The differences in human spinal motoneuron excitability during the foreperiod of a motor task

Changes in excitability of the spinal motoneuron pool during the foreperiod, which was fixed at 0.8 s, in simple and choice reaction time experiments using ankle dorsiflexion and plantar flexion were studied in fourteen healthy normal subjects by combining the visually guided tracking and H-reflex testing methods. Almost all cases showed a significant facilitation in the soleus H-reflex within the time interval between 100 and 300 ms after a warning signal (Phase I), irrespective of movement direction and task modality. The pretibial H-reflex was also facilitated. On the other hand, variable effects were noted in the later half of the foreperiod, particularly within the 200 ms prior to the response signal (Phase II). Using a simple reaction task with dorsiflexion, six cases showed no changes in the soleus H-reflex, while four others showed statistically significant inhibitory changes and the remaining four showed facilitation. The inhibition and facilitation were often accompanied with very weak and unintended EMG activities in the pretibial and So1 muscles respectively. A similar finding was obtained in the simple plantar flexion task and the choice reaction task with dorsiflexion or plantar flexion. We suggest that the facilitation at Phase I represents a perceptual orienting response to a warning signal and the effects seen in Phase II represent the difference in the waiting attitude of each subject anticipating initiation of the coming task, or the preparatory "set" which primed the spinal motor structure in a biased position.