Can short-latency vestibulospinal reflexes in lower leg muscles be elicited by tapping the head?

A gentle tap on the forehead evoked short-latency motor responses in the st ernocleidomastoid muscle, which may be vestibulocollic reflexes with charac teristics similar to those elicited by clicks. We tested this paradigm in s ubjects standing upright, in order to determine the differential effects of taps on the forehead on the patterns and latencies of spinal stretch and v estibulospinal reflexes, which are important for postural control, Taps on the forehead elicited short-latency inhibitory electromyographic responses in both gastrocnemius muscles with a mean latency of 48.3 +/- 3.1 ms (onset ) to 98.3 +/- 6.3 ms (end). Taps on the sternum elicited similar responses from the gastrocnemius muscles, indicating that vestibular stimulation is n ot essential for eliciting these responses. Both responses may play a role in predetermining the strategy for correcting body perturbations. The actua l reflex is probably elicited by somatosensory input from the neck, which c onverges with vestibular input for the multisensory control of posture.