Audio-spinal influence in man studied by the H-reflex and its possible role on rhythmic movements synchronized to sound.

An investigation was made of the time course of audio-spinal influences in man using the H-reflex technique and non-startling sounds. It was found that in all subjects the sound potentiated the H-reflex at a central latency of 80 msec, the peak facilitation (185%) being attained at 110--130 msec. The mean duration of this facilitation was 200 msec ranging from 120 to 460 msec. No inhibition was seen to follow the excitatory period. An habituation study showed a significant drop in peak facilitation after exposure to ten conditioning stimuli but a constant increase of the H-reflex above control level even after 60 presentations. The time course of this audiospinal facilitation was superposed over the EMG events during hopping to a simplified musical stimulus. In this situation, landing occurred some 50 msec prior to the ON beat or strong beat of the music. With this mode of synchronization, the timing of the ON and OFF beats of the musical stimulus would be suitable to potentiate the EMG events related respectively to the peak upwards acceleration determining the take-off and to the landing. It is inferred that during synchronized stereotyped movements to repetitive auditory stimuli, the motor events are timed to make best use of a potential audio-spinal facilitation.