There has recently been a paper in eLife by Whitford etal, Neurophysiological evidence of efference copies to inner speech, Dec 2017 doi 10.7554/eLife.28197.001, examining inner speech. They find it very similar to overt speech.
When we speak a series of motor commands are prepared and executed by the mouth, throat and vocal cords. Copies of these commands, called efferemce copies are used to predict what the auditory area will hear. This prediction is called the internal forward model. When incoming sounds match the prediction, the auditory area lowers its response to the speech. This efference copy mechanism applies to other motor commands and is why we cannot tickle ourselves. The sensory pattern that will result from an action is predicted so that self-generated sensory input is attenuated compared to input that is not self-generated. In the case of speech, the actual sounds are predicted and when input arrives at the right time that matches the expected sound, the sound is dampened. This dampening can be measured. The sounds result in a particular brain wave which has an amplitude that matches the volume of the sound and it can be seen in EEG traces. It is called N1 indicating that it is the first event produced negative wave. This wave has less amplitude for sounds in self-generated speech than for identical sounds that were not self-generated.
In their introduction the author say “…the central aim of the present study is to explore whether N1-suppression, which has consistently been observed in response to overt speech, also occurs in response to inner speech, which is a purely mental action. Inner speech - also known as covert speech, imagined speech, or verbal thoughts - refers to the silent production of words in one’s mind. Inner speech is one of the most pervasive and ubiquitous of human activities; it has been estimated that most people spend at least a quarter of their lives engaged in inner speech. An influential account of inner speech suggests that it ultimately reflects a special case of overt speech in which the articulator organs (e.g., mouth, tongue, larynx) do not actually move; that is, inner speech is conceptualized as ‘a kind of action’. Support for this idea has been provided by studies showing that inner speech activates similar brain regions to overt speech, including audition and language-related perceptual areas and supplementary motor areas, but does not typically activate primary motor cortex. While previous data suggest that inner and overt speech share neural generators, relatively few neurophysiological studies have explored the extent to which these two processes are functionally equivalent. If inner speech is indeed a special case of overt speech - ‘a kind of action - then it would also be expected to have an associated internal forward model.” The researcher show that thinking of a particular sound (such as ba) attenuates the N1 signal of an external sound if they are the same sound at the matching timing. The inner speech efference copy and its internal forward model are produced in inner speech and can dampen an external sound if it matches the internal one.
Here is their abstract. “Efference copies refer to internal duplicates of movement-producing neural signals. Their primary function is to predict, and often suppress, the sensory consequences of willed movements. Efference copies have been almost exclusively investigated in the context of overt movements. The current electrophysiological study employed a novel design to show that inner speech – the silent production of words in one’s mind – is also associated with an efference copy. Participants produced an inner phoneme at a precisely specified time, at which an audible phoneme was concurrently presented. The production of the inner phoneme resulted in electrophysiological suppression, but only if the content of the inner phoneme matched the content of the audible phoneme. These results demonstrate that inner speech – a purely mental action – is associated with an efference copy with detailed auditory properties. These findings suggest that inner speech may ultimately reflect a special type of overt speech.”
This probably explains the nature of ‘hearing voices’. If this mechanism failed and inner speech was not properly predicted, it would appear to be external speech. It would not be ‘owned’ by the individual.