Human standing is modified by an unconscious integration of congruent sensory and motor signals

We investigate whether the muscle response evoked by an electrically induced vestibular perturbation during standing is related to congruent sensory and motor signals. A robotic platform that simulated the mechanics of a standing person was used to manipulate the relationship between the action of the calf muscles and the movement of the body. Subjects braced on top of the platform with the ankles sway referenced to its motion were required to balance its simulated body-like load by modulating ankle plantar-flexor torque. Here, afferent signals of body motion were congruent with the motor command to the calf muscles to balance the body. Stochastic vestibular stimulation (±4 mA, 0–25 Hz) applied during this task evoked a biphasic response in both soleus muscles that was similar to the response observed during standing for all subjects. When the body was rotated through the same motion experienced during the balancing task, a small muscle response was observed in only the right soleus and in only half of the subjects. However, the timing and shape of this response did not resemble the vestibular-evoked response obtained during standing. When the balancing task was interspersed with periods of computer-controlled platform rotations that emulated the balancing motion so that subjects thought that they were constantly balancing the platform, coherence between the input vestibular stimulus and soleus electromyogram activity decreased significantly (P < 0.05) during the period when plantar-flexor activity did not affect the motion of the body. The decrease in coherence occurred at 175 ms after the transition to computer-controlled motion, which subjects did not detect until after 2247 ms (Confidence Interval 1801, 2693), and then only half of the time. Our results indicate that the response to an electrically induced vestibular perturbation is organised in the absence of conscious perception when sensory feedback is congruent with the underlying motor behaviour.