Interaction of vestibular and proprioceptive inputs.

The study investigates the interaction of leg proprioceptive and vestibular afferents for human self-motion perception in space. Stimulation consisted of sinusoidal (0.025-0.4 Hz) and transient horizontal rotations of trunk and head in space (vestibular stimulus, VEST) and of the feet relative to the trunk (leg proprioceptive stimulus, LEG-PROP). Measures of the perception were obtained with the help of a pointing procedure. Leg proprioception. The perception of relative motion between feet and trunk during LEG-PROP was veridical across the frequencies tested and had a low detection threshold (0.2 degree/s). Perception of trunk turning in space. Trunk turning during VEST was underestimated, especially at low frequencies, and the threshold of the perception was > or = 1.0 degree/s. LEG PROP evoked an illusion of trunk turning, which reached a considerable magnitude at low frequencies. During VEST-LEG-PROP combinations, the perception varied monotonously as a function of both inputs. Reflecting the deficiencies of its constituents, it was erroneous with 1 exception. During trunk rotation about the stationary feet, the perception was approximately veridical across frequency and its threshold was down to 0.2 degree/s, suggesting that it was determined essentially by leg proprioception in this condition. These findings resemble those previously obtained for neck proprioception and, therefore, were incorporated into a conceptual model of vestibular-proprioceptive interaction in general. In this model, first an internal notion of foot in space is created by summing the following high-threshold signals: head in space (vestibular), trunk relative to head (neck proprioceptive), and foot relative to trunk (leg proprioceptive). Second, further addition of low-threshold proprioceptive signals of trunk on foot and head on trunk yields the perception of trunk in space and head in space, respectively. Not included in the model is the finding that subjects' perceptual mode may change in certain conditions. When foot excursion exceeds a certain magnitude, for instance, vestibular input alone may determine the self-motion perception.