ABSTRACT
Research devoted to functional changes in the motor system during preparation for movement has been guided by the emerging conception of some isomorphism between models of motor organization proposed by neurophysiologists and psychologists. A sequence of three functional stages, each associated with activity in a particular neuronal system, would be responsible for goal planning, motor programming and movement execution respectively.
Experimental facts were mainly drawn from studies combining the experimental procedures used by psychologists to manipulate the functional state of a particular stage of motor organization, while simultaneously using neurophysiological methods to analyze changes in the activity of the neural structures presumed to be involved in the execution of that stage.
In a first experiment conducted in monkeys, single-cell activity was recorded in the precentral motor cortical structures during the foreperiod of a between-hands choice-reaction time paradigm in which the probability for the arm contralateral to the recording site to perform the response movement was varied. “Presetting” cells, that is, those firing at a rate which was related to response probability and, thus, to reaction time, were found, possibly expressing changes in the activity of associative cortical areas.
In a second experiment conducted in humans, late electromyographic responses triggered by suddenly stretching a muscle were analyzed during the foreperiod of a hand-pointing task, in which the warning signal provided information about the directional parameter of the response movement. Differential amplitude changes, according to the involvement of the muscle as either an agonist or an antagonist, were found for the later response only, which presumably involves cortical structures.
260The fact that such functional presetting was not found for the earliest monosynaptic response confirms that was previously shown by studying changes in the reactivity of reflex pathways during preparation. These changes influence a large set of spinal motor structures and appear to make the motoneuron pools less accessible to peripheral inputs. However, with adequate procedures, the reactivity of spinal reflex pathways was found sensitive to the force of the movement to be performed.
These results, together with other data collected in the field, are discussed in the frame of the hierarchical three-stage model proposed for motor organization, each stage being the target for specific preparatory processes.
