The Inner Ear

The inner ear is incredibly useful in maintaining our sense of balance and orientation. Especially when vision is limited, it can be very useful. However, there are limitations in what it is able to do, and it is subject to illusions. Beyond knowing the types of spatial illusions, it is important to understand how the inner ear actually works.

Semicircular Ducts

The three semicircular ducts connected to the cochlea called the anterior, lateral, and posterior. As they are each perpendicular to each other, they are able to sense in all three axes, similar to how we would think of pitch, roll, and yaw.

At the center of the canal is the cupola, which sits on top of sensory hairs located at the end of the vestibular nerve.

When the body experiences a rotation, fluid moves through the semicircular ducts, which causes the har cells to move. That motion is picked up by the vestibular nerve and gives a sensation of rotation.

Below is a diagram of entering and exiting a constant rate turn and how it effects the semicircular ducts:

Otolith Organs

The otolith organs sense gravity and linear acceleration.

The otolith organ is filled with a gelatinous membrane containing chalk-like crystals which cover sensory hairs.

The weight of the crystals cause the membrane to shift due to gravity, and are detected by the sensory hairs. Acceleration causes a similar shift.

Below is a simplified diagram of how the organ shifts to detect acceleration. Notice how there is a similar shift from accelerating forward and tilting the head backwards. This is what we would refer to as the somatogravic illusion.