NEURONAL CORRELATES OF THE SPECIFICATION OF MOVEMENT DIRECTION AND FORCE IN 4 CORTICAL AREAS OF THE MONKEY

Single-neuron activity was recorded in several areas of the cerebral c ortex when monkeys performed a movement-precueing reaction time task. In such a task, information provided by a first signal ('preparatory s ignal', PS) refers to what has to be done in response to a second sign al ('response signal', RS). Two monkeys were trained to rotate a handl e by performing wrist flexion/ extension movements while two levels of frictional resistance were applied to the manipulandum. The PS provid ed 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 mov ement parameter shortened reaction time (RT) - RT being shorter when m ovement direction than movement force was precued -, as compared to th e condition in which no prior information was provided, the analysis o f changes in neuronal activity during the preparatory period (PP), i.e ., the instructed delay between PS and RS, makes the study of the neur onal mechanism underlying the specification of movement parameters pos sible. 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 activit y in relation to movement direction than in relation to movement force , not only during PP, but also during RT and movement time (MT). The n umber of purely direction-related neurons increased, whereas the numbe r 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 popu lation of neurons changed its activity in relation to both movement di rection and force, especially in MI. These data provide further eviden ce for the clustering of distinct neuronal populations responsible for programming movement direction and force.