1. We studied the effect of the continuous vibration of symmetrical dorsal
neck muscles in seven normal subjects during (a) quiet standing, (b) steppi
ng in place movements and (c) walking on the treadmill. The experiments wer
e performed in a darkened room and the subjects were given the instruction
not to resist the applied perturbation. In one condition the velocity of th
e treadmill was controlled by feedback from the subject's current position.
Head, trunk and leg motion were recorded at 100 Hz.
2. In normal standing, neck vibration elicited a prominent forward body swa
y. During stepping in place, neck vibration produced an involuntary forward
stepping at about 0.3 m s(-1) without modifying the stepping frequency. If
the head was turned horizontally 45 and 90 deg to the right or to the left
, neck muscle vibration caused stepping approximately in the direction of t
he head naso-occipital axis. For lateral eye deviations, the direction of s
tepping was roughly aligned with gaze direction.
3. In treadmill locomotion, neck vibration produced an involuntary step-lik
e increase of walking speed (by 0.1-0.6 m s(-1)), independent of the initia
l walking speed. During backward locomotion, the walking speed tended to de
crease during neck vibration.
4. Thus, continuous neck vibration evokes changes in the postural reference
during quiet standing and in the walking speed during locomotion. The resu
lts suggest that the proprioceptive input from the neck is integrated in th
e control of human posture and locomotion and is processed in the context o
f a viewer-centred reference frame.