The involvement of articulatory motor cortex in motor imagery of speech

Motor imagery of speech is thought to involve motor planning and simulated execution of speech actions. However, the precise cortical mechanisms subserving motor imagery are poorly understood. For instance, it is unclear to which extent articulatory motor cortex is involved during motor imagery of speech. We investigated the involvement of motor cortex during articulatory motor imagery through transcranial magnetic stimulation (TMS) to determine its contribution to motor imagery processing. We collected motor-evoked potentials (MEPs) to assess motor cortex excitability in three conditions: speech motor imagery, auditory speech perception, and baseline (no action). MEPs were collected at two time points after trial onset (200ms and 500ms), to measure the relative engagement of tongue primary motor cortex. The results showed that MEPs were larger for the motor imagery than for the baseline condition at 500ms post trial onset only, with no differences between the auditory speech perception and baseline conditions. These results suggest greater facilitation of tongue motor cortex during motor imagery of speech compared to rest, supporting the idea that motor cortex is engaged in motor simulation of complex speech actions.