Motor control of rapid sequential finger tapping in humans

We investigated in 29 healthy subjects a simple model of rapid independent finger movement: the rapid sequential tapping of adjacent fingers. Inter-tap interval (ITI) was measured for adjacent pairs of fingers in each direction. ITI was shorter in the ulnar-->radial direction than in the reverse direction [P < 0.001 for middle to index (M-->I) compared with index to middle (I-->M)]. There was a gradient across the hand such that in the ulnar-->radial direction, little to ring (L-->R) tapping was fastest and M-->I slowest; in the radial-->ulnar direction, the reverse was the case. Rectified surface electromyography (EMG) from finger extensors and flexors was averaged with respect to either the first or second tap. The interval between the flexor EMG burst and the tap was similar irrespective of the order of finger tapping, excluding a mechanical explanation of the timing difference. Transcranial magnetic stimulation (TMS) was applied at 0- to 50-ms intervals after the first tap. Interposed TMS delayed the second tap significantly more (P < 0.001) in the M-->I direction than in the I-->M direction. Motor-evoked potentials (MEPs) evoked by TMS interposed between taps showed a greater facilitation in the M-->I than in the I-->M direction (P < 0.001). Increasing intensity of TMS rendered subjects unable to produce the second tap, more frequently in the I-->M direction than in the M-->I direction. We have demonstrated a consistent pattern across the hand and postulate that finger-order-dependent differences in ITI and the gradient of these across the hand may reflect the mechanism of grasping and further that the cortical programming of finger tapping differs depending on finger order.