EMG responses in lower leg muscles elicited by head-taps.

Forehead-taps elicited short-latency motor responses in the lower leg muscle which, together with vestibulocollic reflexes, might contribute to multisensory control of posture. We tested this paradigm in human subjects standing upright in order to determine if the responses differed depending on whether the taps were to the forehead or temporal bone. Forehead-taps elicited short-latency inhibitory EMG responses in both gastrocnemius muscles with a mean latency of 48 to 99 ms. The right or left temporal bone-taps elicited short-latency inhibitory EMG responses in the ipsilateral gastrocnemius muscle of 47 to 77 ms, short-latency excitatory EMG responses in the contralateral gastrocnemius muscle of 46 to 78 ms, and short-latency excitatory EMG responses in the contralateral tibialis anterior muscle of 48 to 78 ms. The temporal bone-taps elicited short-latency inhibitory EMG responses in the ipsilateral gastrocnemius muscle and excitatory EMG responses in the contralateral gastrocnemius muscle, which were thought to be generated by vestibular afferents and convergent with vestibular input for the multisensory control of posture.