Distinct neural mechanisms support inner speaking and inner hearing

Area of Science:

• Cognitive Neuroscience
• Neuroscience
• Speech Production

Background:

• Inner speech, a covert form of speech production, is accompanied by sensory percepts.
• The cognitive and neural mechanisms generating inner speech percepts remain debated.
• Inner speech is proposed to have two distinct components: inner speaking and inner hearing.

Purpose of the Study:

• To investigate if distinct neural mechanisms support the phenomenologically distinct processes of inner speaking and inner hearing.
• To test the hypothesis that inner speaking relies on motor-to-sensory simulation, while inner hearing relies on memory retrieval.
• To determine the role of the speech motor system, specifically the primary motor cortex, in inner speech components.

Main Methods:

• Utilized transcranial magnetic stimulation (TMS) to probe neural activity.
• Measured modulations of TMS-evoked responses at the muscle level.
• Stimulated the lip primary motor cortex in 31 participants during inner speaking and inner hearing tasks.

Main Results:

• Inner speaking significantly increased primary motor cortex excitability compared to inner hearing.
• This effect was more pronounced when participants silently produced syllables that strongly recruit lip movements.
• Data corroborated the prediction of differential involvement of the speech motor system.

Conclusions:

• The findings support models where the primary motor cortex is involved in inner speech.
• Distinct neural mechanisms underlie inner speaking (motor simulation) and inner hearing (memory retrieval).
• Clarifies the neural implementation of silently speaking in one's mind.