Time course of determination of movement direction in the reaction time task in humans

The primary motor cortex produces motor commands that include encoding the direction of movement. Excitability of the motor cortex in the reaction tim e (RT) task can be assessed using transcranial magnetic stimulation (TMS). To elucidate the timing of the increase in cortical excitability and of the determination of movement direction before movement onset, we asked six ri ght-handed, healthy subjects to either abduct or extend their right thumb a fter a go-signal indicated the appropriate direction. Between the go-signal and movement onset, single TMS pulses were delivered to the contralateral motor cortex. We recorded the direction of the TMS-induced thumb movement a nd the amplitude of motor-evoked potentials (MEPs) from the abductor pollic is brevis and extensor pollicis brevis muscles. Facilitation of MEPs from t he prime mover, as early as 200 ms before the end of the reaction time, pre ceded facilitation of MEPs from the nonprime mover, and both preceded measu rable directional change. Compared with a control condition in which no vol untary movement was required, the direction of the TMS-induced thumb moveme nt started to change in the direction of the intended movement as early as 90 ms before the end of the RT, and maximum changes were seen shortly befor e the end of reaction time. Movement acceleration also increased with maxim a shortly before the end of the RT. We conclude that in concentric movement s a change of the movement direction encoded in the primary motor cortex oc curs in the 200 ms prior to movement onset, which is as early as increased excitability itself can be detected.