Proprioception and the Pinocchio Illusion

Proprioception — from Latin proprius, meaning “one’s own,” and perception — is one of the human senses. Rather than sensing external reality, proprioception is the sense of the orientation of one’s limbs in space. This is distinct from the sense of balance, which derives from the fluids in the inner ear, and is called equilibrioception. Without proprioception, we’d need to consciously watch our feet to make sure that we stay upright while walking. Proprioception doesn’t come from any specific organ, but from the nervous system as a whole. Its input comes from sensory receptors distinct from tactile receptors — nerves from inside the body rather than on the surface. Proprioceptive ability can be trained, as can any motor activity. Learning any new motor skill involves training our proprioceptive sense. Anything that involves moving our arms or legs in a precise way without looking at them invokes it — baseball, basketball, painting. Proprioception is often overlooked as one of the senses because it is so automatic that our conscious mind barely notices it. It is one of the oldest senses, probably even more evolutionarily ancient than smell.

In 1906, Charles Scott Sherrington published a landmark work that introduced the terms “proprioception“, “interoception“, and “exteroception“. The “exteroceptors” are the organs responsible for information from outside the body such as the eyes, ears, mouth, and skin. The interoceptors then give information about the internal organs, while “proprioception” is awareness of movement derived from muscular, tendon, and articular sources.

Proprioception is occasionally impaired spontaneously, especially when one is tired. One’s body may appear too large or too small, or parts of the body may appear distorted in size. Similar effects can sometimes occur during epilepsy or migraine auras. These effects are presumed to arise from abnormal stimulation of the part of the parietal cortex of the brain involved with integrating information from different parts of the body.

Proprioceptive illusions can also be induced, such as the Pinocchio illusion.

The Pinocchio illusion is an illusion that one’s nose is growing longer, as happened to the literary character Pinocchio when he told a lie. It is an illusion of proprioception, reviewed by Lackner (1988). To experience the illusion, a vibrator is applied to the biceps tendon while one holds one’s nose with the hand of that arm. The vibrator stimulates muscle spindles in the biceps that would normally be stimulated by the muscle’s stretching, creating a kinesthetic illusion that the arm is moving away from the face. Because the fingers holding the nose are still giving tactile information of being in contact with the nose, it appears that the nose is moving away from the face too, in a form of perceptual capture. The illusion involves activity in the parietal cortex of the brain responsible for integrating information from different parts of the body (e.g., Ehrsson, Kito, Sadato, Passingham, & Naito, 2005). Distortions of the size of parts of the body can sometimes occur spontaneously, through application of psychedelic chemicals or during epilepsy or migraine auras.

People that have a limb amputated may still have a confused sense of that limb existence on their body, known as phantom limb syndrome. Phantom sensations can occur as passive proprioceptive sensations of the limb’s presence, or more active sensations such as perceived movement, pressure, pain, itching, or temperature. There are a variety of theories concerning the etiology of phantom limb sensations and experience. Jack Tsao, MD. at Walter Reed Hospital has advanced a theory based on the concept of “proprioceptive memory.” This theory argues that the brain retains a memory of specific limb positions and that after amputation there is a conflict between the visual system, which literally sees that the limb is missing, and the memory system which remembers the limb as a functioning part of the body. Phantom sensations and phantom pain may also occur after the removal of body parts other than the limbs, such as after amputation of the breast, extraction of a tooth (phantom tooth pain), or removal of an eye (phantom eye syndrome).

David Bohm expands the idea of proprioception to include the “self perception of thought” in which thought is aware of its movement.

Information found at: http://en.wikipedia.org/wiki/Proprioception and http://www.wisegeek.com/what-is-proprioception.htm both accessed 01/01/2012

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