Neural representations of imagined speech revealed by frequency-tagged magnetoencephalography responses

Speech mental imagery is a quasi-perceptual experience that occurs in the absence of real speech stimulation. How imagined speech with higher-order structures such as words, phrases and sentences is rapidly organized and internally constructed remains elusive. To address this issue, subjects were tasked with imagining and perceiving poems along with a sequence of reference sounds with a presentation rate of 4 Hz while magnetoencephalography (MEG) recording was conducted. Giving that a sentence in a traditional Chinese poem is five syllables, a sentential rhythm was generated at a distinctive frequency of 0.8 Hz. Using the frequency tagging we concurrently tracked the neural processing timescale to the top-down generation of rhythmic constructs embedded in speech mental imagery and the bottom-up sensory-driven activity that were precisely tagged at the sentence-level rate of 0.8 Hz and a stimulus-level rate of 4 Hz, respectively. We found similar neural responses induced by the internal construction of sentences from syllables with both imagined and perceived poems and further revealed shared and distinct cohorts of cortical areas corresponding to the sentence-level rhythm in imagery and perception. This study supports the view of a common mechanism between imagery and perception by illustrating the neural representations of higher-order rhythmic structures embedded in imagined and perceived speech.