ARE EXTENT AND FORCE INDEPENDENT MOVEMENT PARAMETERS - PREPARATION-RELATED AND MOVEMENT-RELATED NEURONAL-ACTIVITY IN THE MONKEY CORTEX

Movement extent and movement force can be independently controlled in motor performance. Therefore, independent representations of extent an d force should exist in the central nerve us system (CNS). To test thi s hypothesis, microelectrode recordings were made in sensorimotor cort ex of monkeys trained to perform visually cued wrist flexion movements of two extents, against two levels of frictional resistance. An initi al preparatory signal(PS) provided complete, partial or no information about extent and/or force of the movement, which had to be performed in response to a second, response signal (RS). The activity of 511 neu rons of the primary motor cortex (MI), the premotor cortex (PM), the p ostcentral cortex (PC), and the posterior parietal cortex (PA) was rec orded in two monkeys. Both reaction time (RT) and neuronal data sugges t that there exist independent neuronal mechanisms responsible for the programming of either parameter. On the one hand, partial information about either movement parameter shortened RT when compared with the c ondition of no prior information. On the other hand, there were, among others, two discrete populations of neurons, one related only to exte nt, the other only to force. Preparatory changes in activity related t o either movement parameter were mainly located in the frontal cortex, especially in the PM. After occurrence of the RS, the percentage of s elective changes in activity increased and tended to extend to the par ietal cortex. In particular during the movement, force-related changes in activity have been encountered in PA. Furthermore, we conducted tr ial-by-trial correlation analyses between RT and preparatory neuronal activity for all conditions of prior information. The mean correlation coefficient was significantly higher in the condition of information about movement extent than of information about movement force and it was significantly higher in MI/PM than in PC/PA.