THE ORIGIN AND USE OF POSITIONAL FRAMES OF REFERENCE IN MOTOR CONTROL

A hypothesis about sensorimotor integration (the lambda model) is desc ribed and applied to movement control and kinesthesia. The central ide a is that the nervous system organizes positional frames of reference for the sensorimotor apparatus and produces active movements by shifti ng the frames in terms of spatial coordinates. Kinematic and electromy ographic patterns are not programmed, but emerge from the dynamic inte raction among the system's components, including external forces withi n the designated frame of reference. Motoneuronal threshold properties and proprioceptive inputs to motoneurons may be cardinal components o f the physiological mechanism that produces positional frames of refer ence. The hypothesis that intentional movements are produced by shifti ng the frame of reference is extended to multi-muscle and multi-degree s-of-freedom systems with a solution of the redundancy problem that al lows the control of a joint alone or in combination with other joints to produce any desired limb configuration and movement trajectory. The model also implies that for each motor behavior, the nervous system u ses a strategy that minimizes the number of changeable control variabl es and keeps the parameters of these changes invariant. Examples are p rovided of simulated kinematic and electromyographic signals from sing le- and multi-joint arm movements produced by suggested patterns of co ntrol variables. Empirical support is provided and additional tests of the model are suggested. The model is contrasted with others based on the ideas of programming of motoneuronal activity, muscle forces, sti ffness, and movement kinematics.