Even skimmers subvocalize key words. This is detectable, even among speed readers who think they don't do it, by the placement of electromagnetic sensors on the throat which pick up the faint nerve impulses sent to the muscles. Our brains just don't seem to be able to completely divorce reading from speaking. NASA has even built systems to pick up these impulses, using them to browse the web or potentially even control a spacecraft. Chuck Jorgensen, who ran a team at NASA in 2004 developing this system, said:
"Biological signals arise when reading or speaking to oneself with or without actual lip or facial movement. A person using the subvocal system thinks of phrases and talks to himself so quietly, it cannot be heard, but the tongue and vocal chords do receive speech signals from the brain."