Why internal feedback is necessary in the perception-action loop

Animals can move reliably in unpredictable environments. The study of sensorimotor control has assumed that sensory information from the environment leads to actions, which then act back on the environment, creating a single, unidirectional perception-action loop. This loop contains internal delays in sensory and motor pathways, which can lead to unstable control. We show here that these delays can be compensated by internal feedback signals that flow backwards, which are ubiquitous in neural sensorimotor systems. Recent ad-vances in control theory show that internal feedback plays a key role in compensating internal delays. Based on these theoretical ad-vances, we use a basic, mathematically tractable control model to show that internal feedback has an indispensable role in state estimation, can explain localization of function – why different parts of cortex control different parts of the body – and how attention can improve motor performance, all of which are crucial for effective sensorimotor control. Control theory can explain anatomical, physiological and behavioral observations, including motor signals in visual cortex, heterogeneous kinetics of sensory receptors and the presence of giant Betz cells in motor cortex.