In this paper, we describe the neural changes observed in the primary motor cortex of two monkeys while they learned a new motor skill. The monkeys had to adapt their reaching movements to external forces that interfered with the execution of their arm movements. We found a sizable population of cells that changed their tuning properties during exposure to the force field. These cells took on the properties of neurons that are involved in the control of movement. Furthermore, the cells maintained the acquired activity as the monkey readapted to the no-force condition. Recent imaging studies in humans have reported the effects of motor learning in the primary motor cortex. Our results are consistent with the findings of these studies and provide evidence for single-cell plasticity in the primary motor cortex of primates.I n a number of recent studies investigators have shown that when networks of neurons are repeatedly exposed to sensorymotor associations, learning of motor tasks occurs. Primates learn a new task as the result of repeated exposures to sensory signals coming from a variety of visual and proprioceptive sources. The sensory inputs are funneled to the motor areas of the central nervous system each time a movement is produced. The current view is that learning results from a change in the internal structure of the cortical and subcortical networks brought about by sensory and recurrent signals.Presumably, the iterative sensory-motor process leads to the establishment of an internal model of the controlled dynamics through a gradual change of the synaptic strength (1) of the neurons of the cortical and subcortical motor areas. The internal model is embedded in the newly formed connectivity of a group of neurons, and the activity of these trained neurons generates the neural impulses necessary for the execution of the learned motor task. According to this view, motor learning and the control of dynamics are two facets of the same process. In this paper, we describe the cellular changes in the circuitry of the primary cortical motor area of the monkey during the acquisition of a motor skill.In the experiments described here, a key feature of the task to which the monkeys were exposed involved a change in the mechanical environment with which their hand interacted. Because of this change, the neural representation of the arm would have to develop a new model to deal with the new dynamics of the environment. In this paper, we present psychophysical evidence for the formation of this new internal model and we describe the neural changes observed in the primary motor cortex as the new model was formed. MethodsDescription of the Motor Task. Two monkeys (Macaca nemestrina) were trained to grasp the manipulandum of a 2-df, lightweight, low-friction robot with a force-torque transducer mounted on the handle. Two torque motors were mounted on the base of the robot and produced force fields upon the hand of the monkey as the animal performed reaching movements (see ref. 2 for details).During an experimental sessio...