Subvocalization, or silent speech, is defined as the internal speech made when reading a word, thus allowing the reader to imagine the sound of the word as it is read. This is a natural process when reading and helps to reduce cognitive load, and it helps the mind to access meanings to enable it to comprehend and remember what is read. Although some people associate Subvocalization with moving one's lips, the actual term refers primarily to the movement of muscles associated with speaking, not the literal moving of lips. Most Subvocalization is undetectable (without the aid of machines) even by the person doing the subvocalizing.

There is no evidence that normal non-observable subvocalizing will negatively affect any reading process. At the more powerful rates (memorizing, learning, and reading for comprehension), subvocalizing is very detectable by the reader. At the less powerful, faster rates of reading, (skimming, and scanning) Subvocalization is less detectable. For competent readers, subvocalizing to some extent even at scanning rates is normal.

Advocates of speed reading generally claim that Subvocalization"places extra burden on the cognitive resources, thus, slowing the reading down." These claims are currently backed only by controversial, sometimes non-existent scientific research; in some cases, concepts are drawn from pseudo science and urban myths about the brain. Speed reading courses often prescribe lengthy practices to eliminate subvocalizing when reading. Normal reading instructors often simply apply remedial teaching to a reader who subvocalizes to the degree that they make visible movements on the lips, jaw, or throat.

It may be impossible to totally eliminate Subvocalization because people learn to read by associating the sight of words with their spoken sounds. Sound associations for words are indelibly imprinted on the nervous system—even of deaf people, since they will have associated the word with the mechanism for causing the sound or a sign in a particular sign language. Subvocalizing is an inherent part of reading and understanding a word, and micro-muscle tests suggest that subvocalizing is impossible to eliminate. Attempting to stop subvocalizing is potentially harmful to comprehension, learning, and memory. At the more powerful reading rates (100-300 words per minute), subvocalizing can be used to improve comprehension.

Subvocalization involves actual movements of the tongue and vocal cords that can be interpreted by electromagnetic sensors. Since 1999 NASA has been working on a system that can interpret a limited number of English words using nervous signals gathered from sensors placed on the throat's exterior. NASA initiated the research as part of its Extension of the Human Senses program and according to Dr. Chuck Jorgensen, Chief Scientist for Neuroengineering at NASA Ames Research Center who heads the research it could have potential applications for rescue operations people, security and special operations forces, people with vocal cord problems, and might even find a place in gaming.