Neck muscle responses to abrupt free fall of the head: comparison of normal with labyrinthine-defective human subjects.

1. EMG responses from sternocleidomastoid (SCM) and orbicularis oculi were recorded in subjects who lay supine with their heads cradled in a sling. When the sling released abruptly, their heads fell freely. Normal and bilateral labyrinthine-defective subjects (LDs) were studied. 2. The normal response in SCM was a small burst of excitation at 22-25 ms latency, of 18 ms duration. This merged into a larger, later burst. The drop also produced eye blinks at 22-38 ms. 3. The onset of the SCM response in LDs was delayed (56-73 ms) even though the latency of their eye blinks was normal. 4. We conclude that the early response at approximately 22 ms in normal subjects is mediated by a vestibulocollic reflex. The delayed activity in LDs may be a stretch reflex. This is the first demonstration of the latency of the vestibulocollic pathway to natural stimulation in man.