Vestibular System

The vestibular sense interprets cues from gravity and acceleration. In practice, those cues are equivalent, meaning that the vestibular organ cannot tell them apart. The vestibular organ is in the inner ear, but it has nothing to do with hearing. It consists of three semicircular canals filled with fluid. That fluid moves under acceleration or deceleration (but not under constant velocity). That fluid movement provides the cues for perceptions about gravity and acceleration.

Flying an airplane is a good example of that equivalence. Fighter pilots who "bank and yank," or who turn and climb at the same time, will feel a strong feeling of heaviness. That feeling, caused by their acceleration, will overwhelm the pull of gravity. So, if you asked them to tell you which way was down using only their vestibular organ, they would point to the cues provided by the turn, not to the cues provided by the earth's gravity.

Another example involving flying occurs when pilots fly in clouds without instruments. In such conditions, it only takes a few seconds before a pilot becomes completely disoriented. Many airplane crashes have occurred because of this phenomenon. These two examples illustrate why pilots are taught to rely both on visual cues and on their instruments while flying.

Another interesting example of when the vestibular system cannot tell you which way the earth's gravity is pulling you occurs underwater. Scuba divers in brightly lit deep water may become disoriented. One scuba diver reported that he thought his head was pointed up. When he tested that hypothesis by releasing a stream of bubbles, he found that he was lying on his back instead! (Notice that in this case, air bubbles were smarter than we are.) Again, this example illustrates why scuba divers are taught to execute that maneuver.

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