Abstract
Although SMA is thought to contribute to the generation of anticipatory postural adjustments (APAs), which act to stabilize supporting body segments prior to movement, its precise role remains unclear. Non-invasive brain stimulation, such as transcranial direct current stimulation (tDCS), over the primary motor cortex (M1) has been shown to modulate cortical excitability, thereby influencing motor behavior and learning. However, only a few studies have reported that application of tDCS over the SMA changes the excitability of the M1 and motor learning process. This study was performed to elucidate whether cathodal tDCS applied over the SMA modifies the function of APAs. Cathodal tDCS and sham tDCS (2 mA) were applied over the SMA or left of the leg area of the M1 of 11 healthy subjects for 15 min. Subjects performed the task of self-paced rapid shoulder flexion on before, immediately after, and 15 min after tDCS. Electromyographic (EMG) activity was recorded from the deltoid anterior (DEL_A), as the prime mover muscle, and biceps femoris (BF), as the postural muscle during tasks. Latency differences (ΔEMG onset) with the two types of stimuli were calculated by subtracting the EMG burst onset of the BF from that of the DEL_A. Following cathodal tDCS over the SMA, the ΔEMG onset was significantly shortened compared to after sham tDCS, while tDCS over the M1 did not produce any significant effect. These results suggest that the SMA may play a critical role in the generation of voluntary movement accompanied by APAs. Cathodal tDCS could have beneficial method to investigate the function of the SMA in virtual lesion studies.
