The Processing of Human Ballistic Movements Explored by Stimulation over the Cortex

1. When seated human subjects abducted one arm rapidly in response to a tone there was successively a burst of electromyographic (EMG) activity in the deltoid and latissimus dorsi muscles followed by another burst in the deltoid muscle. This triphasic pattern is typical of a ballistic 'focal movement'. There were also bursts of EMG activity in the contralateral latissimus dorsi, pectoralis major and abdominal muscles, which were assumed to be 'associated postural adjustments'. The same bilateral pattern of muscle activity occurred in a deafferented subject. 2. When subjects abducted the left arm rapidly, magnetic stimulation over the left motor cortex delayed the onset of the EMG burst in the right latissimus dorsi relative to the initial burst in the left deltoid. When subjects abducted the right arm rapidly, magnetic stimulation over the left motor cortex delayed the onset of the initial EMG burst in the right deltoid relative to the burst in the left latissimus. In each case, the delay of an EMG burst was greatest (about 80 ms) when the stimulus was given just before the burst was expected to occur. The inhibition of voluntary movements by transcranial stimulation was not associated with a reduction in the excitability of spinal motoneurons. 3. We conclude that focal ballistic movements and their associated postural adjustments are generated in exactly the same way. We postulate that these movements are preprogrammed, held in a memory until the 'go' signal and then released through both motor cortices to spinal motoneurons.