Motor Pool Excitability to a Reaction Time Stimulus Preceded by a Varied Foreperiod

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Abstract
Motor pool excitability, H-reflex amplitude, was studied in RT responses preceded by varied foreperiods. Rapid plantar flexion of the foot followed auditory warning signals, varied foreperiods, and visual RT stimuli. Response speed was latency of EMG onset in the soleus muscle (PMT). The H-reflex stimulus was either omitted (none), or presented at one of three intervals in the movement response period: 0 (simultaneous with RT stimulus), 100 or 200 ms following stimulus. Control H-reflexes were taken before and after RT trials. The motor pool was not differentially excited following the random foreperiods. In fact it was not elevated over control values until late into the preparatory period (200 ms) which probably reflects the direct expression of the motor command. Using the H-reflex paradigm in combination with a RT response strongly altered the PMT when the H-reflex was presented early in the preparatory period (0). Since motor pool excitability changes were not evident early in the movement preparation period, they poorly predicted PMTs.