Starts and Stops of Rhythmic and Discrete Movements: Modulation in the Excitability of the Corticomotor Tract During Transition to a Different Type of Movement

Movement is arguably one of the most important functions that the human brain has control over. Externally, we can divide all movements into rhythmic, continuous movements or discrete, one-time movements. To ensure smooth and purposeful behavior, the motor areas of the brain must have some way of controlling these two types of movements. Recent MRI (functional magnetic resonance imaging) studies show the preferential activation of contra-lateral M1 (primary motor cortex) in response to rhythmic wrist rotations in the opposite arm, whereas similar discrete wrist movements had much greater activation and in other areas of the brain. To understand the control of these two types of movements, we stimulated the M1 while having the subject perform either a rhythmic, discrete, or transition movement. We found that the motor evoked potentials (MEPs) amplitude increases in degree from regular discrete movement to discrete movement to transition movements (from rhythmic to discrete and vice versa). We anticipated that MEP amplitudes of regular discrete movements will be significantly greater those of regular rhythmic movements, which indicates that discrete movements involve more direct transmission of neurons from the cortical motor system than rhythmic movement due to their physiological nature. We confirmed that fact in our study. However, we were surprised to find out that the MEP amplitude after transition to discrete or rhythmic movements were almost identical, with an average difference of approximately 1%. This similarity between the transition movements suggested that the transition caused by voluntary inhibition operated by the frontal part of the brain, which corresponds to the characteristics of discrete movements. That might explain why the MEP amplitude of transition movements is greater than both rhythmic and discrete movements and why there is no stark distinction within the transition movements.