Influence of expectation on postural disturbance evoked by proprioceptive stimulation.

Recent experiments have shown that the vestibular channel of balance control differs fundamentally from the visual channel. Whereas the response to a visual perturbation can be suppressed if the subject has awareness that an upcoming disturbance is likely to be caused by an external agent rather than by self-motion, a similar assumption cannot be made concerning the vestibular system. The present experiment investigated whether postural responses evoked by a proprioceptive perturbation (vibration of the Achilles' tendon at 90 Hz for 2.2 s) are either automatic and immune to expectation (similarly to vestibular responses) or cognitively penetrable (similarly to visual responses). Subjects (n = 12) stood on a force platform while stimuli were delivered either by the subject himself (self-triggered condition) or by the experimenter. For the latter condition, the stimulus was delivered either without warning (unpredictable condition) or at a fixed interval (500 ms) following an auditory cue (precue condition). Results showed that the backward CoP displacement induced by vibration was delayed by approximately 500 ms in the expected and self-triggered conditions compared to the unexpected condition. However, once initiated, the velocity of the backward displacement was higher in the self-triggered condition as compared to the unexpected condition. After a period of 2.2 s of vibration, the amplitude of this backward CoP displacement was similar in the three experimental conditions. Therefore, although expectation appears to delay the upcoming of the main backward body sway, it does not appear to be able to weight the impact of the proprioceptive stimulation. This suggested that afferents provided by the different sensory channels involved in postural control are not similarly susceptible to high level processes such as expectation.