Cortical activity increases in speech motor areas as a function of the subjective loudness of inner speech

Inner speech, sometimes referred to as an inner monologue or silent verbal thinking, is a common mental phenomenon, often experienced as a faint auditory image of words as spoken in one's own or a more generic voice. Brain-scanning research has shown that inner speech activates many of the same cortical areas responsible for spoken speech. This neural overlap has been leveraged with considerable success recently to decode the content of inner speech from cortical activity using cutting-edge data-analytic methods so that totally paralyzed patients can communicate. The hypothesis underlying the current study is that efforts to create subjectively louder inner speech will be associated with greater neural activity in cortical areas associated with overt speech.

While they were situated in an MRI scanner, we asked eight participants to repeat simple syllables as either soft or loud inner speech in a randomized order. For comparison purposes, the inner speech trials were followed by participants listening to a recording of the same syllables they made earlier, and a phase during which they repeated those syllables aloud.

As expected, we found significantly greater neural activity in cortical areas associated with speech motor activity during loud as compared to soft inner speech. We also that found greater suppression of neural activity in auditory perception areas was associated with louder inner speech.

The results are discussed with respect to their implications for the identification of inner speech in totally paralyzed individuals and the possibility of using neurofeedback to reduce the volume of negative inner speech.