Figure 2. A, A trial of the BTT starts with a display of a blue target line (TT), which was presented for an entire trial. An auditory IS (525 Hz, 126 ms) was presented 2000 ms (preparatory phase) after the onset of TT. Concurrently with IS, a white target dot started to move along the target line from the starting (middle of the screen) to the end of the TT at a constant speed (duration = 7000 ms). Participants were required to trace the moving white dot along a target line by simultaneous cyclical rotation of two dials with left and right index fingers. Concurrent visual feedback was provided by means of a red cursor displaying the actual tracking trajectory based on the contribution of both limbs. B, Participants were required to rotate the dials clockwise with same or different interhand frequency ratios (1:1, 1:3, 3:1) to be able to follow the target template (blue line). The angle of the blue line denotes the frequency ratio. For example, 1:3 notation indicates that the right hand is required to move three times faster than the left hand. C, The modulation of interhemispheric interactions between the DLPFC and contralateral primary motor cortex (M1), dorsal premotor and contralateral M1, and M1 and M1 during movement preparation were investigated. In a trial, the test stimulus was delivered either at the onset of the target template (TT0 ms) or at 50 ms before the onset of the auditory imperative signal (IS-50 ms), whereas the conditioning stimulus was applied to DLPFC, PMd, or M1 before TS with a designated IHI (i.e., 60 ms for DLPFC, 8 or 40 ms for PMd, and 10 or 40 ms for M1).