Do Corollary Discharges Contain Information About the Volume of Inner Speech? An ERP Study

When we move our articulator organs to produce overt speech, the brain generates a corollary discharge that acts to suppress the neural and perceptual responses to our speech sounds. Recent research shows that inner speech-the silent production of words in one's mind-is accompanied by a corollary discharge that contains information about the timing and content of inner speech. The aim of the present study was to determine whether this corollary discharge contains information about the volume of inner speech. To investigate this, participants watched an animation that provided them with precise knowledge about when they should produce a "loud" or "quiet" inner sound. At the same time, they heard an audible sound that was either similar or dissimilar to the volume of the inner sound. We found that producing the inner sound attenuated the N1-an ERP signature of auditory processing-and enhanced the slow negative wave-an ERP signature of anticipation and motor preparation-compared with listening, regardless of whether the volume of the inner sound was similar or dissimilar to the audible sound. We also found that the volume of the inner sound did not differentially modulate the N1 or slow negative wave. We speculate that this might be because one of the functions of corollary discharge is to protect our auditory receptors from desensitization caused by audible sounds, and this might be redundant in the context of inner speech because inner speech does not produce an audible sound. We conclude that there might be a functional difference between the neural processes that underlie the production of inner and overt speech.