<P> Georgopoulos and colleagues suggested that muscle force alone was too simple a description . They trained monkeys to reach in various directions and monitored the activity of neurons in the motor cortex . They found that each neuron in the motor cortex was maximally active during a specific direction of reach, and responded less well to neighboring directions of reach . On this basis they suggested that neurons in motor cortex, by "voting" or pooling their influences into a "population code", could precisely specify a direction of reach . </P> <P> The proposal that motor cortex neurons encode the direction of a reach became controversial . Scott and Kalaska showed that each motor cortex neuron was better correlated with the details of joint movement and muscle force than with the direction of the reach . Schwartz and colleagues showed that motor cortex neurons were well correlated with the speed of the hand . Strick and colleagues found that some neurons in motor cortex were active in association with muscle force and some with the spatial direction of movement . Todorov proposed that the many different correlations are the result of a muscle controller in which many movement parameters happen to be correlated with muscle force . </P> <P> The code by which neurons in the primate motor cortex control the spinal cord, and thus movement, remains debated . </P> <P> Some specific progress in understanding how motor cortex causes movement has also been made in the rodent model . The rodent motor cortex, like the monkey motor cortex, may contain subregions that emphasize different common types of actions . For example, one region appears to emphasize the rhythmic control of whisking . Neurons in this region project to a specific subcortical nucleus in which a pattern generator coordinates the cyclic rhythm of the whiskers . This nucleus then projects to the muscles that control the whiskers . </P>

The primary motor cortex in the left hemisphere controls