Speech imagery as corollary discharge

• Main Author: Scott, Mark
• Language: English
• Published: University of British Columbia 2012
• Online Access: http://hdl.handle.net/2429/42231

This thesis tests the theory that the sensory content of inner speech is constituted by corollary discharge. Corollary discharge is a signal generated by the motor system and is a “prediction” of the sensory consequences of the motor system’s actions. Corollary discharge normally functions in the nervous system to segregate self-caused sensations from externally-caused sensations. It does this, partially, by attenuating the nervous system’s response to self-caused sensations. This thesis argues that corollary discharge has been co-opted in humans to provide the sensory content of speech imagery. The thesis further tests the claim that the sensory detail contained in speech imagery is sufficiently rich and sufficiently similar to the representations of external speech sounds that the perception of external speech sounds can be influenced by inner speech. This thesis claims that the perception of external speech is altered because corollary discharge prepares the auditory system to hear those sensory features which the corollary-discharge signal carries. These claims were tested experimentally by having participants engage in specific forms of speech imagery while categorizing external sounds. In one set of experiments, when external sound and speech imagery were in synchrony and were similar in content, the perception of the external sound was altered — the external sound came to be heard as matching the content of the speech imagery. In a second set of experiments, the presence of corollary discharge in speech imagery was tested. When a sensation matches a corollary discharge signal, the sensation tends to have an attenuated impact. This attenuation is a hallmark of corollary discharge. In this set of experiments, when participants’ speech imagery matched an external sound, the perceptual impact of the external sound was attenuated. Proper controls ensured that it was the degree of match between the speech imagery and the external sound that was responsible for this attenuation, rather than some extraneous factor.