How The Body Knows Where It Is

Like the instruments on the dashboard of a car, the body's somatosensory system reports on its internal state—temperature, position, and similar data—and analyzes information collected by the senses. The somatosensory system is the infrastructure behind the brain's image of the body. In the past, experiments have found that when a person sees a fake hand being stroked at the same time that they feel, but can't see, their own hand being stroked in the same place, their brains identify the fake had as theirs. This phenomenon is due to somatosensory input and was an important step in science's understanding of how the system works. In particular, it involves a feedback process between the brain and nerve endings in the skin that respond to touch, pressure, and changes in temperature and also incorporates information from the eyes and ears. The system is responsible for, among other things, our knowledge of where we are and where we're facing.

Now another breakthrough has demonstrated the role of the motor neurons in this process. The primary function of the motor neurons is to transmit signals from the brain to the muscles. Now scientists have found that they also respond to visual cues—the parts of the brain that are responsible for the motor neurons become active in response to visual stimulus. These brain regions get no direct input from the visual cortex, the section of the brain that processes and interprets visual data from the eyes. However, what a person sees appears to affect their sensation of being touched.

This has important implications for neurological medicine and beyond. Stroke patients sometimes struggle to regain somatosensory functioning if that part of the brain has been damaged, but these studies may help point to techniques and treatment protocols that can be used to help them recover that function. In addition, people with neuroprosthetic limbs—prostheses that include biotechnological replacements for damaged nerves and are connected directly to the nervous system—can be helped to assimilate their prostheses into the brain's image of the body by involving the somatosensory system. This helps the body and the prosthetic function as a unit as people do with healthy limbs.

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