Perspectives on the role of afferent signals in control of motor neuroprostheses.

K.O. Johnson reviews the architecture and low level neural mechanisms by which the external environment is transduced and encoded into the neural system, summarizing work that correlates neurophysiological and psychophysical testing with isolation of sensory components. The slowly adapting Type I afferent system is responsible for form and texture perception; the rapidly adapting afferent system is responsible for motion perception; and the Pacinian corpuscle system is responsible for vibratory sensation. R.R. Riso reviews the current level of understanding of the major factors to be considered in the design of a functional neuromuscular stimulation (FNS) grasp controller that uses cutaneous sensory feedback to detect slip. The elegant natural control scheme that matches the ratio of grip and lift forces to frictional conditions provides a model for implementing a slip-based control algorithm. D. Popović discusses the possible use of recordings from more proximal peripheral nerves to determine needed information for synthesis of locomotion. The discussion is illustrated with an animal model where rule-based closed-loop control is used for the ankle joint during treadmill locomotion. Neural signals from the tibial and superficial peroneal nerves were employed to substitute for missing afferent input from cutaneous and proprioceptive sensors. The feasibility of more invasive intraneural electrodes for distinguishing sensory from motor information in mixed nerves is considered. M. Koris raises surgical and functional issues relevant to developing clinical FNS systems. C. Van Doren suggests alternative neurophysiological and engineering approaches.