reach a threshold for vocalization.

“The signal from the paper goes through your eyes. It goes to the back of your brain, to your visual cortical regions eventually. And then you now got to interpret that signal in your visual pathway of what you're reading. How are you going to do that in terms of speech? That visual signal then goes to your speech pathway in the motor cortex in front here, in Broca's area. And you silently speak what you read in your brain without moving your muscles. And so your speech pathway is now speaking what you're reading. Now to finish it off, that signal is sent to your auditory pathways so you can hear what you're speaking in your own head.”