Neuronal correlates of the specification of movement direction and force in four cortical areas of the monkey.

Single-neuron activity was recorded in several areas of the cerebral cortex when monkeys performed a movement-precueing reaction time task. In such a task, information provided by a first signal ('preparatory signal', PS) refers to what has to be done in response to a second signal ('response signal', RS). Two monkeys were trained to rotate a handle by performing wrist flexion/extension movements while two levels of frictional resistance were applied to the manipulandum. The PS provided complete, partial or no prior information about movement direction (flexion or extension) and/or the level of the frictional force (weak or strong). Since providing partial prior information about either movement parameter shortened reaction time (RT)--RT being shorter when movement direction than movement force was precued--, as compared to the condition in which no prior information was provided, the analysis of changes in neuronal activity during the preparatory period (PP), i.e., the instructed delay between PS and RS, makes the study of the neuronal mechanism underlying the specification of movement parameters possible. The activity of 411 neurons of the primary motor (MI), premotor (PM), somatosensory (SI) and parietal (PA) cortex was recorded during task performance. Many more neurons changed selectively their activity in relation to movement direction than in relation to movement force, not only during PP, but also during RT and movement time (MT). The number of purely direction-related neurons increased, whereas the number of purely force-related neurons decreased from SI to PA, then to MI and finally to PM. During PP, selective activity changes were related only to one movement parameter, whereas during RT and MT, a large population of neurons changed its activity in relation to both movement direction and force, especially in MI. These data provide further evidence for the clustering of distinct neuronal populations responsible for programming movement direction and force.