The vestibular sense is commonly known as our sense of balance. It is commonly misunderstood to be the same thing as proprioception. The vestibular sense is detected by an organ in the inner ear, and proprioception refers to detector cells in muscles, tendons and joints that provide information about body position and accuracy of movements. Additionally, proprioception is processed in the primary somatosensory cortex, while the vestibular sense is processed in the medulla and temporal cortex. Proprioception is considered part of the kinesthetic/somatosensory/touch sense.

The vestibular system has two parts: vestibular sacs and semicircular canals. Vestibular sacs detect the position of the head with respect to gravity, and the semicircular canals detect movement of the head. The semicircular canals contain fluid that moves when the head is moved. The movement of this fluid is detected by hair cells that eventually synapse with the cochlear nerve.

This sense helps to adjust the eyes when the head changes position. Incongruities between the visual and vestibular systems produce nystagmus and motion sickness. Motion sickness can also be caused when the vestibular system detects movement that is inconsistent with information from the motor system. This is what happens when you get seasick. From an evolutionary perspective this may be a way for an organism like a mouse to ward off potential predators by becoming sick when the predator attempts to move them, or attack them while their eyes are closed.

Source: Klein, Stephen. Biological Psychology. New Jersey: Prentice Hall, 2000.

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