Motor Functions of the Posterior Parietal Cortex in Monkeys

Recently we recorded readiness potentials for self-paced movements of various body parts in the posterior parietal cortex (PPC) as well as in the motor cortex. To study involvement of the PPC in muscle force control, we analyzed readiness potentials recorded by electrodes implanted in many cortical areas in two monkeys performing self-paced hand movements to lift a lever either with or without additional load. Readiness potentials in the PPC increased in amplitude as required muscle force in hand movements increased as well as in the premotor, motor and somatosensory cortices. Further, we investigated in which body part the PPC activates muscles and how easily. Electromyograms induced in muscles in various body parts after cortical stimulation in three monkeys were analyzed. Stimulation of areas 5 and 7 produced a somatotopic pattern in muscle activities, as in the motor cortex. Stimulus intensity in area 5 for inducing muscle activity was slightly higher than that in the motor cortex, while much higher stimulus intensity was required in area 7. Taken together, preparative muscle force control was found in the PPC, which could activate muscles in various body parts, similar to the motor cortex but with a higher threshold. These suggest that the PPC plays important roles in motor functions.