Cerebellar control of motor activation and cancellation in humans: an electrophysiological study.

Execution of rapid ballistic movement is characterized by triphasic, alternating electromyographic bursts in agonist (AG) and antagonist (ANT) muscles. The ability to rapidly initiate movement and cancel ongoing action is a basic requirement for efficient control of motor function. Normal functioning of the cerebellum is necessary for the generation of AG and ANT muscle activity that should be both of appropriate magnitude and timing to control the dynamic phase of arm movements. We studied AG, ANT reaction time (RT), and RT differences in both motor activation (MA) and motor cancellation (MC) tasks, in response to an auditory stimulus. The results showed that right cerebellar transcranial magnetic stimulation (TMS) with a horizontally applied focal coil resulted in decreased AG RT and increased latency difference between AG RT and ANT RT (DIFF) in the ipsilateral upper limb during MC. No effect was apparent during sham stimulation, MA tasks, left upper limb recording, and other coil orientations. While the high correlation between AG and ANT RT suggests a close relationship in both MA and MC, significant DIFF changes point to an alteration of this relationship by TMS during MC. Although TMS resulted in significantly increased DIFF during MC tasks, this was not due to delayed ANT RT. This suggests that the short ANT burst observed invariably during MC may not be a cerebellum-generated response, but is derived from the cortical or subcortical level. The focal nature of our TMS coil and the horizontally effective orientation supports the hypothesis of interference with the parallel fiber system. Our findings contribute to the understanding of cerebellar neural networks involvement in movements, in particular, those pertaining to cessation of an ongoing action not previously addressed.