REM-sleep twitching in adults and the maintenance of specialized sensorimotor systems.

Twitching, an abundant and spontaneous infant behavior and a defining feature of rapid eye movement (REM) sleep, supports development of the sensorimotor system. For unknown reasons, twitching persists into adulthood. Whereas infantile twitches are distributed across all body parts, adult twitching appears to occur more selectively in those appendages used for active sensing, such as elephant trunks, rat whiskers, pig snouts, and raccoon hands. Notably, these specialized appendages also have disproportionately large representations in sensorimotor cortex, a phenomenon known as cortical magnification. We propose that active sensing and cortical magnification - or, more broadly, neural magnification - helps to explain the persistence of twitching in adults. These associations between twitching, active sensing, and neural magnification are likely a subcategory of a broader need to maintain and update vital sensorimotor functions. We contextualize this proposal within an evolutionary-developmental framework, with the functions of twitching shifting across the lifespans of diverse species, including invertebrates that lack a cerebral cortex. This comprehensive theory of twitching may yield new insights into sleep-dependent processes that support the development, calibration, maintenance, and repair of neuroplastic systems across the animal kingdom, and perhaps even inspire new therapeutic approaches for humans after limb amputation, stroke, or other injuries.