Activity of neurons in area 5 during a simple arm movement in monkeys before and after deafferentation of the trained limb.

Unit recordings were performed in the postcentral cortex and focused on area 5 of awake monkeys during the execution of a learned movement of the contralateral forearm so that the time relationship between the motor act and any modification of neuronal activity could be precisely correlated. Recordings were obtained from intact animals (561 neurons) and after deafferentation (C1-T7) of the trained limb (344 neurons). Of the movement-related neurons in normal animals, 243 cells were located in area 5 and these cells were divided into two populations. The first population (66% of movement-related neurons) presented modifications of activity after the onset of movement and receptive fields, often complex, were identifiable for these somaesthetic-like cells. No such neurons were found in the same cortical area after deafferentation. The second population (34% of movement-related neurons) presented modifications of activity related to movement but these changes occurred well before the onset of movement, up to 280 ms before. These cells were also characterized by an absence of sensory modulation and they represented the entire population of movement-related neurons recorded in area 5 after deafferentation (124 neurons). The first population appears to subserve a complex somaesthetic function. The second population is subject to purely central influences which, in part, may be due to corollary discharge or internal feedback. However, this population most likely represents a command apparatus for movement located 'upstream' to the motor cortex.