Definition of proprioceptionThe term was introduced by Sherrington (1906), although this basic feeling of ourselves had always been present. Proprioception, or the perception of body awareness, is a that people are frequently not aware of, but greatly rely upon. More easily demonstrated than explained, proprioception is the unconscious awareness of where the various regions of the body are located at any given time. For example, with closed eyes, we can say where our hands or legs are at this moment. Without proprioception we could not bring a spoon bearing soup into the mouth, ride a bicycle, or change the gears of a car without looking at our hands or feet.Proprioception as described by SechenovIn his 1863 work Refleksi golovnogo mozga [Reflexes of brain], the renowned Russian physiologist Sechenov called proprioception a dark muscle sense (Sechenov, 2013, originally published 1863), and described the role of that muscle in the training of vision, hearing and other senses, especially in his work Elements of Thoughts (Sechenov, 2013). He demonstrated that spatial vision is formed first of all with the help of proprioceptors of the eye muscles, and secondly, due to multiple evaluation and combination of distance by eyes or legs. As for distance measurements, we still conserve in some countries (related to the length of body parts) units like feet, inch, or old ones such as ell (originally a cubit, i.e., approximating to the length of a man's arm from the elbow to the tip of the middle finger, or about 18 inches), dactilus or digit, and palm in ancient Greece.As Sechenov thought, the muscle is not only analysing components of space, but also of time: Near, far, height of subjects, their traces and velocities -- all are the products of the muscle sense... The same muscle sense, being partial (frac- tioned) in periodical movements, becomes a partial measurement instrument of space and time (Sechenov, 2013).Proprioception is tested by Russian neurologists when they ask patients to touch their nose with their finger or walk with their eyes closed along a straight line drawn on the floor. It is also checked by American police officers by having a suspect touch their nose with their finger, with eyes closed, to gauge alcohol intoxication: people with normal proprioception make an error of no more than twenty millimetres.Effects of alterations in proprioceptionWhen proprioception is altered, sensitive ataxia can take place, as in the clinical case described by Wingenshtein (Schmidt, 1984), when a patient after an operation had lost her proprioception and gradually forgot how to move or eat, and even stopped breathing. To remain functional, she would compensate for proprioceptive loss by other senses, principally by vision, as in the Ian Waterman case (BBC movie, 1998, The man who lost his body). If proprioceptive impairment takes place due to some body part being missing from one's mental self-image, we need to check it visually (to look down at our limbs, for example) or by touch (to pinch ourselves to feel this part); however, under a complete loss of proprioception in all or a part of body, we simply cannot feel it and may guide ourselves only visually, as in the Ian Waterman case. In order to learn to walk again, he used his eyes and needed to always look at his feet while moving (BBC documentary, 1998, The man who lost his body).Small proprioceptive alterations are felt when one catches cold or is simply tired. Under vibration or other external/internal stimuli (real and virtual) proprioceptive changes can appear, such as the Pinocchio effect (Kilteni, Normand, Sanchez-Vives, & Slater, 2012; Lackner, 1988) when body size perception is altered (perceived as too large or too small). To experience the Pinocchio Illusion, you need to apply a vibrator to the biceps tendon while one holding your nose with the ipsilateral hand. …