How many senses do we have? The common answer is five: taste, touch, smell, vision, and hearing. If that is so, then which of those five senses is responsible for the sensation of falling? Or the feeling of acceleration and deceleration while riding in a car (even if your eyes are closed)? What about leaning to the right or left with your eyes closed—how do you know which way you are leaning?
The answer is that none of the traditional five senses can account for your sense of falling, acceleration, or tilt. In this chapter, we will explore a sixth sensory system that is often overlooked—the vestibular system. In Chapter 9 you learned about the structure of the ear, including the inner ear, and its role in hearing. In this chapter we will explore the other structures of the inner ear and the role they play in your vestibular sense—your sense of motion, head position, and the direction of gravity.
In the Vestibular System activity (Activity 12.1), you will take an animated journey through the vestibular organs of the inner ear and learn how they transduce motion into neural energy. In the activity on Sinusoidal Motion (Activity 12.2), you will see how a person’s movement in space can be visualized in several different ways on a graph. The third activity describes Torsional Eye Movements (Activity 12.3), a special kind of eye movement that compensates for head tilt.
Several essays explore topics such as the differences (if any) between our sense of Gravity and Linear Acceleration (Essay 12.1), Space Motion Sickness (Essay 12.3) experienced by astronauts due to the lack of gravity in space, and how the Otolith organs (Essay 12.2) of your vestibular system can sense the difference between translational and tilt motions.