More than aiding balance, vestibular organs provide an on-line movement guidance system

Anyone who’s had to find his or her way through a darkened room can appreciate that nonvisual cues play a large role in our sense of movement. What might be less apparent is that not all such cues come from our remaining four senses.

In a finding that broadens our understanding of human movement control, researchers at the Institute of Neurology in London have shown that the inner-ear vestibular organs provide what is essentially an on-line movement guidance system for maintaining the accuracy of whole-body movements.

The vestibular organs are commonly thought of as sensors that serve balance, the control of visual gaze, and higher spatial functions, such as navigation. However, because these organs respond to head movements, such as accelerations, they also have the potential to signal the accuracy of any voluntary movement that causes the head to move in space. The brain may then use that information for movement control in the same way that it uses sensory feedback information from the eyes, muscles, and skin to assess and adjust a limb movement as it is being executed.

In the new work, appearing in the August 9 issue of Current Biology, Brian Day and Raymond Reynolds of University College London show that the brain uses signals from the vestibular organs to make on-line adjustments to whole-body voluntary movements. The researchers were able to show this by precisely stimulating the vestibular sensory nerves through the skin while volunteers performed a simple upper-body movement. The researchers found that the stimulus altered the normal vestibular response to the upper-body movement and automatically caused the subjects to adjust their movement speed–and did so in a predictable way that depended on how the vestibular sensory nerves were stimulated. As one might expect when perturbing the guidance system, the effect of the nerve stimulation was only apparent in connection with body movement; the same stimulus had almost no effect when the subjects were stationary.

The authors of the study point out that this vestibular mechanism of movement control may be especially valuable when other senses become less reliable–such as in the dark–or for complex, high-precision whole-body movements, such as those of the gymnast or circus performer.