Motor imitation is an innate ability of animals and human beings [1, 2]; it is very important in the acquisition of new motor skills. Motor imitation is widely used in the coaching process of various scenarios, such as physical education, driving, handwork, and so on. In the recent years, it has been applied to the rehabilitation of motor dysfunction caused by central nervous system diseases such as cerebral hemorrhage, cerebral infarction, and brain trauma [3, 4]. Exploring the mechanism of motor imitation is a current research hotspot.
Motor imitation can be divided into two processes namely motor observation and motor execution. The motor observation process primarily involves translating external visual motion information into one's own actions through mirror motor neurons (MNs) [5]. Motor execution is a process in which a conscious decision regarding a movement is made, a motor code is generated by combining sensory information processing in the cortex; the movement is then generated by the effectors through M1. In terms of form, motor imitation can be divided into either anatomical imitation (AI) or specular imitation (SI). In AI, the imitator’s left side corresponds to the demonstrator’s left side, while in SI, the imitator’s right side corresponds to the demonstrator’s left side. A recent study has shown that AI can modulate M1–M1 interhemispheric inhibition (IHI) more effectively than SI, while the plastic effects of AI can last for 30 minutes after the imitation task [6].
It is widely accepted that MNs play an important role in motor observation and execution. Recent studies with macaques have confirmed that the premotor cortex (PMC) is a node in the MNs circuit [5], especially for exogenous information-driven movements such as those associated with vision [7] or given visual goals [8].
The PMC, which includes the anterior margin of the anterior central gyrus, the posterior part of the middle frontal gyrus, and the superior frontal gyrus in the upper surface of the brain, corresponds to Brodmann's partial cellular structure region (BA6). The PMC is anatomically located between the dorsolateral prefrontal cortex (DLPFC) and M1. Functionally, this position in the motor cortex allows the PMC to receive direct input from the DLPFC and posterior parietal cortex, after which the information is processed, the output is projected onto the M1, and the motion is then performed [9].
We hypothesize that the PMC is an important node for information integration in the motor observation and execution network. To test this hypothesis, we delivered continuous theta burst stimulation (cTBS) to the left PMC of healthy participants to create a "virtual lesion", after which the participants performed a motor imitation task. cTBS is a repressive method of modulated repetitive transcranial magnetic stimulation (rTMS) administered for 40 seconds; however, the inhibitory effect lasts for approximately 60 minutes [10]. In this study, we investigate the changes in the cortical excitability to determine the contribution of the PMC during motor imitation training.