MODULATION OF CORTICOSPINAL INFLUENCE OVER HAND MUSCLES DURING GRIPPING TASKS IN MAN AND MONKEY

Transcranial magnetic brain stimulation (TMS) was used to investigate corticospinal influences during a task in which human subjects had to reach out and grasp and lift an object. TMS applied to the hand area o f the motor cortex was delivered during eight different phases of the task. There was a striking phase-related modulation in the amplitude o f the short-latency EMG responses elicited by TMS in six arm and hand muscles. Although several mechanisms probably contribute to this modul ation, one result of their operation is a potentially greater influenc e of the cortex during particular phases of the task. Evidence is prod uced that one factor contributing to this modulation is a phase-relate d change in corticospinal excitability. The results are consistent wit h a strong excitatory corticospinal drive throughout the reach to brac hioradialis and anterior deltoid, which contribute to hand transport, and to the extrinsic hand muscles, which orientate the hand and finger tips. In contrast, the intrinsic hand muscles appear to receive their strongest cortical input as the digits close around and first touch th e object. TMS just before contact delayed the isometric parallel incre ase in load and grip forces necessary to lift the object. The particul arly strong EMG and behavioral effects seen at touch may reflect a pow erful interaction, at the cortical level, between cutaneous inputs sig nalling contact with the object and the effects of TMS. Central intera ctions between tactile afferent input and TMS were tested by deliverin g TMS at different times relative to the application of an unexpected load to an object held between the fingertips. The largest responses o ccurred when TMS was applied 60-80 ms after load onset. The enhanced c orticospinal influence that this represents probably contributes to th e powerful, short-latency boosting in grip force observed when the obj ect was suddenly subjected to an external load. Recording of corticosp inal cells in the primary motor cortex of the awake monkey suggests th at the phasic modulation observed with TMS may reflect the phasic-toni c pattern of corticomotoneuronal cell discharge during the task. Since the activation of corticospinal cells by low-intensity TMS is depende nt upon their level of excitability, EMG responses evoked by TMS durin g the performance of skilled tasks in man may, in part, reflect change s in the excitability of these cells.