Human balance control during cutaneous stimulation of the plantar soles.

Previous work on human postural control of upright stance, performed in the absence of visual and vestibular orientation cues, suggests that somatosensory cues in the feet enable subjects to maintain equilibrium during low-frequency platform tilts. Here we confirm earlier studies which indicated that stimulation of plantar cutaneous mechanoreceptors can lead to postural responses. Yet, this stimulation did not modify considerably the postural reactions of normal subjects and vestibular loss patients during platform tilts. We therefore suggest that it is necessary to differentiate between (i) cues from plantar cutaneous receptors involved in exteroceptive functions, like the evaluation of the support structure or of relative foot-to-surface motion, and (ii) cues from deep receptors which subserve proprioceptive functions like the control of center of pressure shifts within the limits of the foot support base.

Vestibular, visual, and somatosensory contributions to human control of upright stance.

We investigated the changes of human posture control of upright stance which occur when vestibular cues (VEST) are absent and visual and somatosensory orientation cues (VIS, SOM) are removed. Postural responses to sinusoidal tilts of a motion platform in the sagittal plane (+/-2 degrees, f=0.05, 0.1, 0.2 and 0.4 Hz) were studied in normal subjects (Ns) and patients with bilateral vestibular loss (Ps). We found that absence of VEST (Ps, visual reference) and removal of VIS (Ns, no visual reference) had little effect on stabilization of upright body posture in space. In the absence of both VEST and VIS (Ps, no visual reference) somatosensory graviception still provided some information on body orientation in space at 0.05 and 0.1 Hz. However, at the higher frequencies Ps qualitatively changed their behavior; they then tended to actively align their bodies with respect to the motion platform. The findings confirm predictions of a novel postural control model.

Human self-motion perception during translatory vestibular and proprioceptive stimulation.

Self-motion perception in space was studied in normal human subjects during passive vestibular stimulation (lateral translation of whole body in space), proprioceptive stimulation (of feet relative to trunk) and combinations thereof with the eyes closed. Stimulation was sinusoidal, +/- 10 cm, over a frequency range of 0.025-0.4 Hz. Vestibular self-motion perception became increasingly underestimated at low frequency, due to a rather high detection threshold. Proprioceptive stimulation at low frequency elicited a small self-motion illusion. During body translation relative to the stationary feet (vestibular-proprioceptive combination) the magnitude of perceived self-motion was constant across frequency and its threshold was low, as if determined by proprioception alone. Nevertheless, the results can be interpreted in terms of a vestibular-proprioceptive interaction, in analogy to previous findings for rotational stimuli.